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Electronic
Computer
Projects

for Commodore and Atari
Personal Computers

Soon Sivakumaran

COMPUTE! Publications,!^

Part of ABC Consumer Magazines, Inc.
One of the ABC Publishing Companies

Greensboro, North Carolina

Reproduction or translation of any part of this work beyond that permitted by
Sections 107 and 108 of the United States Copyright Act without the permission of
the copyright owner is unlawful.

Printed in the United States of America

10 987654321

ISBN 0-87455-052-1

The author and publisher have made every effort in the preparation of this book to insure the ac-
curacy of the programs and information. However, the information and programs in this book are
sold without warranty, either express or implied. Neither the author nor COMPUTE! Publications,
Inc., will be liable for any damages caused or alleged to be caused directly, indirectly, incidentally,
or consequentially by the programs, information, and/or projects in this book; nor liable for any
accidents or errors in the implementation of the projects, the handling of any materials for the
projects, and for any damage that may result to any individual or equipment.

The opinions expressed in this book are solely those of the author and are not necessarily those of
COMPUTE! Publications, Inc.

COMPUTE! Publications, Inc., Post Office Box 5406, Greensboro, NC 27403, (919)
275-9809, is part of ABC Consumer Magazines, Inc., one of the ABC Publishing
Companies, and is not associated with any manufacturer of personal computers. Atari,
Atari 400, Atari 800, Atari XE, and Atari XL are trademarks of Atari Corporation.
Commodore 64, Commodore 128, and VIC-20 are trademarks of Commodore
Electronics Limited. Radio Shack is a trademark of Tandy Corporation.

Contents

Foreword v

Preface vii

1 Getting Inside Your Computer 1

2 Exploring the Control Port 7

3 Making a Joystick 21

4 Game Paddles 31

5 An Analog Light Sensor 43

6 A Light Pen 51

7 A Digital Light Sensor 65

8 An Electronic Switch 79

9 A Burglar Alarm 95

10 Digital Logic 107

1 1 A Better Logic Probe 121

12 More Ideas 127

1 3 Robotics 1 49

Appendices 163

A COMPUTEI's Guide to Typing In Programs 165

B Integrated Circuits 171

Index 181

Foreword

Your personal computer is a powerful machine. It can calcu-
late faster than any clerk, print documents faster than any typ-
ist, and play chess with the masters. Your software library is
probably full of such applications, along with games, utilities,
and educational software.

But a computer can do a lot more than run software. It can
be a sophisticated controller which turns lights on and off,
monitors your home's windows and doors, and makes robot
limbs move. To accomplish these tasks, however, you need
sensors, motor circuits, and other electronic devices. That's
why you need Electronic Computer Projects.

This book is a step-by-step guide to building a variety of
electronic devices, from the simple to the sophisticated. With
complete and concise instructions anyone can follow, and ac-
companied by detailed photographs and figures, Electronic
Computer Projects is the book with which you can teach your
Commodore or Atari personal computer valuable new tricks.

Parts lists outline what you need, the instructions show you
how to do everything from heating the soldering iron to plug-
ging in the last component, and programs test and adjust each
project.

Learn how to build your own joysticks, game paddles, and
light pens. See how to put together two kinds of light sensors,
even how to create a burglar alarm with an infrared sensor
that "sees" in the dark. Logic probes, which let you "look"
into digital circuits, multiplexers and demultiplexers, even
robotic motor control circuits are all part of this book's intrigu-
ing and educational projects. Each project has been built and
tested — you won't find any surprises when you connect a de-
vice to your computer.

Electronic Computer Projects is a thorough guide to your ad-
venture into computer control of the outside world.

Preface

Computers are versatile machines which can be programmed
to perform tasks quickly and accurately. These tasks, or appli-
cations, run the range from games and word processing to fi-
nancial modeling or controlling the lights and heat in your
home.

Most computer applications, at some point in the process,
involve a program which processes data, or information. A
video arcade game is a good example. Here, the data is en-
tered by the player through a joystick — the processing the
program performs is the motion of the character on the screen.
Another example, word processing, lets you enter text (the
data) and process it (by formatting it properly and printing it
out). These kinds of applications are certainly useful, but they
aren't the end of your computer's capabilities.

A vast majority of computer applications are limited to act-
ing only on data entered by the user, either from the keyboard
or the joystick. Yet some of the most interesting applications
result when computers interact with the outside world. A com-
puter which can examine its surroundings can then use this
information to perform a useful function. Imagine your com-
puter, sensing that it's now dark outside, turning on your out-
side lights before you arrive home from work.

Computers have made possible systems which can control
extremely complex operations. The software, or programs
which instruct a computer to do a specific task, is in fact a
form of intelligence — if written correctly, the programs can re-
act to different conditions in predetermined ways.

This book will show ways you can use your computer to re-
late and react to the outside world. This is done through sen-
sors and actuators. Sensors gather information from the outside
world for the computer to process. Actuators allow the com-
puter to influence outside events. Several of the projects put
sensors and actuators together so that the computer can per-
form a given task.

To help you get started, the first few projects in this book
are relatively easy to complete. As your experience increases,

vii

Preface

so does the difficulty of the projects. If you're new to electron-
ics, then, start with the first projects in the book, developing
your skills as you progress toward the later, more difficult,
projects. But even though the projects increase in difficulty,
you won't need any exotic equipment or advanced skills to
complete them.

Almost all the projects contain less than a half-dozen com-
ponents. You'll have little difficulty with any of the projects,
but you should have some understanding of electronics trouble-
shooting procedures.

Building a Circuit

The basic idea when building a circuit is to wire together the
components' leads correctly. Occasionally, even experienced
builders make mistakes in wiring circuits. It's an excellent idea
to completely read through each section describing the circuit's
construction before starting. Color-coded wires can help in
tracing circuits. Examine the drawing of the breadboard layout
included in the book, and study the schematic diagram. When
you feel you understand what the circuit requires, go ahead
and wire your test circuit.

Several methods of wiring, or breadboarding, circuits are
available and suitable for the projects in this book. One
method involves mounting the components on a perforated
board. The components are positioned on one side of the
board so that their leads extend through the perforations to
the opposite side. The leads on the bottom are then connected
by wires soldered between them.

A similar method, called wire wrapping, requires that each
component be mounted in a socket before being placed on the
perf board. Each socket has metal posts which make contact
with the leads of the component. The posts extend through
the holes. Wire connections are made by wrapping one end of
a wire around one post and the other end of the same wire
around a second post. This is done with a wire wrap tool.

Most commercial electronic circuits, however, are con-
structed using printed circuit boards. Printed circuit boards are
copper plated on one or both sides. When making a circuit

viii

Preface

using printed circuit boards, you must first determine the loca-
tion of components on the board. Then, the interconnections
between the components' leads are marked on the board with
an indelible felt-tip marker. The board is submersed in a
chemical solution which dissolves all copper from the board
except that along the traces you've marked. The copper traces
which remain form the connections between the components.
Commercial manufacturers prefer this technique because
boards can be produced in large quantities quickly and
cheaply with photographic techniques.

Your best bet is to use solderless breadboards, like those
available from Radio Shack. (Radio Shack's Experimenter
Socket, part number 276-174, was used to construct all the
projects in this book.)

A solderless breadboard is a plastic board with a grid of
"holes" called plug points (see Figure 1). The plug points are
internally wired so that the columns of points on each half of
the board are connected. A row of plug points on each half of
the board are connected as well. To hook up a circuit, compo-
nents are just plugged into the solderless breadboard. A con-
nection between two component leads can be made simply by
inserting each lead into plug points that are internally con-
nected. No soldering is required. Alternatively, jumper wires
can be used to bridge one set of connected plug points with
another to complete a connection. Jumper wires should be #22
gauge, as larger wires tend to spread the contacts of a plug
point too far apart, eventually ruining the breadboard. These
solderless boards may be reused for different experiments
without removing solder from connections. Most wiring ex-
planations in this book refer to coordinates on the solderless
breadboard.

Heat the Iron

Even if you use a solderless breadboard when constructing the
circuits in this book, you'll still have to do a little soldering.
While you won't have to do intricate soldering, wires still
must be attached to switches, the connectors which plug into
your computer, and other components that cannot be directly

Preface

• i * • *

•—- • — » — * — •

* e * #

c
o
a

Preface

plugged into a solderless breadboard. Soldering is the only re-
liable method of making these connections.

To make solder connections, you need a pencil soldering
iron (approximately 25 watts) and some electronic resin core
solder. Make sure the solder you use is suitable for electronic
work — some types of solder used in other applications use a
corrosive resin. The steps to make a solder connection are
listed below. Try soldering some practice connections with
scraps of wire before soldering on one of the projects.

1. Make sure the joint you're going to solder is free of dirt and
grease. The solder will not bond properly if it's not clean.

2. Try to make the joint as firm as possible before soldering. If
you're joining two wires, twist them together. When con-
necting to a terminal, like that of a switch, twist the wire
around it. Doing this makes the connection more reliaible,
as a dab of solder by itself will not hold a connection to-
gether for long. If you're using stranded wire, twist the
strands together before making the connection. This will
prevent stray strands from shorting out nearby connections.

3. When you're soldering an electronic component such as a
transistor or IC (integrated circuit) chip, attach an alligator
clip to the lead of the component just above the point
where you're going to make the connection. The metal alli-
gator clip acts as a heat sink, drawing heat away from the
component, preventing damage.

4. Plug in the soldering iron and wait until its tip is hot
enough to melt solder.

5. Touch the tip of the soldering iron to the joint and wait a
couple of seconds for it to heat up.

6. Touch the solder to the joint, not the tip of the soldering
iron. Remember always to heat the work surface, not the
solder. Some solder should flow onto the joint. Remove the
remaining solder from the joint and then remove the solder-
ing iron. A minimum amount of solder should be used
when making a connection. Excess solder not only looks
sloppy, but can cause short circuits.

Preface

7. A good solder connection is bright and shiny. If it's a dull
gray, you have what's called a cold joint. If this happens, re-
make the connection. A poor solder joint can cause an other-
wise perfect circuit to fail.

8. Sometimes it will be necessary for you to tin, or coat, the
leads to a component with a thin coating of solder before
establishing a connection. Tinning helps remove oxidation
and cleans the service, providing a better electrical and me-
chanical connection.

9. When you're finished soldering, wipe the tip of your solder-
ing iron on a damp sponge to remove any excess solder and
resin. The tip should be a silver color, without any trace of
dirt or foreign objects. This practice will increase the useful
life of the tip. Be sure to unplug the soldering iron as well —
an unattended hot iron is a fire hazard.

In Your Toolbox

Besides a soldering iron, there are a few other tools you'll
need. A pair of small wire cutters and a wire stripper are
handy, but you can strip insulation from a wire with a knife if
you're careful not to nick the conductor. Needle-nose pliers
are useful for twisting and untwisting wire, and for reaching
tight spots too small for your fingers. A drill and a screwdriver
are required for some of the projects to mount the circuits in
a case.

Since you'll be working with integrated circuits (ICs), an IC
extractor will be handy for removing the IC chip from the cir-
cuit board. You'll find that the pins of the ICs are very easy
to bend.

If the circuit doesn't give the results you expect after you've
carefully built it, compare your circuit to the diagram in the
book. Are all the pins properly connected? Is the IC inserted
correctly? Have you entered the program designed to use the
circuit properly? Most programs included with this book are
short, so you should have little trouble entering them cor-
rectly. To make this part of the job even easier, use "The
Automatic Proofreader" found in Appendix A — it's an error-

xii

Preface

checking program which insures that you type in the program
correctly the first time.

Each circuit described in this book has been thoroughly
tested. The circuits and the demonstration programs will work
as explained in the text, providing you follow the instructions
carefully.

All projects are designed to operate with the Commodore
64, Commodore 128 (in 64 mode), and VIC-20, and with the
Atari 400, 800, 600XL, 800XL, and 130XE personal computers.
The electronics involved is virtually identical for all these com-
puters, but the programs differ. Be sure to use the correct pro-
gram for your computer.

Have fun building the projects. Experiment with your own
ideas. By the time you've finished the last project, you should
understand how to interface your computer to almost any-
thing. At that point, your own imagination will take control.

Help's on the Way

To make it easier for you to use this book, you'll see small
graphic devices, called icons, throughout the book. These cues
will alert you to specific sections of each chapter:

shows you where the parts list for each project is
located. The number of components, their names,
and their Radio Shack part numbers are provided.

points out where the step-by-step instructions for
building each project begin. Steps are numbered
and are self-explanatory for the most part.

indicates that a testing procedure is described, or
that a program will follow. After you've finished a
project, you'll almost always be shown ways to
test it under working conditions to insure that it
operates as advertised.

calls your attention to various warnings and/or
notes on a project.

xiii

CHAPTER 1

Getting Inside
Your
Computer

Getting Inside Your
Computer

A computer is really nothing more than a series of
switches. Of course, it's a highly complex series of
switches. Thousands and thousands of them. Like
any switches, those you find in a computer can
have two positions, on or off. Storing information
in a computer is simply a matter of setting a se-
quence of switches on or off. If the switch is on, its
value is 1 . If the switch is off, it holds a value of 0.

The digital circuits used in computers are
switches which perform a specified series of ac-
tions, all based on the information in the on and
off states of the circuit. Everything in a digital cir-
cuit is either on or off, unlike analog circuits,
found in things like radios and televisions, which
deal with voltage levels and signals. Analog cir-
cuits may amplify or otherwise modify electronic
signals of various levels.

These on and off states within a computer are
represented as electronic signals of two different
voltage levels. In the computers this book deals
with, an on state corresponds to a signal of about
+ 5 volts, while the off state corresponds with a
signal of about volts. (Actually, a positive volt-
age more than 2.5 volts is considered on, and a
voltage less than 0.5 volts is considered off. This is
called a TTL, or transistor-transistor logic signal
level.)

Since a switch can exist in only two states, on
or off, what information can be represented? Sim-
ple — one of two numbers. The on state can be as-
signed to mean the number 1, and the off state
can be the number — the two digits which make
up the base two binary number system. The bi-
nary number system naturally lends itself for use

CHAPTER 1

with the digital logic circuitry in a computer. Un-
fortunately, we're used to counting by ten. The
number 65, for instance, has far more meaning to
us than its binary equivalent, 1000001.

Dealing with numbers at the binary level is best
left to the computer, but when you're trying to in-
terface your computer to the outside world, it's
often necessary to know and understand how set-
ting a bit in a memory address within the com-
puter affects the machine and its operation.

Number Systems

The binary number system is just another way of
representing numbers. The number system we
normally use, the decimal number system, is a
base ten number system. In other words, there are
ten different symbols (0, 1, 2, 9) which can be
used independently or in combination to represent
a given number. The value of the number repre-
sented depends on both the combination of sym-
bols and their relative positions. Let's take a look
at how a decimal number is made up, for example
37,506.

Position 43210
Decimal number 37506

Decimal

Symbol

Base

Position

Value

= 30,000

7,000

500

A o

37,506

As you can see, the first symbol, 3, represents
three times ten to the fourth power (in other

Getting Inside Your Computer

words, a 3 followed by four zeros). In the same
manner, each symbol represents a number multi-
plied by a power of ten (the power is determined
by the symbol's relative position). Adding all
these values together gives us the number 37,506.

The binary number system has just two sym-
bols, and 1. A Binary digiT, or bit, can thus be
either or 1. One bit by itself can only be used to
count up to one. Larger numbers in binary, as in
the decimal number where you can represent
larger value numbers with more digits (a number
like 245 requires three digits, whereas the number
4 requires just one), are represented by more bits.
As you can see in the following example, a binary
number is made in a similar manner as a decimal
number.

Position 76543210
Binary number 11111001

Decimal

Symbol

Base

Position

Value

128

= 32

= 1
249

Just as with the more comfortable decimal num-
bers, binary numbers are built by combining sym-
bols, then multiplying those values by two (since
binary is base two) raised to the power of the
symbol's position. A 1 in the seventh position,
then, represents the decimal value 128, while a 1
in the first position only represents 1.

CHAPTER 1

Here are a few more examples of how decimal
numbers (what we use) are represented as binary
numbers (what computers use). Try working them
out to see if you come up with the same values.

Decimal Binary

10 = 1010

5 = 101

134 = 10000110

Chock Full of Bits

In computers, bits are usually put together in
groups of eight when stored or otherwise manipu-
lated. A group of eight bits is called a byte. Since
it has eight positions, a byte can hold any number
from 00000000 binary (0 decimal) to 11111111 bi-
nary (255 decimal). The largest number which can
be represented by a byte, then, is 255.

These binary values are used by the computer
for all its operations, from executing an applica-
tions program to representing alphanumeric, even
graphics characters.

Understanding the elements of the binary num-
ber system, bits, and bytes will prove useful as
you move into the heart of Electronic Computer
Projects. The next chapter, "Exploring the Control
Port," is your start toward your own custom-built
computer projects.

u CHAPTER 2

■

Exploring the
Control Port

With a basic understanding of how a computer
operates, you can now begin exploring the com-
puter's ports, the gateways through which infor-
mation flows into and out of your computer. The
ports are often referred to as I/O ports, which
stands for input / output ports. Though there are
several different I/O ports on your computer, the
projects in this book use one port in particular, the
joystick port.

The joystick port provides plenty of access to
your computer for our demonstation circuits. Con-
nectors for this port are also commonly available.
They are the same type as those found on joy-
sticks and game paddles — D-subminiature female
connectors.

Figure 2-1 . The Joystick Port-

Control Port 1 Control Port 2 (Commodore 64)

Type

JOYAO

JOYBO

JOYA1

JOYB1

JOYA2

JOYB2

JOYA3

JOYB3

POT AY

POT BY

Button A/LP

Button B

+ 5 volts

Ground

POT AX

POT BX

CHAPTER 2

The joystick port has nine pins. Pins 1, 2, 3, 4,
and 6 are I/O pins. They are connected to wires
which may be used to communicate binary (on/
off) data. Pins 5 and 9 are used to input analog
data — we'll learn more about these pins later in
the book. Pin 7 is connected to a +5-volt source,
while pin 8 is connected to the power supply
ground.

Once you can identify the pins of a control port,
your next step is to understand how they can be
used. A few homemade "tools" will help.

A Simple Logic Probe

A logic probe is a device that lets you "look" into
what's going on in a digital circuit. It's a device
that tells you whether a 1 (high state) or a (low
state) is present at a point in the circuit. A simple
logic probe can be assembled from two main com-
ponents. This logic probe can be used to see how
the I/O lines of the control port are used to input
information.

To construct the logic probe, you'll need these
parts:

Part

Quantity Part Number

1 Light-emitting diode (LED) 276-041

1 IK ohm resistor 271-8027

2 Alligator clips 270-378

You'll also need some stranded copper wire
(about #22 gauge — Radio Shack part number 278-
1307) to make the probe leads, as well as some
electrical insulation tape.

(In this book, all part numbers are Radio Shack
part numbers.)

Exploring the Control Port

Figure 2-2. Commodore 64, VIC-20, and Atari Joystick Ports

The joystick ports for the Commodore 64 (top). Commodore VIC-20 (middle),
and Atari 800XL (bottom) are shown in these photos. Note that the VIC-20 has
only one joystick port.

CHAPTER 2

Here's the procedure for putting together your
simple logic probe:

1. Cut two pieces of stranded copper wire about
10 inches long. One wire will be the ground
lead, while the other will be the probe lead of
this simple logic probe.

2. Strip about a half inch of insulation from each
end of both wires and attach an alligator clip to
one end of each wire.

3. Solder the free end of one wire to the anode of
the light-emitting diode (LED). Usually, the an-
ode leg of an LED is the shorter of the two legs,
but this may vary with manufacturers. The wire
connected to the LED's anode is the probe lead
of the logic probe.

Note: An LED lights up when current passes
through it in the proper direction, which is from
anode to cathode. LEDs come in various colors,
but the most common are red and green. An LED
will conduct current in only one direction. It's im-
portant that you don't confuse the anode and the
cathode leads. In fact, if an LED fails to work in a
circuit, the first thing to check is whether it's been
installed correctly (see Figure 2-3). The cathode
lead is usually the longer of the two leads.

Figure 2-3. A Typical LED

Anode (+)

Cathode (-)

Exploring the Control Port

4. Solder the cathode of the LED to one end of the
IK ohm resistor.

5. Solder the other end of the IK ohm resistor

(brown, black, and red bands) to the free end of
the second wire. This second wire is now the
ground wire of the logic probe.
6. To complete your simple logic probe, cover your
soldered connections with some electrical insu-
lation tape. This will prevent short circuits from
occurring while you use your probe.

Note: Resistors limit the flow of electricity in a cir-
cuit and are measured in ohms. The value of a re-
sistor is indicated by its colored stripes — usually
there are four. The first two stripes' colors repre-
sent numbers as determined by the resistor color
code (see Table 2-1). These numbers are the first
two digits of the resistor value. The third stripe is
the multiplier. The number corresponding to the
multiplier color is the power of ten by which the
first two numbers are multiplied (an easy way to
remember it is simply to add x number of zeros to
the first two numbers, where x is the multiplier
color's value). All three numbers are combined to
give the value (in ohms) of the resistor.

Thus, a resistor which has as its first three
stripes (as the one you're working on has)

Brown

Black

Red

will have a resistance of 1000 ohms. The brown
stripe provides the 1, the black stripe gives the
first 0, and the red stripe provides the next two 0's
(10 X 10 2 ).

The fourth stripe represents the resistor's toler-
ance. The tolerance gives the range, plus or minus,
which the actual value of the resistor will be
within when compared to the strict value indi-
cated by the color bands. The color of this stripe

CHAPTER 2

can be gold, silver, or salmon. The tolerances are
plus or minus 5, 10, and 20 percent respectively.
Sometimes you won't find a tolerance stripe on a
resistor. This means that the resistor has a toler-
ance of plus or minus 20 percent. If you have a re-
sistor with brown, black, red, and silver stripes,
for example, you have a 1000 ohm (IK ohm) re-
sistor with a 10 percent tolerance. The actual
value of the resistor is thus between 900 and 1100
ohms. The circuits in this book can all use resis-
tors with a 20 percent tolerance or better.

Table 2-1 . Resistor Color Codes

Color Value

Black

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8

White 9

Tolerance Bands
Color Tolerance

Gold ±5 percent
Silver ±10 percent
Salmon ± 20 percent
None ± 20 percent

Resistors also have power ratings, which de-
scribe the amount of energy (measured in watts)
which the resistor can safely handle. For the
projects in this book, you may use 1 /4- or 1 /2-
watt resistors. A resistor, unlike many other circuit
components, may be connected in either direction
in a circuit.

Exploring the Control Port

Figure 2-4. Schematic Diagram of a Simple
Logic Probe

+ o-

1KB

Using the simple logic probe is very easy. Attach
the ground lead of the logic probe to the power
supply ground of the digital circuit you're testing.
Then simply touch the tip of the probe lead to a
point in the circuit. If the point has a voltage of
something greater than the barrier voltage of the
LED, usually about 0.7 volts, the LED will glow.
You can assume the point is at a logic high state (a
1). If the point is at a voltage level less than 0.7
volts, current will not flow through the diode, the
LED stays off, and you can assume a logic low
state (a 0) is present. (Of course, a zero indicates a
voltage level less than 0.5 volts, so a problem
could still exist with the circuit under test and not
be detected by this simple tester.)

CHAPTER 2

Figure 2-5. The Completed Logic Probe

When you're finished building the simple logic probe, it should
look something like this.

To use the logic probe to examine the joystick
port, a connector is required — the next simple
tool.

A Connector for the Joystick Port
The actual connector that plugs into the joystick
port is a D-subminiature female, 9-pin connector.
Stranded copper wire (#22 gauge) should be sol-
dered to the terminals of the connector for easier
access.

Here's what you'll need:

Part

Quantity Part Number

1 9-pin D-subminiature female 276-1538

You'll also need some stranded copper wire
(about 12 feet) or ribbon cable.

The procedure is quite simple.

Exploring the Control Port

1. Cut six pieces of stranded copper wire about 24
inches long and strip about 1 /4 inch of insula-
tion from each end. (You may wish to use rib-
bon cable instead of wire to avoid a tangle.)

2. Solder a wire to the terminals of pins 1, 2, 3, 4,
6, and 8 of the 9-pin plug.

3. Tape a tag to the opposite end of each wire
with the number of the pin it's attached to. This
will save you some time later.

To use your logic probe with the connector, attach
the ground lead of the probe to the wire from pin

While your computer is turned off, insert the plug
into the joystick port. (You should always make
sure that the computer is turned off whenever you
insert a plug into any of the I/O ports.) If you're
using a Commodore 64 or Commodore 128, insert
the plug into joystick port 2 on the right side of
the machine. If you have an Atari computer, insert
the plug into joystick port 1.

Warning: Before turning on your computer, make
sure that none of the wires touch each other. If
any wires are touching, your computer could be
damaged. Be particularly careful to inspect the
wires from pins 7 and 8 — they should never be in
contact. If they are, the computer's power supply
could be damaged.

Touch the end of your logic probe to pins 1, 2, 3,
4, and 6. Each time you do this, the LED should
glow. This glow will be very faint, and you may
need to shield the LED from room light to detect
the glow at all. This is because of the small
amount of current available at the pins.

CHAPTER 2

The I/O lines are connected to registers, or
memory addresses, in your computer. The five
pins (1, 2, 3, 4, and 6) of joystick port 2 corre-
spond to the lower five bits of memory location
56320 ($DC00 in hexadecimal) in the Commodore
64 and Commodore 128. Whether you're using
the port for input or output is determined by the
data direction register located at 56322 ($DC02).
Each bit controls the direction of data on the cor-
responding bit of the port. If a bit is set to 1, the
corresponding bit in the port is set to be used for
data output. Setting the bit to causes the bit to
be used for input. In the VIC-20, these lines corre-
spond to bits in memory locations 37137 ($9111)
and 37154 ($9122). In the Atari computers, you
needn't worry about the memory locations af-
fected, as the BASIC STICK and STRIG com-
mands can be used to see the effect of the five
I/O lines.

The bits in these registers are normally set on
(1) in your computer — that's why the LED of your
logic probe should glow faintly when you touch
the probe to any of these pins. But how can these
pins be used to input information?

Type in and run the appropriate version of Pro-
gram 2-1.

Program 2-1 — Commodore 64/128

A=56320:REM FOR PORT 1 USE 56321

IFPEEK ( A ) AND ( 2 T ) THEN40

PRINT "PIN 1"

IFPEEK ( A ) AND ( 2 T 1 ) THEN60

PRINT "PIN 2"

IFPEEK (A) AND ( 2?2 ) THEN80

PRINT "PIN 3"

IFPEEK ( A ) AND ( 2 T 3 ) THEN100

PRINT "PIN 4"

100

IFPEEK (A) AND ( 2?4 ) THEN120

110

PRINT "PIN 6"

120

GOTO 20

Exploring the Control Port

Program 2-1 —VIC 20

AH 10 A=37137

BS 20 IFPEEK(A)AND( 2T2 )THEN40

PH 30 PRINT "PIN 1"

GF 40 IFPEEK (A) AND ( 2T3 )THEN60

SK 50 PRINT "PIN 2"

SA 60 IFPEEK (A)AND( 2 T4)THEN80

BP 70 PRINT "PIN 3"

BX 80 POKE 37154, : IFPEEK ( 37 152 ) AND ( 2 T7 )

THEN100
ER 90 PRINT "PIN 4"

XE 100 POKE 37154,255 :IFPEEK(A)AND(2T5)T

HEN120
SJ 110 PRINT "PIN 6"
SM 120 GOTO20

Program 2-1 — Atari

FP 10 IF STICK(0X>14 THEN 20

EP 15 PRINT "PIN 1 "

SA 20 IF STICK(0)<>13 THEN 30

FB 25 PRINT "PIN 2"

BA 30 IF STICK<0X>11 THEN 40

FD 35 PRINT "PIN 3"

DH40 IF STICK<0><>7 THEN 50

FF 45 PRINT "PIN 4"

DN 50 IF STRIB(0)<>0 THEN 60

FI 55 PRINT "PIN 6 "

AA 60 SOTO 10

When you run the program for your computer,
nothing should seem to happen at first. Take your
logic probe and touch the probe to the wire con-
nected to pin 8. This is the ground from the com-
puter and should still be connected to the ground
lead of the probe. Obviously, the ground lead and
probe lead are at the same potential. The LED will
not light. Since ground voltage corresponds with
the off state, everything is working correctly.

CHAPTER 2

Now, take the wire from pin 1 and touch its free
end to the wire from pin 8. When you do this,
you force the normally on pin 1 to turn off. Your
program tells you this. If you look at the computer
screen, the message, PIN 1 should be displayed
on the screen. As you touch the wires from pin 2,
3, 4, or 6 to the ground, you'll see a message
showing which pin has been set low. By forcing an
I/O pin to logic low, its corresponding bit in the
register (a memory location) is set low as well.
Since a program can PEEK into a memory location
and check its contents, software can be written to
detect the states (on or off) present at the control
port, then proceed accordingly.

In Program 2-1, your computer is sensing which
pin you're setting low (by touching the wire from
the pin to ground). Then it's taking this input and
processing it; in this case it's used to print out
messages. The next chapter will use this infor-
mation to construct a useful input device, a
joystick.

B CHAPTER 3

■

" Making a

Joystick

Making a Joystick

It may seem that a joystick, with its molded plas-
tic parts, would be impossible to make at home.
Open a Commodore or Atari joystick, though, and
you'll see that it's not that complex. In fact, it only
contains five switches.

Four of the joystick's switches are used to detect
the direction the control stick is pushed. The fifth
acts as a fire button. A switch is pressed if the joy-
stick is moved up, down, left, or right. If the stick
is moved diagonally, two switches are pressed.
Moving the stick diagonally up and to the right,
for example, presses both the "up" and "right"
switches.

These switches are used to make the connection
between one of the five input lines (pins 1, 2, 3, 4,
and 6) of the control port and the ground (pin 8).
Instead of your touching wires together, as you
did while experimenting with the control port,
switches are closed to make the connections.

Since a joystick is basically a set of switches,
you can make your own version of this device. In
your first joystick, four push button switches are
substituted for the control stick. The four buttons
correspond to the directions a joystick can indi-
cate — up, down, left, and right. To get diagonal
directions, you'll press two buttons, the top and
right buttons to indicate a move upward and to-
ward the right, for instance. The fire button is a
fifth push button switch and functions just as does
its counterpart on a commerical joystick.

As you can imagine, the push button joystick
works better with some programs than with others.
The push button joystick lets you move with
greater control in games and in applications where

CHAPTER 3

movement in the four main directions is required.
Applications or games requiring diagonal moves,
however, are trickier than with a standard joystick.

To make a push button joystick for your Com-
modore or Atari computer, you'll need the follow-
ing parts:

Part

Quantity Part

Number

5 Momentary contact normally

open switches

275-1547

1 Plastic case

270-231

1 9-pin D-subminiature female

276-1538

You'll also need some stranded copper wire or

ribbon cable.

1. Cut six pieces of wire about 24 inches long.
Remove about 1/4 inch of insulation from
each end. Solder wires to pins 1, 2, 3, 4, 6,
and 8 of the 9-pin plug.

2. Remove the cover from the plastic case and
drill five holes for the switches (Figure 3-1).

3. Mount a switch in each of these holes.

4. Drill a hole in the top end (the end closest to
the up switch) of the plastic case for the wires
from the connector.

5. Pull the six wires from the 9-pin plug through
the hole you just drilled, and about 10 centi-
meters from their ends, tie them in a knot so
that they can't be pulled back through the hole.

6. Solder the wire from pin 8 to one of the termi-
nals of a switch (probably the up switch, since
it's closest to the hole you drilled in the top
side of the cover). Using several small lengths
of wire, connect this terminal to a terminal of
each of the other four switches in a jumper
fashion (Figure 3-2).

Making a Joystick

Figure 3-1 . Drilling the Cover

Fire

( I

This front view of the plastic case shows the approximate posi-
tions where you should drill the five holes for the joystick's button
switches.

7. Take the wire from pin 1 of the plug and sol-
der it to the free terminal of the up switch (see
Figure 3-2 for a layout view, or Figure 3-3 for
a schematic view).

8. Solder the wire from pin 2 to the free terminal
of the down switch.

9. Solder the wire from pins 3, 4, and 6 to the
free terminals of the left, right, and fire
switches respectively.

10. Replace the cover of the plastic case. Your joy-
stick is complete.

CHAPTER 3

In this back view of the joystick cover, you can see how the switches are wired.
Notice that the wire from pin 8 of the 9-pin connector is first connected to the
up switch, then shorter pieces of wire are used to connect a terminal on each
of the other four switches.

Making a Joystick

Figure 3-3. Joystick Schematic

Pin 1

Pin 2

Pin 3

Pin 4

Pin 6

Up switch
I /

-O

Down switch
I /

Left switch

Right switch

Fire button

To pin 8 of
9-pin plug

Schematic view of the push button joystick wiring.

Figure 3-4. The Finished Push Button Joystick

The completed push button joystick looks like this. Five buttons appear on the
top of the finished case (above), while the wiring can be clearly seen in the view
from the bottom (below).

CHAPTER 3

A Gravity Joystick

You can also use other types of switches to make
a joystick. One alternative is to use tilt switches in
place of the four directional push button switches.

A tilt switch is a switch which closes only when
it's tilted in the proper direction. One type of tilt
switch consists of a plastic case with a metal ball
inside. When the case is held at the proper angle,
gravity causes the ball to roll to one side, where it
touches both terminals of the switch, closing the
contact. Some tilt switches use liquid mercury in
place of the metal ball.

If you're using tilt switches, be sure to mount
them inside the plastic case of your joystick to
prevent them from breaking.

Warning: This is especially important if you're
using mercury switches. Mercury is a potential
health hazard.

The switches can be glued to the inside of the case
using epoxy and should be mounted so that they
close (turn on) when the joystick is tilted in the
proper direction. This means the right switch
should close when the case is tilted to the right,
the left switch should close when the case is tilted
left, and so on.

You tilt the gravity joystick in the direction you
would normally push the control stick if you were
using an ordinary joystick. Because of its unique
feel, the gravity joystick adds another dimension
to game playing. While moving objects on the
screen is easy, keeping them stationary requires
steady hands.

Your joystick, whether push button or tilt
switch, will work with any programs requiring a
joystick. In fact, you may want to try making your
own custom controller, perhaps something as un-
usual as a steering wheel for a racing game.

Making a Joystick

Figure 3-5. Inside a Gravity Joystick

The gravity joystick has only one button (the fire button), since it's operated by
tilting it in the proper direction (above). Tilting the joystick case closes contacts
within one or more of the mercury switches (below).

CHAPTER 4

Game
Paddles

Game Paddles

Game paddles can make arcade-style games like
car racing simulations, tennis, and Ping-Pong
more realistic and fun. Unlike joysticks, game pad-
dles are analog devices. While joysticks provide
the computer with either on or off signals, paddles
provide the computer with a varying signal.

A paddle contains a resistor with a variable
amount of resistance and a switch in one case.
The switch acts as the fire button and operates
like the fire button on a joystick. The electrical re-
sistance of the paddle depends on the position of
the variable resistor, often referred to as a poten-
tiometer, or pot. When the pot is turned all the
way in one direction, the resistance measured be-
tween the potentiometer's side and middle leads is
ohms. When the shaft is turned in the opposite
direction, the resistance is about 500K ohms on
Commodore paddles. Atari paddles require a resis-
tance of about 1M (1 million) ohms.

When paddles are used, current from the com-
puter flows through the potentiometers. The
amount of current that flows depends on the resis-
tance in the pot. The greater the resistance, the
less current flow. The opposite is also true — the
less the resistance, the greater the current flow.

By turning the knob of the potentiometer, the
voltage, measured across the leads of the poten-
tiometer, varies between volts (ground) and +5
volts. The computer senses this voltage and uses it
to determine a value for the position of the
potentiometer.

This voltage, which is an analog value, must be
converted to a digital value to be used by your
computer. Analog-to-digital converters, sometimes
called A/D converters, were once very expensive.

Fortunately, your computer has this hardware
built in. Using its analog-to-digital converter, your
computer interprets the voltage across the variable
resistor of the paddle as a number between and
255 (1 to 228 for the Atari). This value is kept in
special memory locations in your computer. For
the Commodore 64 and 128, these memory loca-
tions are 54297 and 54298 ($D419 and $D41A in
hexadecimal). In the VIC-20, locations 36872 and
36873 ($9008 and $9009) are used. For the Atari
computers, it's not necessary to PEEK the actual
memory locations — Atari BASIC has a command,
PADDLE(X), which lets you see the values re-
turned for the paddle positions. The value of X in
this Atari statement is or 1 (a different X for
each paddle), provided the paddles are plugged
into control port 1 .

To build your own pair of game paddles, you'll
need the following parts:

Part

Quantity Part Number

2 Plastic cases 270-231
2 Momentary contact switches 275-1566

1 9-pin D-subminiature female 276-1538

2 Plastic knobs 274-407
2 500K potentiometers 271-210

(Commodore)

2 1M potentiometers (Atari) 271-211

You'll also need some ribbon cable or stranded
copper wire to connect your paddles to your
computer.

Here's how to put your game paddles together:

1. Cut eight pieces of stranded copper wire about
24 inches long and remove about 1 /4 inch of
insulation from each end.

Game Paddles

2. Connect one end of two of the wires to pin 7 of
the 9-pin plug.

3. Connect another two wires to pin 8 of the plug.

4. Solder the remaining four wires to pins 3, 4, 5,
and 9.

5. Remove the cover from one of the plastic boxes
and drill two holes as in Figure 4-1. One hole is
for the potentiometer, while the other is for the
switch. Drill a hole in one end of the plastic
case for the wires. Repeat these steps for the
second plastic box.

6. Mount the switches and potentiometers in their
respective holes. Attach the knobs to the shafts
of the potentiometers.

7. Take one wire from pin 7, one wire from pin 8,
the wire from pin 3, and the wire from pin 9 of
the 9-pin plug to one of the paddle boxes. Pull
the wires through the hole in the top end of the
box. Turn the box cover with the potentiometer
and switch mounted on it over so that their
leads are facing up (as shown in Figure 4-1).

8. Connect the wire from pin 8 to one lead of the
switch. Solder the wire from pin 3 to the other
switch lead. The wire from pin 7 should be sol-
dered to the left side lead of the potentiometer,
and the wire from pin 9 is connected to the
middle lead of the potentiometer. Mount the
cover on the box. One paddle is completed.

9. The procedure for the second paddle is identi-
cal, except the wires are from pins 7, 8, 4, and 5
of the 9-pin plug. The wire from pin 8 connects
to one lead of the switch. The other switch lead
is connected to the wire from pin 4. The wire
from pin 7 connects to the left lead of the
potentiometer, and the remaining wire, from
pin 5, connects to the middle lead. Close the
plastic box, and your second paddle is finished.

CHAPTER 4

Game Paddles

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CHAPTER 4

Figure 4-3. Under the Paddle

If you've correctly drilled holes, placed components, and wired
leads, the bottom view of the game paddle case should look
something like this.

To try out your paddles, turn off your computer
and plug the paddles' 9-pin connector into control
port 1. Turn on the computer, then type in and
run the appropriate version of Program 4-1.

Program 4-1 — Commodore 64/128

EF 10 POKE5 6333,127 : REM IRQ OFF
HH 20 P0KE56322, 192 :REM SET BITS 6,7 OF
{ SPACE } DATA DIRECTION REGISTER FOR
OUTPUT

EC 30 POKE56320 , 64 : REM 128 FOR PORT 2
HG 40 X=PEEK( 54297 )
DJ 50 Y=PEEK( 54298)

XE 60 POKE56322,255:POKE56333,129:REM RE

STORE REGISTERS
DG 80 PRINTX,Y
SJ 100 GOTO10

Game Paddles

Program 4-1 — VIC-20

GD 10 PRINT1-(PEEK(37151)AND16)/16;PEEK(
36872);

RG 20 POKE37154, 127 :PRINT1- ( PEEK ( 37 152 ) A

ND128)/128; : P0KE3 7 154 , 2 55
PS 50 PRINTPEEK( 36873 )
BD 60 GOTO10

Program 4-1 — Atari

10 PRINT PADDLE(O);" ";PADDLE(1)
20 GOTO 10

When you run the appropriate program, two
columns of numbers should appear on the screen.
Each column of numbers corresponds with a pad-
dle. As you turn the knobs of the paddles, the
numbers should vary between and 255 for the
Commodore computers, and between 1 and 228
for an Atari computer.

Several steps are necessary to read the paddles
with a Commodore 64 computer. Since the key-
board can interfere with values read at address
56321 ($DC01), it's necessary to turn off the IRQ
(Interrupt Request). This prevents normal system
processing from occurring. Program 4-1 does this
in line 10.

Bits 6 and 7 of the data direction register must
be set up for output by setting the proper bits in
address 56322 ($DC02). You want to place a 1 in
bits 7 and 6 of 56322 for data direction register A.
(The decimal equivalent 192 of the binary number
11000000 in line 20 sets bits 6 and 7 for output.)

Line 30 stores the value 64 in the data port reg-
ister A to select reading the paddles at port 1. (To
read port 2, change the 64 in line 30 to 128.)

Lines 40 and 50 PEEK the game paddle POTX
and POTY locations, storing the proper values in
the variables X and Y.
the variables X and Y.

CHAPTER 4

Line 60 restores the IRQ and keyboard.
Line 80 prints the values of X and Y to the
screen.

Since it's necessary to turn off the keyboard for
input to the paddle registers, it's often best to read
the values with a machine language (ML) routine.

Figure 4-4. Paddles Done

Both game paddles are wired to one 9-pin connector.

The following ML routine (Program 4-2) works
only on the Commodore 64 or Commodore 128 in
64 mode. The routine is POKEd into the cassette
buffer and can be called by typing SYS 828 any-
time you want to read the paddles. It's a simple
matter to incorporate this routine's BASIC loader
(omit line 40) into any program you write which
requires paddle reading. To read a paddle value
after calling the routine with SYS 828, simply
PEEK the appropriate location:

Game Paddles

Port 1

Paddle to Read Location to PEEK

X 251
Y 253

Port 2

X
Y

252
254

Program 4-2. ML Paddle Reader

MF 10 1=828 :AD=251

HG 20 READ A: IF A=256 THEN40

RG 30 POKE I,A:I=I+1:GOTO20

QA 40 SYS 828 :PRINTPEEK(AD) ;PEEK(AD+1) ; P

EEK( AD+2 ) ;PEEK(AD+3 ) :GOTO40
RF 50 DATA 76,63,3,162,1,120,173
KH 60 DATA 2,220,133,167,169,192,141
SX 70 DATA 2,220,169,128,141,0,220
EE 80 DATA 160,128,234,136,16,252,173
DS 90 DATA 25,212,149,251,173,26,212
CD 100 DATA 149,253,169,64,202,16,232
FP 110 DATA 165,167,141,2,220,88,96,256

1 CHAPTER 5

■

An Analog
Light Sensor

An Analog Light
Sensor

Sometimes it's handy to be able to detect different
intensities of light. Perhaps you'd like to have a
simple switch to turn on your outside lights at
dusk and then turn them off again at dawn. If you
need to detect different degrees of lighting, you
need an analog light sensor.

The analog light sensor built in this chapter is a
cadmium sulfide photocell. It's a variable resistor
whose resistance changes proportionally to the
amount of light which strikes its light-sensitive
surface. This, in turn, provides the computer with
information regarding just how much light is
present. The resistance increases as the amount of
light striking it decreases. Exposed to bright light,
the cadmium sulfide photocell has a resistance of
a few hundred ohms between its terminals. In
darkness, this resistance increases to approximately
0.5 megohms — about the same value range as the
potentiometer on the Commodore paddles. In fact,
it's possible to connect the cadmium photocell in
much the same manner as a paddle.

Light Sensor — Commodore Computers

To build an analog light sensor for a Commodore
64, Commodore 128, or VIC-20, you'll need the
following parts:

Part

Quantity Part

Number

1 Cadmium sulfide photocell

276-116

1 9-pin D-subminiature female 276-1538

1 0.1 microfarad capacitor

272-1069

1 IK ohm resistor

271-8023

1 Solderless breadboard

276-175

You'll also need some stranded copper

wire, as

well as some solid copper wire (Radio

Shack part

number 278-1306).

To complete the project, follow these steps:

1. Cut two pieces of the stranded copper wire
about 24 inches long and strip about 1/4 inch
of insulation from each end. Cut several small
pieces of solid wire about 2 inches long to use
as jumpers. (You'll need them in this circuit and
most others in this book.)

2. Solder the end of one wire to pin 7, and the
other wire to pin 9 of the 9-pin plug.

3. Connect the wire attached to pin 7, the +5 -volt
lead, to point XI on the solderless breadboard.
Place the other wire (that attached to pin 9) to
point Yl on the board.

4. Install jumper wires between points Y2 and J2,
points X2 and A2, and points J13 and Y13.

5. Place the 0.1 microfarad capacitor between
points F2 and E2.

6. Bend the leads of the IK ohm resistor, and in-
stall it between points B2 and B13.

7. Install the cadmium sulfide photocell between
points E13 and F13.

An Analog Light Sensor

Figure 5-1 . Schematic Drawing of Analog Photocell
(Commodore)

Pin 7 +5v R i

Rj l.OKfl Cadmium sulfide photocell

Cj 0.1|if

Building an analog light sensor for a Commodore personal com-
puter requires several parts, including a cadmium sulfide photo-
cell, a resistor, and a capacitor.

Light Sensor — Atari

Putting together an analog light sensor for the
Atari computer series involves some different
components and different directions.
You'll need:

Part

Quantity Part

Number

1 Cadmium sulfide photocell

276-116

1 9-pin D-subminiature female

276-1538

1 1M potentiometer

271-211

1 Solderless breadboard

276-175

You'll also need some stranded copper wire as

well as some solid copper wire.

CHAPTER 5

Figure 5-2. Breadboard Layout for Atari Analog Photocell

1 2 3 4 5 6 7
» • X2

9 10 11 12 13 14 15

A
B
C
D
E

A2
B2

Cadmium sensor
B6

• E6

jjf IM pot

* F6

The breadboard for the Atari version of the analog light sensor clearly shows
where each component and jumper should be placed.

1. Cut two pieces of the stranded copper wire
about 24 inches long and strip about 1/4 inch
of insulation from each end. Cut several small
pieces of solid wire about 2 inches long to use
as jumpers.

2. Solder the end of one wire to pin 7, and the
other wire to pin 9 of the 9-pin plug.

3. Connect the wire attached to pin 7 , the +5 -volt
lead, to point XI on the solderless breadboard.
Place the other wire (that attached to pin 9) to
point Y2 on the board.

4. Install jumper wires between points Y6 and J6,
and points X2 and A2.

5. Place the IM potentiometer between points E6
and F6.

6. Install the cadmium sulfide photocell between
points B2 and B6.

An Analog Light Sensor

To see how your analog light sensor works, use
Program 4-1 from the previous chapter. If you're
using a Commodore 64, or a Commodore 128 in
64 mode, you can use Program 5-1 below instead.
If you have an Atari computer, enter and run the
two-line program you see below. For a Commo-
dore 64 or 128, the light sensor should be plugged
into port 2. Plug the Atari light sensor into port 1.

Program 5-1 — Commodore 64/128

AD 10 1=828

PE 20 READ A: IF A=256 THEN 50
RG 30 POKE I,A:I=I+1:GOTO20
FX 40 REM ANALOG LIGHT METER
GH 50 SYS 828:A=PEEK(25 2)
DH 60 IFB=ATHEN50

GX 70 B=A: PRINT " {CLRjLIGHT LEVEL IS NOW"
;A

DG 80 GOTO50

KA 90 DATA 76,63,3,162,1,120,173
FR 100 DATA 2,220,133,167,169,192,141
BJ 110 DATA 2,220,169,128,141,0,220
QG 120 DATA 160,128,234,136,16,252,173
QE 130 DATA 25,212,149,251,173,26,212
CS 140 DATA 149,253,169,64,202,16,232
FJ 150 DATA 165,167,141,2,220,88,96,256

Program 5-1 — Atari

10 PRINT PADDLE(O)
20 GOTO 10

When the you run the program, numbers in the
range 0-255 (1-228 for the Atari) will scroll on
your screen. These numbers are related to the
amount of light which is falling on the photocell.
To see this, try reducing the light reaching the
photocell by covering it with your hand. As you
do, the numbers in the column will increase. Con-
versely, increasing the amount of light which hits
the photocell, perhaps by shining a flashlight on it
or by holding it closer to a lamp, causes the num-
bers on the screen to decrease.

CHAPTER 5

Note: On the Commodore light sensor, you may
need to adjust the value of Rl, the IK ohm resis-
tor, to cause a swing through the entire range of

values. In other words, you may need to use a dif-
ferent resistor, one rated higher or lower than IK.
Just experiment until you find the proper value for
your application. That's part of the fun in
breadboarding — making changes is easy.

The computer treats the light sensor as though
it were a game paddle. The varying resistance of
the sensor provides the computer with a signal be-
tween volts and +5 volts at its port. The voltage
is converted to a digit between and 255 by the
analog-to-digital converter in your computer. This
value is stored in a memory location, where it can
be read by a program. It shouldn't be hard, once
you have the sensor built, to take this information
and develop a light meter program which will de-
termine the degree of light present at the sensor.

jl Two analog light sensors can be connected to your
computer through the single 9-pin plug (just like
two game paddles). The second photocell is at-
tached between pins 7 and 5 of the 9-pin plug. To
read the value of this second sensor, simply PEEK
memory location 54298 ($D41A) for the Commo-
dore 64/128, or PEEK location 36873 ($9009) for
the VIC. Don't forget to set the data direction reg-
isters for output. The BASIC command PAD-
DLE(l) can be used to read this value for Atari
computers.

CHAPTER 6

A Light Pen

A light pen is a fascinating input device. By point-
ing a light pen at the computer's screen and press-
ing its input button, you can make selections from
a menu, draw a picture, or perform whatever
function the computer program you're using al-
lows. The biggest attraction of a program which
uses a light pen is that you don't have to use the
keyboard — using a program becomes as simple as
pointing.

To understand how a light pen works, you must
know a bit about how your computer's monitor
displays a picture. Pictures on the screen are made
up of tiny dots of light. These dots are produced
from an electron gun in the monitor which shoots
electrons at the screen. These electrons excite the
phosphor coating on the screen, causing it to emit
light. In this way, the electron gun actually "paints"
the picture, dot by dot, row by row, until the
screen is filled. All of this is done in a fraction of a
second and is updated many times every second.

Your computer keeps track of where the beam
of electrons from the electron gun is at all times.
The light pen, when pointed to a spot on the
screen, triggers the computer to store the location
of the beam as it passes. These x and y coordi-
nates are placed in special registers of your com-
puter. Since the position of the beam stored is the
same as that of the light pen, programs can deter-
mine where you're pointing the light pen by
checking the values contained in these registers.
The program can then use this information to per-
form various functions, like drawing a line or se-
lecting an option.

CHAPTER 6

Here's what you'll need to make a simple light
pen:

Part

Quantity Part

Number

Solderless breadboard

276-175

7400 quad 2-input NAND

276-1801

gate IC

TIL infrared phototransistor

276-145

IK ohm resistor

271-8023

(Commodore only)

9-pin D-subminiature female

276-1538

Push button microswitch

275-016

(normally open)

You'll also need some stranded copper wire, a

ballpoin

t pen, and some solid copper wire.

When building the light pen, refer to Figure 6-1,
the schematic drawing for the light pen circuit,
and Figure 6-2, the solderless breadboard layout,
as you follow these instructions:

1. Solder lengths of solid copper wire to pins 6,
7, and 8 of the 9-pin plug. These wires will
connect the plug with the solderless breadboard.

2. Insert the wire from pin 6 to point El of the
breadboard, the wire from pin 7 to point Yl,
and the wire from pin 8 to point XI.

3. Insert the 7400 IC in the breadboard so that its
pin 1 is at point E16 and its pin 8 at point F10.
You can tell which pin of the IC is pin 1 by
one of two methods: Pin 1 may have a circle
indented in the plastic beside it. Alternatively,
there may be a semicircular indention at the
end of the IC where pin 1 is located. In this
case, pin 1 is the first pin in a counterclock-
wise direction from this indention (looking at
the top of the IC). In either case, the remain -

A Light Pen

ing pins are numbered counterclockwise
around the IC.
4. Insert five solid copper jumper wires as
follows:

First

Second

Wire

Location

A10

X10

A12

A13

C13

D14

C15

D16

J16

Y16

5. Remove the ink cartridge from the ballpoint
pen. The barrel of the pen will act as the body
of your light pen. With your hot soldering
iron, carefully melt a small hole at the end of
the barrel where the ballpoint used to be for
the wires.

Warning: Be sure to clean off the tip of your sol-
dering iron when you're finished. If you intend to
use your soldering iron for serious work, it's a
good idea to change the tip of your iron after
using it to melt plastic. Substances from the burn-
ing plastic will adhere to the tip of your iron,
making it unsuitable for serious soldering work.
[We found that it wasn't necessary to burn a hole
for installing the TIL 414 phototransistor when we
used a BIC ballpoint pen — Editor.]

6. Cut four pieces of stranded copper wire about
30 inches long to connect your light pen with
the solderless breadboard. These wires must
be long enough to allow you to move the light
pen freely.

7. Solder one of the four wires to the emitter
lead of the phototransistor. Another wire
should be soldered to its collector lead. Attach
a small piece of electrical insulation tape
around both the collector and emitter leads so
that they won't come in contact and short out.

CHAPTER 6

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A Light Pen

CHAPTER 6

8. Pull the wires of the phototransistor through
the pen barrel and glue the phototransistor in
place at the "tip" end. Be careful not to get any
glue over the phototransistor's light-sensitive
part.

Figure 6-3. In Reality — The Light Pen Breadboard

'il'iXiiQift-..- '-'AM

I Wl: -^&M : M^

■

Schematics and layouts aside, this is how your light pen solderless
breadboard should look when you're finished.

9. The wire from the emitter plugs into position
XI 6 of the solderless breadboard, while the
wire from the collector inserts into point A16.

10. Solder the remaining two 30-inch wires to the
leads of the microswitch. Pull the wires
through the hole you melted and out the open
end of the pen's barrel. Glue the microswitch
to the barrel of the pen.

11. One of the wires from the switch inserts into
point Al on the breadboard, while the other
goes into point A2.

12. Insert the IK resistor between points C2 and
Cll of the solderless breadboard. (For the
Atari version of this circuit, use a jumper wire
in place of the resistor.)

A Light Pen

To check that your light pen is operating properly,
turn off your computer and insert the 9-pin plug
into control port 1 of the Commodore 64/128 and
Atari computers. (The VIC has but one control
port.) A light pen won't work in control port 2 of
the Commodore 64 and 128 computers. Turn on
your computer, type in, and run the appropriate
version of Program 6-1.

Program 6-1 — Commodore 64/1 28

5 A = 53267:B = 53268

10 PRINT "X=";PEEK(A),"Y=";PEEK(B)

20 GOTO 10

Program 6-1— VIC-20

5 A = 36870:B = 36871

10 PRINT "X = ";PEEK(A),"Y = ";PEEK(B)

20 GOTO 10

Program 6-1 — Atari
5 A = 564:B = 565

10 PRINT "X=";PEEK(A),"Y=";PEEK(B)
20 GOTO 10

Move your light pen around the screen while
pressing the microswitch. The numbers of the two
columns should change, giving you the coordi-
nates of the point of your light pen.

Warning: You may have to adjust the brightness
of your monitor to get your light pen to work
properly.

How It Works

Your computer is set up to store the position of
the electron beam every time pin 6 of the control
port, normally at a logic high state, is set low. The
light pen circuit is designed to set this pin low.
This triggers the computer to save the electron
beam's position when it passes the light pen.

CHAPTER 6

The phototransistor acts as a very fast switch.
Normally, the path between the emitter and col-
lector of the phototransistor is like that of an open
switch. The moment the beam of the electron gun
passes the point at which the phototransistor is
aimed, enough light is emitted to turn the transis-
tor on. When this happens, the connection be-
tween the emitter and the collector of this device
is effectively that of a closed switch. Since the
emitter is connected to ground, the inputs of the
NAND gate, which are attached to the collector,
are grounded as well.

The operation of NAND gates and other logic
building blocks is explained in greater detail in
Chapter 10, "Digital Logic." Let it suffice to say
here that the two NAND gates used in the light
pen circuit act as a noninverting buffer. Non-
inverting buffers are used when a signal isn't suf-
ficient to drive the inputs of other IC gates. The
output of this type of buffer is the same as its
input.

The output of the buffer is connected to pin 6 of
the control port through a resistor and the push
button switch. When the push button switch is
open, the light pen circuit is disconnected from
pin 6 of the control port. The push button switch
must thus be pressed if this pin is to be triggered.

When the light pen is aimed at a point on the
screen and the button is pressed, several things
happen. Because the screen picture is refreshed
many times each second, the electron beam passes
the phototransistor at least once in the time it
takes to press and release the button. Before and
after the beam passes the phototransistor, the in-
put and output of the buffer are logic high, and as
a result, pin 6 remains at that state. The moment
the beam passes the phototransitor, the input and
output of the NAND gate buffer are set low. Thus,

A. Light Pen

pin 6 of the control port is set low for a fraction of
a second, long enough to trigger the computer to
store the location of the electron beam.

Figure 6-4. The Completed Light Pen

The entire light pen system, comprised of the breadboard, the
ink-pen-based light pen, and the 9-pin connector, can be used
for a multitude of programming applications, from drawing pro-
grams to menu selection.

Programming with the Light Pen

As you ran the test program, you probably noticed
that when the pen was held at one spot, the co-
ordinate numbers didn't remain constant, but in-
stead stayed within a relatively small range. This
must be taken into account when writing pro-
grams to work with your light pen.

Another point to remember is that once you use
the light pen, the values remain the same in their
registers until the light pen is used again. This
means your program should have a way of check-
ing that the values in the register are a new selec-
tion rather than a previous selection. For instance,
suppose the registers contain the values 120 and

CHAPTER 6

125 from a previous menu selection. When you
put up your next menu, you expect the user to
make another selection with the light pen. In or-
der to determine where the light pen is pointed,
you might compare the register values to a speci-
fied coordinate range. If the range includes one of
the values from the previous selection, the pro-
gram would immediately proceed as if the pen is
pointed in this range before the user even has a
chance to use the light pen.

One way to avoid this problem is to check
whether the user has pressed the push button
again. This insures that the previous values in the
register have no effect on the selection. Since the
light pen line (pin 6) is the same as that of the fire
button, you can check for it the same way you
would check for the fire button. Another way to
avoid this problem is to make sure two consecu-
tive menus have no ranges in common.

Try entering and running the appropriate version
of Progam 6-2, which shows how a light pen can
be used to make selections from a menu. When
the menu appears, just point to the asterisk (*) be-
side your choice and press the microswitch. The
screen clears and the choice you made appears.
After a short delay, the word CONTINUE is dis-
played with an asterisk in front of it. Point your
light pen to the asterisk and press the microswitch
to return to the menu.

Program 6-2 — Commodore 64/1 28

QK 100 REM * LIGHT PEN DEMO *

HA 150 AS=CHR$(147) :PRINTA$

BB 160 PRINTTAB ( 10 ) "THREE CHOICES"

MQ 170 PRINT: PRINT

GJ 180 PRINTTAB ( 5 ) " * CHOICE ONE "

GF 190 PRINT: PRINT: PRINT: PRINT

DA 200 PRINTTAB (5 ) "* CHOICE TWO"

RC 210 PRINT: PRINT: PRINT: PRINT

FK 220 PRINTTAB ( 5 ) "* CHOICE THREE"

A Light Pen

DD 230 A=PEEK(53267) : B=PEEK ( 53 268 )

FJ 240 IF A>55 THEN 230

EB 250 IF (B>70) AND (B<80) THEN 300

HG 260 IF (B>110) AND (B<120) THEN 330

XP 270 IF (B>150) AND (B<160) THEN 360

HF 290 GOTO230

RF 300 PRINTA?

DA 310 PRINT "CHOICE ONE "

BF 320 GOTO 400

KH 330 PRI NTA$

GD 340 PRINT "CHOICE TWO"

RJ 350 GOTO 400

FK 360 PRINTA$

QS 370 PRINT "CHOICE THREE"

KS 400 FOR 1=0 TO 1500

XP 410 NEXT I: PRINT

CB 420 PRINT"* CONTINUE"

AS 430 IF PEEK( 53267) <55 AND PEEK(53268)

<7012 SPACES jTHEN 150

KX 440 GOTO 430

Program 6-2— VIC-20

XF 10 A$=CHR$ ( 147 ) rPRINTA?

JK 20 PRINTTAB(4) "THREE CHOICES"

CE 30 PRINT: PRINT

BQ 40 PRINTTAB ( 5 ) " * CHOICE ONE"

CF 50 PRINT .-PRINT .-PRINT .-PRINT

HF 60 PRINTTAB (5)"* CHOICE TWO "

JC 70 PRINT: PRINT: PRINT: PRINT

FG 80 PRINTTAB ( 5) "* CHOICE THREE"

RH 90 A=PEEK(36870) : B=PEEK ( 36871 )

CA 100 IF A>65 OR A<40 THEN90

QQ 110 IF (B>30) AND (B<40) THEN150

DH 120 IF (B>50) AND (B<60) THEN180

HJ 130 IF (B>70) AND (B<80) THEN210

SX 140 GOTO90

MQ 150 PRINTA$

CK 160 PRINT "CHOICE ONE"

BR 170 GOTO230

GS 180 PRINTA$

HP 190 PRINT "CHOICE TWO"

AS 200 GOTO2 30

HX 210 PRINTA$

XE 220 PRINT "CHOICE THREE"

DF 230 FOR 1=0 TO 1500

GS 240 NEXT I: PRINT

CHAPTER 6

DP 250 PRINT"* CONTINUE"

QD 260 IF PEEK( 36870 ) <40 AND PEEK( 36871)

<40{2 SPACES } THEN1
CF 270 GOTO260

Program 6-2 — Atari

100

REM LIGHT PEN DEMO

150

CHR* (125)

160

" THREE CHOICES"

1 70

: PR INT .-PRINT

180

"* CHOICE ONE"

1 90

:PRINT :PRINT

200

"* CHOICE TWO "

210

:PRINT .-PRINT

220

"* CHOICE THREE

230

A=PEEK (564) : B=PEEK (565)

240

IF A>90 THEN 230

250

IF (B

>25) AND (B<35)

THEN

300

260

IF (B

>45) AND (B<55)

THEN

330

270

IF (B>60) AND (B<70)

THEN

360

290

BOTO

230

300

CHR* (125)

310

"CHOICE ONE "

320

BOTO

400

330

CHR* (125)

340

"CHOICE TWO "

350

BOTO

400

360

CHR* (125)

370

"CHOICE THREE"

400

FOR I

=1 TD 15:PRINT :

410

420

"* CONTINUE"

430

IF PEEK ( 564 ) < 90 AND PEEK(565)

>B0 THEN 150
SI 440 BOTO 430

CHAPTER 7

A Digital Light
Sensor

The light pen you built in Chapter 6 is a type of
light sensor — it detects light from the computer's
screen and sends out an appropriate signal. But
what if you want to be able to tell whether it's
dark outside, or whether somebody opened your
closet door (letting light inside)? Or perhaps you
want to build an electronic timer that starts and
stops timing when a beam of light is broken? To
detect different levels of light, you need a slightly
more complex sensor.

This chapter will show you how to build and
use a variable digital light sensor. This sensor pro-
vides the computer with either a high (1) or low
(0) digital signal. When the light sensor is adjusted
for a certain level of light, the computer receives a
logic low (0) signal as long as this level of light, or
more, is maintained. However, if the level of light
falls below this setting, the computer gets a logic
high (1) signal from the sensor. This type of sen-
sor is useful for applications such as light beam
timers and counters, two applications which will
be demonstrated here as well.

To build the digital light sensor, you need these
parts:

Part

Quantity Part

Number

1 3900 op amp IC

276-1713

1 500K potentiometer

271-210

(Commodore)

1 1M potentiometer

271-211

(Atari)

1 100K ohm resistor

271-8045

CHAPTER 7

Part

Quantity Part Number

1 10K ohm resistor 217-8034

1 IK ohm resistor 271-8023

1 TIL 414 infrared photo- 276-130
transistor

1 9-pin D-subminiature female 276-1538

1 Solderless breadboard 276-175

You'll also need some solid copper wire for
jumper connections and some stranded copper
wire for connections to the phototransistor.

These steps will take you through the process of
building your digital light sensor:

1. Cut three pieces of solid copper wire and re-
move about 1 /4 inch of insulation from each
end.

2. Connect one wire each to pins 1, 7, and 8 of
the 9 -pin plug.

3. Connect the wire from pin 8 to location XI of
the solderless breadboard.

4. Connect the wire from pin 7 to location Yl.

5. Connect the wire from pin 1 to location F5.

6. Plug the 3900 op amp IC into the solderless
breadboard so that pin 1 goes to point El 5,
and pin 8 goes to point F9.

7. Cut two pieces of solid copper wire about 2
inches long and remove about 1 /4 inch of insu-
lation from each end. Solder one wire to an out-
side lead of the potentiometer. Solder the other
wire to the middle lead of the potentiometer.

8. The wire from the outside lead connects to
point X22 of the solderless breadboard. The
wire from the potentiometer's middle lead
connects to point A22.

A Digital Light Sensor

9. Cut two pieces of stranded copper wire about
12 inches long and remove about 1/4 inch of
insulation from each end. Solder one wire to
the emitter of the phototransistor and the
other wire to the collector.

10. The wire from the collector plugs into point
Y22 of the solderless breadboard, while the
wire from the emitter inserts into point J22.

11. Insert the resistors as follows on the solderless
breadboard:

Resistor From To

IK ohm B4 J4

10K ohm B22 B13

lOOKohm D13 D12

12. Connect five solid copper wire jumpers on the
solderless breadboard as follows:

Jumper

From

E22

F22

Y15

J15

X14

A14

C12

Figure 7-1 . The Final Sensor Board

Your final light sensor breadboard will look like this.

CHAPTER 7

A Digital Light Sensor

CHAPTER 7

Inside the Sensor

The phototransistor in this circuit acts as a switch.
It's in series with the 500K potentiometer. When
light strikes the light-sensitive surface, current will
flow through the emitter-collector junction. The
sensitivity is set by adjusting the 500K ohm poten-
tiometer in series with the phototransistor. The
operational amplifier's (op amp) inverting input is
connected between the potentiometer and the
phototransistor. The voltage at the op amp's input
is determined by the resistances of the potentiom-
eter and phototransistor.

The op amp is set up using the 10K ohm and
100K ohm resistors so that when the input voltage
is a certain level, its output goes to the logic low
level. The output of the op amp is connected to
pin 1 of the control port through a IK ohm
resistor.

The phototransistor must be set for the desired
level of light. This is done by aiming the light-
sensitive part of the phototransistor at a light
source, then adjusting the potentiometer for trig-
gering at the desired light level. Program 7-1 pro-
vides the necessary lines for testing and adjusting
this circuit.

Adjust the potentiometer so that the output to
the computer turns logic low. Once this is done, if
the phototransistor receives less light, the input
voltage of the op amp no longer will be the cor-
rect voltage to force its output low. As a result,
pin 1 of the 9-pin plug returns to its normal logic
high level.

Putting It to Use

To use the light sensor, first turn off your com-
puter. Insert the 9-pin plug into port 2 of the
Commodore 64 or 128 (there's only one port on

A Digital Light Sensor

the VIC), or into port 1 of any Atari computer.
Turn your computer back on, type in, and run the
version of Program 7-1 for your machine. For this
demonstration, the sensor is set to the lighting in
the room, so make sure that the light-sensitive
portion of the phototransistor is uncovered and
facing up.

Program 7-1 — Commodore 64/128

JE 10 IFPEEK(56320)AND(2T0)THEN20
XR 15 PRINT " {CLRjLIGHT IS ON":GOTO10
EP 20 PRINT "{CLR} LIGHT IS OFF":GOTO10

Program 7-1— VIC-20

QD 10 IF(PEEK(37151) AND 4 ) THEN PRINT "

{CLR} LIGHT IS OFF"
AS 20 IF (PEEK( 37151 ) AND 4)=0THEN PRINT

{ SPACE }" {CLR} LIGHT IS ON"
SB 30 GOTO10

Program 7-1 — Atari

10 IF STICK(0)=15 THEN PRINT "OFF"
20 IF STICK(0) = 14 THEN PRINT "ON "
30 GOTO 10

When you run the program, a column of mes-
sages should appear on the screen. Either the mes-
sage LIGHT IS ON or LIGHT IS OFF will be
displayed.

If the message is LIGHT IS ON, it means the
sensor is detecting light at the level of or greater
than the potentiometer setting. When the sensor
detects the light level, it forces pin 1 of the control
port low, just as connnecting it to pin 8 (ground)
does. If the sensor does not detect the light level
it's set for, pin 1 of the control port remains at
logic high.

Adjust the setting of the potentiometer by turning
its shaft. Doing this controls the sensitivity of the
light sensor. You should find a point as you turn

the potentiometer shaft where the messages in the
column change from LIGHT IS OFF to LIGHT IS
ON. In order to set the light sensor to detect the
light in the room, leave the potentiometer set at
the point where the messages just change from
LIGHT IS OFF to LIGHT IS ON.

Just as with a joystick, programs can check the
control port for the light sensor's signal. All that's
necessary is to PEEK the appropriate register, or
use the STICK command in the case of the Atari,
then check the bit (corresponding to pin 1, in this
case) for a 1 or 0.

A Digital Light Beam Timer

This circuit, quite similar to the light sensor, can
be used as a timer circuit for slot cars, or for any
application involving measuring time over a dis-
tance. This project is essentially the same as the
digital light sensor circuit you just completed. The
following parts are required:

Part

Quantity Part

Number

3900 op amp IC

276-1713

500K potentiometer

271-210

(Commodore)

1M potentiometer

271-211

(Atari)

100K ohm resistor

271-8045

10K ohm resistor

217-8034

IK ohm resistor

271-8023

Infrared emitting diode

276-142

Infrared detecting diode

276-142

9-pin D-subminiature female

276-1538

Solderless breadboard

276-175

You'll also need some solid copper wire and some
stranded copper wire.

A Digital Light Sensor

CHAPTER 7

Construct the circuit as you did for the digital light
sensor, but position the infrared emitting diode
and the infrared detecting diode so that the emit-
ting surface and detecting surface face each other.
Tests indicate that the maximum usable distance
between these diode pairs (the two parts come in
one package, listed as Radio Shack part number
276-142) is less than one foot.

Connect the infrared emitter's anode to the y
line on the solderless breadboard, and the cathode
to the x line. Use the appropriate version of Pro-
gram 7-1 to adjust and focus the diodes so that
the circuit triggers when the beam of light from
the emitter is broken, not when ambient room
light is interrupted. The light in the infrared is in-
visible to the naked eye. You'll see no indication
that the diode is turned on. Be sure you've in-
stalled it correctly in the circuit.

When you've tested the circuit and are satisfied
that it's functioning properly, type in and run Pro-
gram 7-2. Adjust the potentiometer as you did
with the digital light sensor, then press the X key
to continue the program.

Start the timer by breaking the beam between
the infrared emitting and infrared detecting di-
odes. When the beam is broken a second time, the
elapsed time is displayed and the timer stops.
Press the X key to reset the timer and run the pro-
gram again.

This circuit could be useful as a burglar alarm,
one that detects when an object or intruder breaks
the beam of infrared light.

Program 7-2 — Commodore 64/1 28

DX 110 REM TIMER PROGRAM

MH 120 PRINT" ICLR} ADJUST THE INFRARED BE

AM AND THE"
FA 130 PRINT "POTENTIOMETER UNTIL THE MES

SAGE JUST"

A Digital Light Sensor

GQ 140 PRINT "CHANGES FROM OFF TO ON"
MG 150 PRINT : PRINT "PRESS X TO CONTINUE
I DOWN j "

AQ 160 A$=CHR$(145)+CHR$(145)
PM 165 A=PEEK( 56320)

JM 170 IF (A AND 1) THEN PRINT "LIGHT IS
OFF"

GE 175 IF (A AND 1)=0 THEN PRINT "LIGHT

I SPACE} IS ON "
KE 180 GET B$

BQ 190 IF B$="X"THENGOTO220
DS 200 PRINTA$
AD 210 GOTO 165

HD 220 PRINT" I CLR] BREAK BEAM TO START TI
MER"

QX 230 IF (PEEK( 56320) AND 1)=0 THEN 230
DF 240 TI$="000000":PRINT"{CLR}TIMER STA
RTED"

MD 250 IF (PEEK(56320) AND 1) THEN 250
KH 260 IF (PEEK( 56320) AND 1)=0 THEN 260
RC 270 PRINT "TIME IS" ;TI/ 60; "SECONDS"
XH 280 PRINT" {DOWN J PRESS X TO RESET TIME
R"

DG 290 GETB$ :IFB$=" "THEN290
CD 300 IFB$="X"THEN220
MK 310 GOTO290

Program 7-2— VIC-20

SH 10 PRINT" {CLR } ADJUST THE INFRARED
{3 SPACES} BE AM AND THE CIRCUIT
QS 20 PRINT "UNTIL THE MESSAGE JUST"
SR 30 PRINT "CHANGES FROM OFF TO ON"
KQ 40 PRINT: PRINT "PRESS X TO CONTINUE"
GM 50 A$=CHR$(145)
PB 60 A=PEEK( 37137 )

AE 70 IF (A AND 4) THEN PRINT "LIGHT IS

{ SPACE }OFF"
PQ 80 IF (A AND 4)=0 THEN PRINT "LIGHT I

S ON "
XA 90 GET B$
MH 100 IF B$="X"THEN130
QB 110 PRINTA$A$
SQ 120 GOTO60

PH 130 PRINT "{CLR} BREAK BEAM TO START"
RP 140 IF ( PEEK ( 37137 )AND4)=0 THEN140
KS 150 TI$="000000"

RX 160 IF (PEEK (37 137) AND 4) THEN160
JD 170 IF (PEEK( 37137) AND 4)= THEN170

CHAPTER 7

MQ 180 PRINT "TIME IS " ; Tl/60 ; "SECONDS "
HD 190 PRINT "PRESS X TO RESET TIMER"
XA 200 GETG$:IFG$=""THEN200
QX 210 GOTO130

Program 7-2 — Atari

AP110 REM TIMER PROBRAM

KP 120 PRINT "TURN ON FLASHLIGHT AND

ADJUST THE "
LF 130 PRINT "POTENTIOMETER TILL THE

MESSAGE JUST"
KO 140 PRINT "CHANGES FROM OFF TO ON

FE 150 PRINT "PRESS X TO CONTINUE"
GH 155 FOR J=l TO 2500:NEXT J
KE 170 IF STICK<0>=15 THEN PRINT "OF
F"

BK 175 IF STICK(0>=14 THEN PRINT "ON

MB 180 REM CHECK IF X KEY PRESSED
KI 190 IF PEEK<764)=22 THEN 215
SE210 GOTO 170

B6 212 REM CLEAR OUT PREVIOUS KEYSTR
OKE

0ft215 POKE 764,255

JB 220 PRINT "BREAK BEAM TO START TI
MER"

IJ 230 IF STICK(0)=14 THEN 230

OA 240 POKE 1S,0:POKE 19,0:POKE 20,0

10 250 IF STICK(0)=15 THEN 250

IP 260 IF STICK<0)=14 THEN 260

BG 270 PRINT "TIME IS " ; < ( PEEK < 1 8 ) * 2

55*255) + (PEEK < 19) *255) + < PEEK (

20) ) ) /60; " SECONDS"

For something a little different, modify Program 7-
2 to count (COUNT = COUNT + 1) every time
the beam is interrupted.

CHAPTER 8

An Electronic
Switch

An Electronic Switch

So far, all the projects you've completed have
been sensors — devices that transfer information
about the outside world into your computer. But
your computer can also be used to control outside
devices through electronic signals.

While exploring the control port, you saw how
information in the form of electronic on/off sig-
nals can be sent to your computer using the data
(I/O) lines. These same wires can also be used by
your computer to send digital signals out. Whether
your computer is using the data lines for input or
output, two registers are involved.

The first register, called the data port, contains
the actual data being transferred. The signal on
each data line corresponds with a bit stored at this
location. When receiving data, your program sim-
ply checks the bit which corresponds to the af-
fected I/O line. Suppose you wish to see whether
pin 1 of control port 1 is set low, as it is if the joy-
stick is pushed to the up position. For the Com-
modore 64/128, you could use the following
statement:

10 IF (PEEK(56321) AND 2T0) = THEN PRINT "YES"

On the VIC-20, you'd use this instead:

10 IF (PEEK(37137) AND 2T0) = THEN PRINT "YES"

Since the zero bit in the memory location (the reg-
ister) corresponds with pin 1 (the I/O line), the
word YES is printed if this bit is set low.

When outputting data, your program must alter
the contents of the data port and set the port for
output. Suppose you want to set pin 1 of port 2 to
output a logic high signal. To do this, your pro-
gram must set bit in the data direction register
to a 1 for output:

CHAPTER 8

For Commodore 64/128:

10 POKE 56321,PEEK(56321)OR(2T0)

For Commodore VIC-20:

10 POKE 37137,PEEK(37137)OR(2t0)

Each I/O pin of the control port has a cor-
responding bit in the data direction register. When
a control port pin is to be used for input, its bit in
this register must be set low. Setting a data direc-
tion register bit high, on the other hand, makes its
corresponding control port pin an ouput line. The
data lines are normally set for input — that's why
you could ignore this register when you were first
exploring the control port.

However, when you want the computer to out-
put signals, this register must first be set. The data
direction registers are located at memory locations
56322 ($DC02 in hexadecimal) for joystick port 2
and 56323 ($DC03) for joystick port 1 on the
Commodore 64 and 128, and location 37139
($9113) for the VIC-20.

The Atari Computers

The Atari computers are a little different when it
comes to sending and receiving data. A data regis-
ter and data direction register are still used, but
unlike the Commodore computers, the Atari's reg-
isters are both at the same memory location.

Though you previously used the BASIC STICK
command to check input to the control port, you
can also use one of the computer's registers di-
rectly to obtain the same information. Bits 0-3 of
memory location 54016 ($D300) correspond to
pins 1-4 of control port 1, respectively, while bits
4-7 correspond to pins 1-4 of control port 2. By
PEEKing this memory location, you can see what
data has been input through the control ports.
This same register is used to send information out
of the computer using the control ports' lines. This

An Electronic Switch

is done by POKEing values into the register to set
the bits corresponding to the control ports' pins
either high or low. Memory location 54016 is used
as a data register and contains the information be-
ing either sent or received.

Memory location 54016 acts as a data register,
accepting incoming data, as long as bit 2 of mem-
ory location 54018 ($D302) is set to 0. If bit 2 of
memory location 54018 is set to 1, location 54016
acts as a data direction register. Outgoing data can
now be sent. The control ports' pins are set as
either input or output lines, depending on
whether their corresponding bits of the data direc-
tion register are 0's or l's. You can set memory lo-
cation 54016 to be the data direction register by
POKE 54018,48

and return it to acting as a data register with
POKE 54018,52

Note: With Atari BASIC, you cannot mask or force
bits using ANDs and ORs in POKE statements as
with Commodore computers. To set an individual
bit in a memory location high, you can POKE the
value directly, such as

POKE location, Tbit

where location is the memory address you wish to
change and bit is the bit within that address that
you wish to set. For example, to set bit 2 of loca-
tion 54016, you would use
POKE 54016,4

Without AND and OR, there's no easy way to
change a single bit without affecting the current
settings of the other bits in the location. For ex-
ample, if location 54016 already has bits 6 and 7
set (corresponding to a value of 192 in the loca-
tion) and you wish to set bit 3 without affecting
the others, then the value you must POKE into
54016 is 200, 192 + (2 A 3).

Using a Computer's Output Signals

How can an on/off signal from the computer con-
trol events in the outside world? An electronic cir-
cuit is required to take the digital signals that the
computer outputs and perform some useful task.
One example of this type of circuit is the elec-
tronic switch you'll make. It allows your computer
to switch things on and off under the control of a
computer program.

Building the Electronic Switch

You'll need these parts to construct your electronic
switch:

Part

Quantity Part

Number

1 9-pin D-subminiature female 276-1538

1 2N2222 NPN transistor

276-1617

1 2.2K ohm resistor

271-8027

1 IN914 diode

276-1620

1 5 -volt SPDT DIP relay

275-243

1 LED

276-041

1 Solderless breadboard

276-175

1 7400 IC (Atari)

276-1801

You'll also need some solid copper

wire for con-

nections to the 9-pin plug, as well

as for jumper

connections on the solderless breadboard.

Follow these steps to build your electronic
switch if you have a Commodore computer (the
steps are identical for the Atari, though some ad-
ditional steps are listed below).

1. The first step in constructing the electronic
switch is to wire the 9-pin plug. Cut three
pieces of wire about four inches long and re-
move about 1/4 inch of the insulation from
each end.

An Electronic Switch

2. Solder wires to pins 1, 7, and 8.

3. Connect the three wires as follows: The wire
from pin 8, the ground wire, connects to point
Yl on the solderless breadboard. The +5- volt
wire from pin 7 connects to point XI of the
solderless breadboard. The data line from pin
1 connects to location B4 on the board (except
for the Atari version — see below).

4. Connect the 2.2K ohm resistor between points
A4 and F4 on the solderless breadboard.

5. Mount the 2N2222 transistor so that its base
inserts into point H4, its emitter into point H3,
and its collector into point H5.

Figure 8-1 . Pin Diagrams for Typical 2N2222 NPN
Transistors

1. Emitter

2. Base

3. Collector

Here are three of the most common 2N2222 NPN transistor con-
figurations you'll find.

6. The diode is connected so that its cathode lead
inserts into point X5 of the solderless bread-
board and its anode lead connects to point B5.
(A band around one end of the diode is gener-
ally used to identify the cathode lead.)

7. The SPDT relay is inserted so that its normally
open and normally closed pins attach to points
F9 and E9 respectively. The common pin con-
nects to point E14.

Continue with step 8 on page 88.

CHAPTER 8

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An Electronic Switch

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CHAPTER 8

8. The following jumper connections are required
to complete the electronic switch.

Wire From To

1 J3 Y3

2 J5 J10

3 E5 F5

4 X10 A10

Additional Steps for Atari Version Only

9. Insert the 7400 IC into the solderless bread-
board so that its pins 1 and 8 go into plug
points F16 and E22 respectively.

10. Insert the wire from pin 1 of the 9-pin plug
into point J16 of the solderless breadboard.

11. Add the following jumper connections.

Wire From To

5 X16 A16

6 J22 Y22

7 H16 G17

8 G18 G19

9 H19 G20
10 G21 B4

Switch Is On

The 9-pin plug connects to control port 1 of your
computer. The circuit draws its power from the
computer's power supply through the wires from
pins 7 and 8. The wire from pin 1 provides the
circuit with a digital signal. This signal can be set
by the program to be either logic high or logic low.

Pin 1 of the control port is connected to the
base of the transistor through a current-limiting
resistor. If the signal from the computer is +5
volts (logic high), current flows from this pin
through the base of the transistor. This base cur-
rent causes the transistor to act as a closed switch
between its emitter and collector leads. When the

An Electronic Switch

CHAPTER 8

transistor acts as a closed switch, current flows
from the + 5 volts of the power supply through
the coil of the relay to ground. The relay turns on
due to this coil current. The Atari version of the
circuit uses two NAND gates as a noninverting
buffer. This buffer is required since the signal
from the Atari's control port is not sufficient to
turn on the transistor. Refer to Chapter 10, "Digi-
tal Logic," for more on buffers and gates.

When a logic low signal (0 volts) is present at
pin 1, no base current flows in the transistor. As a
result, the transistor acts as an open switch across
its emitter and collector leads. Consequently, the
relay assumes its off position since no current
flows through its coil.

The relay is an electromechanical switch. The
device to be controlled is connected to the relay
via its common, normally open and normally
closed terminals. The circuit which controls the
device is attached to the coil terminals of the relay.

When the relay is off, a metal lever bridges the
gap between its common and normally closed ter-
minals. The coil of the relay is part of an electro-
magnet. When current flows through this coil (to
turn the relay on), the electromagnet pulls the
metal lever so that it bridges the gap between the
common and normally open terminals instead.

Many electric devices can be switched on and
off by this circuit. A description of using the cir-
cuit to turn on an LED appears below. To use this
circuit to control other devices, connect the de-
vices between points G9 and D14, the two points
marked A in Figure 8-3. Other devices should
have their own power supply, such as a battery
connected in series with point G9 or D14 and the
device itself. (You can see the connection from the
breadboard to a 6-volt battery in Figure 8-4.) Cau-
tion should be exercised not to exceed the current
rating of the SPDT DIP relay.

An Electronic Switch

Figure 8-5. Through with the Breadboard

. . ■

* * * w *■

iyi if

* * v •«.«» ...

* «' » ■

The electronic switch can be used to control a variety of devices.

Using the Electronic Switch — Flashing an LED

Here's one application for your electronic switch —
a computer-controlled LED. The computer pro-
gram, with the aid of the electronic switch, turns
on the LED after a specified time delay. For this
application, you'll need to connect two jumper
wires and the LED.

1. Connect a wire between points X8 and G9 on
your solderless breadboard.

2. Also connect a jumper between points Y15 and
J15.

3. Plug the cathode of the LED into point D15 and
its anode into D14.

First of all, insert the 9-pin plug of the electronic
switch circuit into control port 1 of your computer.
Next, turn on your computer.

CHAPTER 8

Enter and run the appropriate version of Program
8-1. The program prompts you to enter the hours,
minutes, and seconds of the delay. After you enter
the delay, the computer will wait that length of
time before turning on the LED — the LED will re-
main on for only about five seconds.

To turn on the LED, the program sets pin 1 of
the control port to logic low. This turns off the re-
lay, closing the connection between its common
and normally closed terminals.

Program 8-1 — Commodore 64/128

KR 10 PRINT "ENTER HOURS";
BK 20 INPUTA

SS 30 PRINT "ENTER MINUTES";
DP 40 INPUTB

SG 50 PRINT "ENTER SECONDS";
RR 60 INPUTC

GP 70 D=(A*216000)+(B*3600)+(C*60)
QE 80 REM START TIMER
QR 90 TIS="000000"
RM 100 IFTI > DTHEN1 30
QH 110 GOTO100

KX 120 REM SET DATA LINES FOR OUTPUT
JJ 130 POKE 56323, PEEK(56323)OR(2T0)
DH 140 REM SET DATA LINES TO LOGIC LOW T

O TURN ON BUZZER
CF 150 POKE 56321 , PEEK ( 56321 )ANDNOT( 2l0)
FX 160 REM KEEP BUZZER ON FOR FIVE SECON

AA 170 TI$="000000"

DK 180 IF TI>300 THEN210

HB 190 GOTO180

SQ 200 REM SET DATA LINE TO LOGIC HIGH T

O TURN OFF BUZZER
XJ 210 POKE 56321, PEEK ( 5632 1 ) OR{ 2 ?0 )
FC 220 REM RESET DATA LINE FOR INPUT
QQ 230 POKE 56323, PEEK ( 563 2 3 ) ANDNOT ( 2 t )

Program 8-1 — VIC-20

KR 10 PRINT "ENTER HOURS " ;

BK 20 INPUTA

SS 30 PRINT "ENTER MINUTES";

DP 40 INPUTB

An Electronic Switch

SG 50 PRINT "ENTER SECONDS";
RR 60 INPUTC

GP 70 D=(A*216000)+(B*3600)+(C*60)
QE 80 REM START TIMER
QR 90 TI$="000000"
RM 100 IFTI>DTHEN130
QH 110 GOTO100

KX 120 REM SET DATA LINES FOR OUTPUT
BM 130 POKE 37139, PEEK ( 37 1 39 ) OR( 2? 2 )
DH 140 REM SET DATA LINES TO LOGIC LOW T

TURN ON BUZZER
BD 150 POKE 37137, PEEK ( 371 37 ) ANDNOT ( 2T2 )
FX 160 REM KEEP BUZZER ON FOR FIVE SECON

AA 170 TI$="000000"

DK 180 IF TI>300 THEN210

HB 190 GOTO180

SQ 200 REM SET DATA LINE TO LOGIC HIGH T

O TURN OFF BUZZER
GM 210 POKE 37137, PEEK( 37137 )OR( 2?2 )
FC 220 REM RESET DATA LINE FOR INPUT
RM 230 POKE 37139, PEEK ( 37 139 ) ANDNOT ( 2T 2 )

Program 8-1 — Atari

IS 100 REM TIME BUZZER
KO120 PRINT "ENTER HOURS";
6F 130 INPUT A

EE 140 PRINT "ENTER MINUTES";
GI 150 INPUT B

DA 160 PRINT "ENTER SECONDS";
GL 170 INPUT C

PP 1S0 D= (A*2 16000) + (B*3600) + (C*60)
OE190 POKE 18,0:POKE 19,0:POKE 20,0
OJ 200 IF ( (PEEK ( IB) #255*255) + (PEEK <

19) *255) + (PEEK (20) 5 ) >D THEN 2

FO210 SOTO 200

NF215 REM SET DATA LINES FOR OUTPUT
FN 220 POKE 54018,48
JA 225 POKE 54016,255
FJ 230 POKE 54018,52

UK 235 REM SET DATA LINES LOGIC LOW

TO TURN ON BUZZER
CB 240 POKE 54016,0

OF 245 POKE 18,0:POKE 19,0:POKE 20,0
EC 250 IF < (PEEK ( 18) *255*255) + (PEEK (
19) #255)+ (PEEK (20) ) ) >300 THEN
270

CHAPTER 8

SI 260 GOTO 250

ML 265 REM SET DATA LINES HISH TO TU

RN OFF BUZZER
JA270 POKE 54016,255

AN 280 REM RESET DATA LINES FOR INPU
T

6E290 POKE 54018,48
BO 300 POKE 54016,0
FI 310 POKE 54018, 52

Other Applications

The LED flasher is just one application of the elec-
tronic switch. You can use your electronic switch
to turn on any electric device, anything from an
alarm to a light, so long as the ratings of the relay
and solderless breadboard are not exceeded. Sim-
ply wire the device and its power supply to the
relay as if you were using an ordinary switch.

Warning: If you're planning on using the switch to
turn on lights, a coffee pot, or any appliance con-
nected to house line voltage, remember that the

relay must be rated to handle the current needed
by the appliance. Potentially lethal voltages will be
present at the relay. The relay should be housed in
a container to prevent accidental contact, and
good electrical practices should be employed. For
additional safety, a buffer circuit should be em-
ployed to further isolate your computer (and your-
self) from high voltages.

A Burglar Alarm

A burglar alarm is supposed to detect an intruder.
To do that, though, the alarm must be as sophisti-
cated as possible to detect and act upon a variety
of situations. A computer, equipped with the
proper sensors and actuators, can do just this and
become an effective burglar alarm.

Intruders may reveal their presence to a
computer-controlled burglar alarm in a number
of ways. First of all, the intruder must somehow
enter the building — a door or window is the most
logical means of entry. If the computer's sensors
are set to detect use of these entrances, an alarm
can be triggered when someone passes through
them.

Sensors could also be set to detect anyone in
areas considered off-limits. For instance, if a sen-
sor detects someone in the living room when all
rightful occupants are sleeping upstairs, the bur-
glar alarm can safely assume that an unauthorized
person is in the protected area.

Another possibility is to have the alarm sensors
detect movement of particular objects. Most car
alarms work on this principle. The sensors can be
set to tell whether your stereo, television, or any-
thing else has been moved. If an intruder tries to
steal something, the burglar alarm can alert you,
or even alert the police.

Using What You Know
The previous projects in this book laid the
groundwork for a burglar alarm. In particular, the
joystick project in Chapter 3 and digital light sen-
sor project in Chapter 7 will be used as the sen-

CHAPTER 9

sors. The electronic switch project, found in
Chapter 8, is the actuator required by the alarm. If
you didn't put them together earlier, turn back
now and try them. Be sure you have a basic un-
derstanding of these circuits before proceeding.
You'll need to refer to these chapters as you con-
struct this burglar alarm.

As you saw in Chapters 2 and 3, a computer
joystick is actually a set of switches. Many types
of burglar alarm sensors are switches as well.
These sensors can be connected to your computer
in the same way as a joystick — through the I/O
lines of the control port.

One type of switch sensor is a thin, adhesive,
metallic tape that can be stuck on windows and
other glass. This tape normally acts as a closed
switch, allowing current to flow through a circuit.
If the glass is broken, the fragile tape breaks, too,
and the circuit is interrupted. Another type of
switch sensor is a magnetic switch. This sensor is
generally used to detect doors and windows being
opened or closed, but it could also be installed on
desk drawers. A typical application is a magnet
mounted on a door, with the magnetic switch
mounted on the door frame. When the door
closes, the magnetic switch either closes or opens,
depending on the type of switch. When the door
opens, the magnetic switch changes.

Other types of sensor switches include vibration
detectors, ultrasonic sensors, and even infrared
sensors. Most of these "switch" alarm sensors can
be purchased at electronic stores such as Radio
Shack.

Your alarm will use the digital light sensor
project built earlier to detect entry into an off-limits
area. The light source and sensor should be set up
so that any light in the controlled area will trigger
the sensor.

A Burglar Aiarm

The computer will use the electronic switch to
turn on the alarm itself. The alarm can be a buzzer
or a louder alarm (providing it doesn't exceed the
ratings of the relay or solderless breadboard). With
the proper programming, your computer could
even use a modem to call the police, and then
play a tape recording announcing your address
and reporting a break-in.

Setting Up the Alarm

To set up your computer as a burglar alarm, plug
the electronic switch into control port 1.

Before placing the digital light sensor's 9-pin plug
into control port 2, solder wires for switch sensors
to its pins 2, 3, 4, and 6.

Figure 9-1 . Atari and Commodore 64/1 28 Burglar Alarm
Switches

Normally closed switches
represent switch sensors

Pin 2

9-pin

Plug J
of \
light

Pin 3

Pin 4

Pin 6

To Pin 8

"Q l_£T

(or any ground connection)
Notes

Digital light sensor plug inserts into control port 2.

Only additions to this circuit are sensors illustrated above.

Electronic switch circuit plugs into control port 1 unaltered.

The alarm sensors are wired to the digital light project's 9-pin plug.

CHAPTER 9

If you have a VIC-20, you need a new connec-
tor since both the electronic switch, the digital
light sensor, and the other switch sensors must be
plugged into the single control port.

To do this, you need to make a new connector:

1. Solder wires to pins 1, 2, 3, 4, and 6 of a 9-pin
plug. Solder two wires each to pins 7 and 8.

2. Remove the old connectors from the electronic
switch and light sensor cicuits.

3. The wire from pin 1 of the new connector in-
serts into point A4 of the electronic switch cir-
cuit. It provides the digital signal required to
turn the electronic switch on or off.

4. The wire from pin 2 connects to point 14 of the
light sensor circuit to receive its digital output
signal.

5. The wires from pins 7 and 8 provide the power
to the two circuits. One set of two wires from
pins 7 and 8 connects to points Yl and XI re-
spectively of the light sensor circuit board. The
other set of two wires from pins 7 and 8 con-
nects to points XI and Yl respectively on the
electronic switch circuit. The remaining wires
from the control port are used for switch sensors.

Whichever type (or types) of switch sensors you
choose for your burglar alarm, they must be nor-
mally closed. They're set up so that they ordinarily
form a closed loop between the ground pin of the
control port and one of the I/O pins. The Commo-
dore 64, 128, and Atari computers can each have
four switch sensors connected to them in addition
to the light sensor. The VIC-20 can have three
switch sensors in addition to the light sensor. For
any of the machines, there's no need to wire one
end of the switch sensors directly to pin 8 of the
control port — any connection to ground, such as
the x row of the light sensor circuit, will do.

100

A Burglar Aiarm

Figure 9-2. VIC-20 Burglar Alarm Switches

To point A4 of electronic
switch solderless breadboard

Pin 1

To point 14 of digital light
sensor solderless breadboard

Pin 2

. Normally closed switches represent
jf switch sensors

Pin 3 -

Pin 4

Pin 6

Pin 8

fl —

(or any ground)

Notes

One wire from pin 7 inserts to point Yl of electronic switch solderless breadboard.
One wire from pin 8 inserts to point XI of electronic switch solderless breadboard.
Second wire from pin 7 inserts to point XI of digital light sensor.
Second wire from pin 8 plugs into point Yl of digital light sensor.

You must have special connector to build the burglar alarm for the VIC-20 since
the computer has only one control port.

CHAPTER 9

Since, in normal conditions, the switch sensors
form a closed loop between ground and their par-
ticular I/O lines, these lines are at a logic low
level. Similarly, the digital light sensor circuit out-
puts a logic high signal as long as no light is de-
tected. The computer program of the burglar
alarm can tell if things are as they should be just
by checking to make sure that the bits in the data
register corresponding to the sensors are off.

If one of the sensor bits of the data register has
changed, this means one of the sensors has been
activated — its corresponding I/O line no longer
forced to ground. The burglar alarm program must
detect if this happens and send a signal to the
electronic switch to turn on the alarm.

You may want to experiment with infrared
emitters and detectors, such as those used in the
digital timer project of Chapter 7. In a darkened
hallway, you might be able to use the digital timer
as a switch for an intruder alert. Other infrared
detectors and emitters could be purchased.

Program 9-1 is a simple demonstration program il-
lustrating the use of your computer-controlled
burglar alarm. Once you have the basic circuitry
working, you can modify the program and circuit
to add additional sensors as necessary. If you need
more sensors than this simple alarm allows, refer
to the multiplexer project of Chapter 12.

Program 9-1 —Commodore 64/1 28

MH 120 PRINT " { CLR } ADJUST THE INFRARED BE

AM AND THE"
FA 130 PRINT "POTENTIOMETER UNTIL THE MES

SAGE JUST"
GQ 140 PRINT "CHANGES FROM OFF TO ON"
KX 150 PRINT: PRINT "PRESS X TO CONTINUE"
SF 160 AS=CHR$(145)
PM 165 A=PEEK( 56320)

BJ 170 IF (A AND 2?0) THEN PRINT "OFF"
XM 175 IF (A AND 2?0)=0 THEN PRINT "ON "

102

A Burglar Alarm

KE 180 GET B$

FK 190 IF B$="X"THEN220

PH 200 PRINTA$A$

AD 210 GOT0165

EK 220 PRINT "TRIGGER A SENSOR TO START A
LARM "

HQ 225 REM CHECK SENSOR STATUS
MX 230 B=PEEK(56320)
QH 240 IF B=A THEN 230

JQ 250 PRINT "WARNING-ALARM TRIGGERED"
AG 260 FOR 1=0 TO 4

AA 270 IF (B AND 2TD THEN PRINT "SENSOR

";I+1;" DETECTS VIOLATION"
EH 280 NEXT I
MM 340 TI$=" 000000"
QM 370 REM TURN ON ALARM

SQ 375 REM SET DATA DIRECTION REGISTER F

OR OUTPUT
XJ 380 POKE 56323,PEEK(56323)OR(2T0)
AM 385 REM SET DATA LINE LOGIC LOW TO TU

RN ON ALARM
HF 390 POKE 56321, PEEK ( 56321 ) ANDNOT ( 2T0 )
QC 395 REM ALARM ON FOR 2.5 SECONDS
KE 400 IF TI>3600/24 THEN 420
GP 410 GOTO 400

BP 420 REM SET DATA LINE LOGIC HIGH TO T

URN OFF ALARM
AK 430 POKE 56321, PEEK ( 5632 1 ) OR ( 2 T0 )
JR 440 REM RESET DATA DIRECTION REGISTER
FOR INPUT

PM 450 POKE56323,PEEK(56323)ANDNOT(2T0)

Program 9-1 — VIC-20

GE 10 PRINT "TURN ON FLASHLIGHT AND ADJUS
T THE"

GG 20 PRINT "POTENTIOMETER TILL THE MESSA
GE JUST"

SR 30 PRINT "CHANGES FROM OFF TO ON"
KQ 40 PRINT: PRINT "PRESS X TO CONTINUE"
GM 50 A$=CHR$(145)
PB 60 A=PEEK( 37137 )

HS 70 IF (A AND 2T3) THEN PRINT "OFF"

FH 80 IF (A AND 2?3)=0 THEN PRINT "ON "

XA 90 GET B$

MH 100 IF B$="X"THEN130

QB 110 PRINTA$A$

SQ 120 GOTO60

103

CHAPTER 9

DX 130 PRINT "TRIGGER A SENSOR TO START A
LARM "

QG 140 Z? = " DETECTS VIOLATION"
MG 150 REM CHECK SENSOR STATUS
ES 160 B=PEEK( 37137)

FS 170 POKE37154,0:IF(PEEK(37152)AND(2T7
) ) THEN PRINT "SENSOR 3 " ; Z$ :GOTO230
EF 180 IF B=A THEN160

RP 190 PRINT "WARNING- ALARM TRIGGERED"
GG 200 IF (B AND 2?3) THEN PRINT "SENSOR
1";Z$

MM 210 IF (B AND 2?4) THEN PRINT "SENSOR
2";Z$

DX 220 IF (B AND 2T5) THEN PRINT "SENSOR
4";Z$

HB 230 POKE 37154,255

XE 240 TI$="000000"

PC 250 REM TURN ON ALARM

GH 260 REM SET DATA DIRECTION REGISTER F
OR OUTPUT

HB 270 POKE 37139,PEEK(37139)OR(2T2)

PR 280 REM SET DATA LINE LOGIC LOW TO TU

RN ON ALARM
PM 290 POKE 37137,PEEK(37137)ANDNOT(2T2)
RM 300 REM ALARM ON FOR 2.5 SECONDS
BC 310 IF TI>3600/24 THEN330
SG 320 GOTO310

HB 330 REM SET DATA LINE LOGIC HIGH TO T

URN OFF ALARM
CB 340 POKE 37137,PEEK(37137)OR(2T2)
AC 350 REM RESET DATA DIRECTION REGISTER
FOR INPUT

EF 360 POKE37139,PEEK( 37 1 39 ) ANDNOT ( 2T2)

Program 9-1 — Atari

PL 110 REM ALARM PROGRAM

KP120 PRINT "TURN ON FLASHLIGHT AND

ADJUST THE"
LF 130 PRINT "POTENTIOMETER TILL THE

MESSAGE JUST "
K0 140 PRINT "CHANGES FROM OFF TO ON

FE 150 PRINT " PRESS X TO CONTINUE"

6H 155 FOR J = l TO 2500:NEXT J

BP 160 IF STICK(1)=1 THEN PRINT "OFF

DB 170 IF STICK<1>=0 THEN PRINT "ON

104

A Burglar Alarm

(IB 180

KD 190

6C 200

ML 2 1

CH 220

DD 230

NI 240

611 250

LS 260

OC 270

OF 280

OJ 290

PA 292

ON 294

CF 300

NK 305

NO 310

NC 320

FP 330

10 340

Ffl 350

OG 360

CF 370

AM 380

HA 390
OF 400

IH 410
AJ 420

6A 430
CD 440
FN 450

REM CHECK IF X KEY PRESSED
IF PEEK(764)=22 THEN 210
GOTO 160

REM CLEAR OUT LAST KEYSTROKE
POKE 764,255

PRINT "TRIGGER A SENSOR TO ST
ART ALARM "

A=STICK(1) :B=STRIG(1)

IF (A = AND B=0) THEN 240

PRINT "WARNING - ALARM TRIGGE

RED"

IF A=l THEN PRINT "SENSOR 1"
IF A=2 THEN PRINT "SENSOR 2"
IF A=4 THEN PRINT "SENSOR 3"
IF A=8 THEN PRINT "SENSOR 4"
IF B=l THEN PRINT "SENSOR 5"
PRINT "DETECTS VIOLATION"
REM INITIALIZE CLOCK
POKE 18,0:POKE 19,0:POKE 20,0
REM SET DATA LINES FOR OUTPUT
POKE 54018,48
POKE 54016,255
POKE 54018,52

REM SET DATA LINES LOW TO TUR
N ON ALARM
POKE 54016,0

IF ( (PEEK ( 18) #255*255) + (PEEK (
19) *255> + (PEEK (20) ) ) >3600/24
THEN 400
GOTO 380

REM TURN OFF ALARM BY SETTING

DATA LINES LOGIC HIGH
POKE 54016,255

REM RESET DATA LINES FOR INPU
T

POKE 54018,48
POKE 54016,0
POKE 54018,52

Before mounting sensors, you should first check
your burglar alarm. The switch sensors can be
simulated by connecting their I/O pin wires to the
ground of either of the circuits. When you run the
appropriate version of Program 9-1, you're in-
structed to carry out the necessary adjustment to
the digital light sensor. Once you do this, your
burglar alarm is operational.

105

CHAPTER 9

To trigger the alarm, shine a light on the light
sensor or disconnect a switch sensor wire from
ground. The program prints on the screen which
sensor was triggered and turns on the alarm after
a delay of several seconds (so that an authorized
person could disable the alarm). The alarm stays
on for a couple of seconds before shutting off.
You'll probably want to increase the delay of tim-
ers when you install the system. (Of course, a
power failure will disable the alarm. The program
will have been removed from your computer's
memory.)

Other Applications

Not all the applications for this project are as seri-
ous as a burglar alarm. You could mount a mag-
netic switch on the refigerator door to make sure
someone isn't cheating on a diet. If you have a
small store, you could use this project to sound a
buzzer when a customer enters. With some
thought, you can modify the program and circuit
for almost any application.

106

CHAPTER 10

Digital Logic

Until now, whenever a digital output signal was
required, you took it directly from one of the com-
puter's control port I/O lines. Similarly, all the
sensors sent their digital output signals directly to
a control port pin. Every digital signal, whether
input or output to the computer, needed its own
I/O line.

Advanced projects cannot be interfaced to your
computer like this — there simply are not enough
I/O lines available. The control port gives you ac-
cess to five I/O lines. But what if you need to
control 12 electronic switches or monitor 15 sen-
sors? Or even do both at the same time? The solu-
tion to this predicament is to use digital circuitry
to manipulate the signals the computer sends and
receives.

Digital circuitry is the same type of circuitry
your computer uses. The uses of these types of
circuits include performing calculations, storing
information, and controlling the flow of digital
signals. The behavior of digital circuits is de-
scribed by what is known as Boolean algebra.

Even the most complex digital circuits, like the
microprocessor of your computer, are created from
basic building blocks. These building blocks are
called gates. A gate has one or more input lines
and only a single output line. The input lines are
used to input binary data in the form of on/off
signals. A gate combines these signals to produce
an output. The output signal is either logic high
(on) or logic low (off). The relationship between
the input signals and the output signal of a gate is
given by its truth table.

109

CHAPTER 10

In the following descriptions, you'll see H and L
listed. They represent:

H = Logic high On 1 signal
L = Logic low Off signal

The main types of gates, along with their truth
tables, follow.

Noninverting Buffer

This is the simplest of all gates. The signal that
appears at its input is the same as the signal on its
output.

Figure 10-1 . Symbol for a Noninverting Buffer

NOT Gate

NOT gates are often called inverters. A NOT
gate's output signal is the opposite of its input sig-
nal. Note that the symbol of the NOT gate is the
same as that of the buffer, except for the small cir-
cle near its output. Small circles at input or output
lines of a gate's schematic symbol denote the fact
that the signal is inverted.

Figure 1 0-2. Symbol for a NOT Gate

Output

Input

L
H

Output

L
H

Output

110

Digital Logic

Input Output

L H
H L

AND Gate

This gate has two or more inputs. Its output goes
logic high only if all the inputs are logic high.

Figure 10-3. Symbol for an AND Gate

Input Output
A B

L L L
L H L
H L L
H H H

Output

NAND Gate

This gate has two or more inputs and only one
output. Its output goes logic low only if all the in-
puts are logic high. For all other input conditions,
the output is logic high. Note that the output of
the NAND gate is the opposite of that of the AND
gate for indentical input signals.

Figure 1 0-4. Symbol for a NAND Gate

Output

Input

Output

A B

L L

L H

H L

H H

111

CHAPTER 10

OR Gate

The output of an OR gate goes high if any of its
two or more input lines are set high.

Figure 1 0-5. Symbol for an OR Gate

Output

Input Output
A B

L L L
L H H
H L H
H H H

NOR Gate

A NOR gate is identical to an OR gate with its
output inverted. The output of a NOR gate is logic
high only if all the inputs to the gate are logic
low.

Figure 1 0-6. Symbol for a NOR Gate

Output

Input Output
A B

L L H
L H L
H L L
H H L

Exclusive OR Gate

The output of this gate goes high if one input (but
not more) goes logic high.

112

Digital Logic

Figure 10-7. Symbol for an Exclusive OR Gate

Output

Input Output
A B

L L L
L H H
H L H
H H L

Exclusive NOR Gate

The exclusive NOR gate has a logic low output
when one (but not more) of the input lines is logic
high. Its output is the same as that of an exclusive
OR gate with its output inverted.

Figure 10-8. Symbol for an Exclusive NOR Gate

Output

Input Output
A B

L L H
L H L
H L L
H H H

Combining Gates

Logic gates can be combined in various ways. Two
input gates can be combined to make three (or
more) input gates. Figure 10-9 shows how three
two-input AND gates can be used to produce a
four-input AND gate.

113

CHAPTER 10

Figure 1 0-9. Three Two-Input AND Gates Form a Four-Input
AND Gate

Output

Its function can be verified by using the following
truth table:

Input

Intermediate

Output

A B

L L

L H

H L

H H

Logic gates can even be combined to form other
logic gates. A NAND gate or a NOR gate acts as
an inverter (NOT) if both the input lines are
joined together.

Figure 1 0-1 0. NAND Gate As a NOT Gate

Input

A
6

C
D

Output

114

Digital Logic

Figure 10-11. NOR Gate As a NOT Gate

Input

Output

Three two-input NAND gates can be hooked up
to form an OR gate.

Figure 10-12. Three Two-Input NAND Gates As OR Gate

Output

NAND and NOR gates are extremely useful
since they can be wired to operate as any type of
gate.

These gates can be combined in an unlimited
number of ways. Once you understand the basic
functions of the gates, putting together a digital
circuit to meet specific requirements is fairly easy.

Decoders and Encoders

Also referred to as demultiplexers and multiplexers,
decoders and encoders can be used by your com-
puter to send or receive digital signals on many
different lines using fewer pins of the control port.
These circuits can help you solve the problem of
controlling and sensing several different devices
using a limited number of I/O lines. Like all digi-
tal circuits, decoders and encoders are constructed
from the basic gates discussed earlier.

The decoder circuit in Figure 10-13 lets your
computer output digital signals on four lines using

115

CHAPTER 10

only two lines of the control port. The two control
port lines can output the binary numbers 00 (0
decimal) to 1 1 (3 decimal) in the form of high and
low signals. One of the four output lines of the
demultiplexer outputs a logic high signal depend-
ing on the input combination received from the
control port. The other three outputs remain logic
low. While the decoder allows you to turn on four
electronic switches (or whatever) using two con-
trol port lines, there is a catch — you can switch on
only one device at a time. In many applications
this tradeoff between control port lines and flex-
ibility will work to your advantage.

Try tracing the logic of the demultiplexer in Fig-
ure 10-13. You'll see that the inputs of the AND
gates are wired to the input lines and inverted in-
put lines from the control port in a way that al-
ways produces the expected output.

The number of outputs you can design in a de-
coder depends upon the number of control port
lines you have available. If you have X control
lines, you can make a demultiplexer with 2~X out-
puts. Of course, you would also need X inverters
(NOT gates) and TX AND gates to build it.

Multiplexers, or encoders, perform the opposite
function of decoders. They let you input digital
signals from several lines to a single control port
pin. As you can see from Figure 10-14, the cir-
cuitry for an encoder is similar to a decoder, with
some additions. Each output of the demultiplexer
section (the part on the left in Figure 10-14) is con-
nected to an AND gate. The other lead of the
AND gate is connected to one of the four lines
carrying information from the sensors to the
computer.

116

Figure 10-13. Decoder (Demultiplexer) Circuit

(from computer's control port)

Input A

Input B

Output signals
(to control port)

Output Z

Output Y

Output X

Output W

Truth Table

Input Output

A B Z Y X W

L L H L L L
L H L H L L

H L
H H

L L H L
L L L H

CHAPTER 10

The demultiplexer part of this circuit is used to
select which of the four sensor lines is effectively
connected to the control port pin. The line con-
nected to the AND gate whose other input is logic
high (from the demultiplexer part) has its signal
passed to the OR gate and through to the control
port. The signals from the lines connected to AND
gates whose other inputs (from the demultiplexer
section) are low are in essence blocked from the
inputs of the OR gate.

The only data line signal therefore "seen" by
the control port is the one selected by the demulti-
plexer circuit. A multiplexer with X control port
pins available can be designed to allow 2*(X — 1)
digital input lines to be connected to the com-
puter. The value X— 1 comes from the fact that
one pin must be used to input the actual data,
while the remaining pins are used to select the
data line.

These encoder and decoder circuits may seem
too large to set up on a solderless breadboard.
Using individual chips, they would be. Fortunately
entire multiplexers and demultiplexers are avail-
able on integrated circuit "chips." All you need to
do is properly connect their pins to the control
port. In fact, this procedure is described in Chap-
ter 12, "More Ideas."

Experimenting with Digital Logic
The best way to learn and understand how digital
circuitry works is to do some hands-on experi-
menting. All the different types of gates are avail-
able on integrated circuit (IC) chips. All you need
are a few chips, a solderless breadboard, a power
supply, some wire, and a logic probe to get
started.

118

Digital Logic

3
3

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Dh Ph Dh Ph

c c c c

nj nj nj nj

-*— ' -4— ' -4— » -4— »

'5b'5b'5b'3b
QQQQ

£ "3 PQ
3

Et=tJ

.£P§ §

CHAPTER 10

The integrated circuits you should use are TTL
or LS types. These chips usually contain one or
more gates. The 7400, for instance, is four two-
input NAND gates in one IC package. It's a TTL
(Transistor-Transistor Logic) IC. The 74LS00 is
identical to the 7400 IC, except it uses less power
(LS stands for Low-power Schotkey). In fact, most
TTL ICs have LS versions.

TTL IC names start with the digits 74. Low-
power Schotkey ICs follow this identifier with the
letters LS. The remaining part of the names are the
same for TTL and LS ICs which are functionally
identical.

The pin layouts for the integrated circuits used
in this book, as well as some additional basic ICs,
appear in Appendix B.

Simply connect the ground and Vcc pins of the
chip to a power supply. Be careful wiring the ICs
to the power. If you reverse the connections, you
could destroy the IC.

If you're using only a couple of chips, you can
tap your computer's control port for power. Pin 7
can supply +5 volts with up to 50 mA (milliam-
peres) of current. Pin 8 is attached to the ground
terminals of the computer's power supply.

If you're using several ICs, you'll need an exter-
nal 5 -volt power supply. When you use a separate
external power supply for a digital circuit and in-
terface it with your computer, remember to pro-
vide a common ground line by connecting the
external power supply ground terminal to pin 8 of
the control port.

120

a CHAPTER 1 1

" A Better Logic

Probe

A Better Logic
Probe

To experiment with digital logic circuits, you'll
need a logic probe better than the simple one you
built earlier. The logic probe illustrated here is ca-
pable of detecting both logic high and logic low
states in a circuit. (If you're really serious about
experimenting with digital circuits, you should
definitely consider buying a commercial logic
probe.)

To put together this more sophisticated logic
probe, you'll need these components:

Part

Quantity Part

Number

IK ohm resistors

271-8023

Green LED

276-022

Red LED

276-041

Alligator clips

270-378

74LS367 IC

None

Solderless breadboard

276-175

You'll also need some solid copper wire

for the

jumpers

and some stranded copper wire

for probe

leads.

Here's how to put together the "better" logic
probe:

1. Cut three pieces of stranded copper wire about
8 inches long and remove about 1 /4 inch of in-
sulation from each end. Attach an alligator clip
to one end of each of these wires.

2. Connect the other end of one wire to point Yl
of the solderless breadboard. This lead is the
ground wire.

123

CHAPTER 1 1

<N
(S

3. Insert one end of another wire, the +5 -volt
lead, to point XI of the breadboard.

4. The third wire is the probe lead and is inserted
into point 16 of the board.

5. Insert the 74LS367 IC into the breadboard so
that its pin 1 goes into point F6, and its pin 9
goes into point E13.

6. Connect three jumper wires as follows:

Jumper From To

1 G7 X7

2 J13 Y13

3 X5 A6

7. The IK ohm resistors bridge the following
points:

Resistor From To

1 H2 H6

2 H7 H14

8. The red LED is mounted on the breadboard so
that its cathode plugs into point Y2 and its an-
ode into point J2.

9. The cathode of the green LED inserts into point
Y14, while its anode plugs into point J14.

Figure 11-2. Better Logic Probe

• \. • • • •

It's easy to see the various parts which make up this better logic
probe.

125

CHAPTER 1 1

Probing

To use the logic probe, connect the ground wire
and +5 -volt wire to the power supply ground and
+ 5-volt terminals of the circuit you're checking.
Touch the probe lead to the point of the circuit
you want to check. If the green LED turns on, the
point is at a logic low level, but if the red LED
lights, the point is at a logic high level. (If neither
LED lights, the point is neither at a logic high nor
low state.)

When the probe is touched to a logic high point
of the circuit, current flows through the red LED,
causing it to light. The probe is also connected to
the enable input pin of the 74LS367 IC. When this
pin is high, all the buffer outputs in the chip enter
the high impedance state. Since the green LED is
connected to the output of a buffer, it remains off.
The green LED is effectively disconnected from
the circuit since one of its leads is connected to
the buffer output, which is acting like a very large
resistor (open circuit).

When the probe is touched to a logic low point
of the circuit, no current flows through the red
LED since both its leads are at the same voltage
(ground). The low signal, however, enables the
74LS367 IC so that its buffers act normally. Since
the buffer (whose output is connected to the green
LED) input is attached to + 5 volts, the green LED
lights up. This happens because the output of the
buffer is logic high, and as a result, a current
flows through the green LED causing it to
illuminate.

Note that due to low current levels, the LEDs
may be dim and difficult to see at times.

This circuit is very handy for exploring and
troubleshooting digital circuits. It's also a worthy
candidate for "permanent" mounting on a printed
circuit board, perhaps, as you'll probably use it
often.

126

CHAPTER 12

More Ideas

You've built the simple circuits in this book, tried
out the programs that accompanied them, and
you're ready for more. Now what?

Since you've done the work already let's take a
look at some more ways to put these circuits to
work. Once you start thinking about it, you'll
quickly find that there's a very wide range of pos-
sible applications for these circuits. And the circuits
can even be expanded to perform additional, more
complex functions. Here are just a few projects
you can create with the circuits you've built.

A Two-Beam Digital Timer Circuit

To make a two-beam digital timer, you'll need
two digital light sensors (see Chapter 7).

The two-beam timer begins timing when the
first light beam is broken and stops timing when
the second light beam is broken. A light sensor is
required to monitor each of these light beams. The
elapsed time between the two events (breaking
the paths of the two light beams) is displayed on
the computer's screen. This project is ideal for tim-
ing races. It may also be useful in classroom sci-
ence experiments where the speed of an object
must be measured by timing how long it takes for
something to move from point A to point B.

You've already built one sensor — the second
sensor can be put on the same solderless bread-
board as your original circuit. Both light sensors
operate in the same way, and the digital sensors
even share one major circuit component, the 3900
op amp IC. The 3900 IC has four operational am-
plifiers (op amps) built in. Each digital light sensor

129

CHAPTER 12

requires the use of one operational amplifier. The
second sensor just uses one of the three unused
op amps. In fact, you could build four digital light
sensors using a single 3900 IC.

Figure 12-2 is the schematic diagram of the two-
beam digital timer circuit. For a more reliable
switch, infrared sources are recommended.

Assuming you have the first digital light sensor
at hand (see Chapter 7), here's a list of additional
parts you'll need to build this circuit.

Part

Quantity Part

Number

500K potentiometer

271-210

(Commodore)

1M potentiometer

271-211

(Atari)

100K ohm resistor

271-8045

10K ohm resistor

271-8034

IK ohm resistor

271-8023

TIL 414 infrared photo-

276-130

transistor

Here's how to construct your two-beam digital
timer.

1. Take the solderless breadboard you used to cre-
ate the digital light sensor in Chapter 7. Make
sure all the components are still on the board
and in their proper places.

2. Solder a 4-inch length of copper wire to pin 2
of the 9-pin plug. This wire will provide the
computer with the digital signal from the sec-
ond sensor.

3. Insert the other end of this wire into point 16 of
the solderless breadboard.

4. Solder two wires to the middle and one of the
outside leads of the 500K potentiometer.

130

More Ideas

5. The other end of the middle wire goes to point
A2 of the breadboard.

6. The wire from the outside lead goes to location
X2 of the board.

7. Solder wires to the emitter and collector of the
TIL 414 infrared phototransistor. The wires
should be long enough so that the phototransis-
tor can be mounted properly. Insert the emitter
and collector leads into points J2 and Y2
respectively.

8. Bridge the three additional resistors between the
following sets of points:

Resistor From To

Rl (10K ohm) G2 G9

R3 (100K ohm) H9 H10

R4 (IK ohm) 16 110

9. Make the following jumper wire connections (in
addition to the jumpers which already exist on
the digital light sensor solderless breadboard
which you built in Chapter 7) to complete the
hardware for the two-light-beam timer:

Jumper From To

1 G14 X19

2 F2 E2

Figure 1 2-1 . Two-Beam Timer, the Final Look

This mass of wires and electronic components is actually a two-
beam timer.

131

CHAPTER 12

More Ideas

CHAPTER 12

Using the Two-Beam Timer

Plug the 9-pin connector into control port 2 for
the Commodore 64/128, or into control port 1 for
the Atari computers. To use the timer, set the sen-

sors so that the light detect is interrupted when
the object being timed passes them. Each sensor
must be calibrated using its own potentiometer.

Program 12-1 monitors the two digital inputs to
the computer from the sensors. When the first
sensor is triggered by the passing object, it starts
timing. When the object blocks the light to the
second sensor, the program stops timing and dis-
plays the results.

Program 1 2-1 —Commodore 64/128

JF 10 FOR I=0TO1

KF 20 PRINT "TURN ON FLASHLIGHT AND ADJUS
T THE "

XP 30 PRINT "POTENTIOMETER ";I+1;" TILL T

HE MESSAGE JUST"
DR 40 PRINT "CHANGES FROM OFF TO ON"
CR 50 PRINT: PRINT "PRESS X TO CONTINUE"
PM 60 A$=CHR$(145)
HM 70 A=PEEK( 56320)

DX 80 IF (A AND 2TD THEN PRINT "OFF"

BE 90 IF (A AND 2tl)=0 THEN PRINT "ON "

JS 100 GET B$

HE 110 IF B$="X"THEN140

JA 120 PRINTA$AS

CR 130 GOTO70

AS 140 NEXT I

BB 150 PRINT "BREAK BEAM 1 TO START TIMER

GM 160 REM CHECK DATA REGISTER TILL FIRS

T SENSOR TRIGGERED
MD 170 IF (PEEK(56320)AND(2T0) )=0 THEN17

BH 180 REM START TIMER
JD 190 TI$="000000"

AG 200 REM CHECK DATA REGISTER TILL SECO

ND SENSOR TRIGGERED
EM 210 IF (PEEK(56320) AND(2U))=0 THEN2

KS 220 PRINT "TIME IS "; TI / 60 ;" SECONDS "

134

More Ideas

Program 1 2-1 — VIC-20

SH 10 FOR I=2T03

KF 20 PRINT "TURN ON FLASHLIGHT AND ADJUS
T THE"

XP 30 PRINT "POTENTIOMETER ";I+1;" TILL T

HE MESSAGE JUST"
DR 40 PRINT "CHANGES FROM OFF TO ON"
CR 50 PRINT: PRINT "PRESS X TO CONTINUE"
PM 60 A$=CHR$(145)
GB 70 A=PEEK( 37137 )

DX 80 IF (A AND 21 I) THEN PRINT "OFF"

BE 90 IF (A AND 2?I)=0 THEN PRINT "ON "

JS 100 GET B$

HE 110 IF B$="X"THEN140

JA 120 PRINTA$A$

CR 130 GOTO70

AS 140 NEXT I

BB 150 PRINT "BREAK BEAM 1 TO START TIMER

GM 160 REM CHECK DATA REGISTER TILL FIRS

T SENSOR TRIGGERED
BM 170 IF (PEEK(37137)AND(2T2) ) =0 THEN17

BH 180 REM START TIMER
JD 190 TI$="000000"

AG 200 REM CHECK DATA REGISTER TILL SECO

ND SENSOR TRIGGERED
QC 210 IF (PEEK(37137) AND(2?3))=0 THEN2

KS 220 PRINT "TIME IS "; Tl/60 ; "SECONDS "

Program 12-1 — Atari

FL110 REM TWO LIGHT SENSOR TIMER PR
OGRAM

FJ112 REM CALIBRATE BOTH SENSORS
SI 115 FOR 1=0 TO 1

KP 120 PRINT "TURN ON FLASHLIGHT AND

ADJUST THE"
BE 130 PRINT "POTENTIOMETER ";I + 1;"

TILL THE MESSAGE JUST "
KQ 140 PRINT "CHANGES FROM OFF TO ON

BL 150 PRINT : PRINT " PRESS X TO CONT
INUE"

6B 155 FOR J=l TO 1000.-NEXT J

BE 160 A = STICK (0)

KK 162 IF 1 = 1 THEN 174

135

CHAPTER 12

PRINT
PRINT
PRINT
PRINT
X KEY
= 22 THEN 220

"OFF
"OFF
"ON
"ON

"OFF 1
"OFF '
"ON
"ON

PRESSED

OJ 164 IF A= 1 5 THEN PRINT
OJ 166 IF A=13 THEN PRINT
KO 168 IF A=14 THEN PRINT
KF170 IF A=12 THEN PRINT
6N 172 GOTO 190
OK 174 IF A=15 THEN
OL 176 IF A=14 THEN
KO 178 IF A=13 THEN
KG 180 IF A=12 THEN
MC 190 REM CHECK IF
JN 200 IF PEEK<764>=
GD210 SOTO 160
NE 220 REM CLEAR LAST
CN 230 POKE 764,255
BO 240 NEXT I

NF 250 PRINT "BREAK BEAM
TIMER"

AO 260 REM CHECK FIRST SENSOR

RIG6ERED
HN 270 IF STICK(0X>13 THEN 270
IN 280 REM START TIMER

OF 290 POKE 18,0:POKE 19,0:POKE 20,0
JE 300 REM CHECK SECOND SENSOR UNTIL

TRIGBERED
II310 IF STICK(0)=14 THEN 320
S6315 GOTO 310

BC 320 PRINT "TIME IS " ; < < PEEK ( 1 8 ) *2
55*255)+(PEEK<19)*255)+<PEEK<
20) ) ) /60; " SECONDS"

KEYSTROKE

1 TO START

TILL T

Light Switch

This project combines the analog light sensor from
Chapter 5 with the electronic switch you built in
Chapter 8. The analog light sensor detects the
level of outside light. As night approaches, the
computer detects the change in light conditions
and turns on a light via the electronic switch.
Near dawn, the sensor detects the increase in
light, and the computer switches off the light. (Of
course, other things, from dark clouds during the
day to car headlights at night, may also activate
the switch.)

136

More Ideas

The analog light sensor, a cadmium sulfide photo-
cell, is connected between pins 9 and 5 of the
electronic switch's 9-pin plug. Use leads long
enough so that the sensor can be aimed easily.
The electric light is connected to the relay of the
electronic switch across its normally closed and
common terminals (remember, the relay turns on
as soon as you plug the circuit into the computer's
port). Be sure that the ratings of the solderless
breadboard or relay are not exceeded by the elec-
tric light. The circuit is connected to control port 1
of your computer.

Program 12-2— Commodore 64/128

GX 10 PRINT "SET LIGHTING LEVEL TO SWITC
H ON"

EG 20 PRINT "PRESS X TO CONTINUE"

RA 30 A=PEEK( 3687 2 )

HR 40 GET B$

RR 50 IF BS="X" THEN70

XE 60 GOTO30

QG 70 REM

HP 80 PRINT "SET LIGHTING LEVEL TO SWITC
H OFF "

PD 90 PRINT "PRESS X TO CONTINUE"

AX 100 B=PEEK(36872)

ES 110 GET B$

DF 120 IF B$="X" THEN140

EJ 130 GOTO100

MJ 140 PRINT "PRESS X TO START"

FB 150 GET B$

HQ 160 IF B$="X" THEN180

PS 170 GOTO150

PR 180 REM CHECK IF LIGHT LEVEL LOWER TH

AN FIRST SETTING
FK 190 IF PEEK( 36872) <A THEN190
SP 200 REM SET DATA DIRECTION REGISTER F

OR OUTPUT

XG 210 POKE 37139, PEEK ( 37139 ) OR( 2T 2 )
CC 220 REM SET OUTPUT LINE LOGIC LOW
CG 230 POKE 37137, PEEK( 37137 )ANDNOT ( 2T2 )
HQ 240 REM CHECK IF LIGHTING LEVEL HIGHE

R THAN SECOND SETTING
FP 250 IF PEEK( 36872) >B THEN250
HQ 260 REM SET OUTPUT LINE LOGIC HIGH

137

CHAPTER 12

RG 270 POKE 37137, PEEK ( 371 37 ) OR( 2T2 )
DG 280 REM RESET DATA DIRECTION REGISTER
FOR INPUT

FB 290 POKE 37139, PEEK( 37139 )ANDNOT ( 2T2 )

Program 12-2— VIC

GX 10 PRINT "SET LIGHTING LEVEL TO SWITC
H ON"

EG 20 PRINT "PRESS X TO CONTINUE"

HD 30 A=PEEK( 54297 )

HR 40 GET B$

RR 50 IF B$="X" THEN70

XE 60 GOTO30

QG 70 REM

HP 80 PRINT "SET LIGHTING LEVEL TO SWITC
H OFF "

PD 90 PRINT "PRESS X TO CONTINUE"

MB 100 B=PEEK( 54297)

ES 110 GET B$

DF 120 IF B$="X" THEN140

EJ 130 GOTO100

MJ 140 PRINT "PRESS X TO START "

FB 150 GET B$

HQ 160 IF B$="X" THEN180

PS 170 GOTO150

PR 180 REM CHECK IF LIGHT LEVEL LOWER TH

AN FIRST SETTING
CM 190 IF PEEK( 54297 ) <A THEN190
SP 200 REM SET DATA DIRECTION REGISTER F

OR OUTPUT
ED 210 POKE 56323, PEEK( 5632 3 ) OR ( 2 T0 )
CC 220 REM SET OUTPUT LINE LOGIC LOW
CJ 230 POKE 56321, PEEK ( 56321 ) ANDNOT ( 2 ?0 )
HQ 240 REM CHECK IF LIGHTING LEVEL HIGHE

R THAN SECOND SETTING
BE 250 IF PEEK( 54297 ) >B THEN250
HQ 260 REM SET OUTPUT LINE LOGIC HIGH
GD 270 POKE 56321 ,PEEK( 56321 )OR( 2T0)
DG 280 REM RESET DATA DIRECTION REGISTER
FOR INPUT

ED 290 POKE 56323, PEEK ( 563 2 3 ) ANDNOT ( 2 T )

138

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Program 12-2 — Atari

LP 100 REM LIQHT SWITCH
HS 1 1 REM

FC 120 PRINT "SET LIGHT LEVEL TO SWI
TCH ON"

FC 130 PRINT "PRESS X TO CONTINUE"
DO 140 A=PADDLE (0)

LO 150 REM CHECK IF X KEY PRESSED
KG 160 IF PEEK (764) =22 THEN 180
66 170 GOTO 140

NN 175 REM CLEAR LAST KEYSTROKE
DB 180 POKE 764, 255

HF 190 PRINT "SET LIGHTING LEVEL TO

SWITCH OFF"
FA 200 PRINT "PRESS X TO CONTINUE"
DN210 B = P ADDLE ( )

Lfl 220 REM CHECK IF X KEY PRESSED
KD 230 IF PEEK(764)=22 THEN 260
6C 240 GOTO 210

NH 250 REM CLEAR LAST KEYSTROKE
DA 260 POKE 764,255

EI 262 PRINT :PRINT "PRESS X TO STAR
T"

LC 264 IF PEEK(764)=22 THEN 268

HD 266 GOTO 264

01 268 POKE 764, 255

LE270 REM CHECK IF LIGHT LEVEL LOWE

R THAN FIRST SETTING
JK 280 IF PADDLE ( ) < A THEN 280
SD290 REM SET DATA DIRECTION REG 1ST

ER FOR OUTPUT
FM 300 POKE 54018,48
IL310 POKE 54016,255
FJ 320 POKE 54018,52

FL 330 REM SET DATA LINES LOGIC LOW
CC 340 POKE 54016,0

PD 350 REM CHECK IF LIGHTING LEVEL H

IGHER THAN SECOND SETTING
JN 370 IF PADDLE (0)>B THEN 370
10 380 REM SET DATA LINES LOGIC HIGH
JD 390 POKE 54016,255

AH 400 REM RESET DATA LINES FOR INPU
T

FO410 POKE 54018,48
CB 420 POKE 54016,0
FL 430 POKE 54018,52

139

CHAPTER 12

Enter and run the appropriate version of Program
12-2. You'll be prompted to set the lighting condi-
tion which will indicate that the switch is to turn
on. Do this by exposing the analog light sensor to
the amount of light at which you want the light to
turn itself on. While the sensor is still exposed to
this light level, press the X key on the computer.
The computer stores this level of light as a num-
ber and will turn on the electric light when the
sensor sees less than this amount of light.

The program also prompts you to set the light
off condition. After the electric light has been
turned on, when this second light level (or more
light) is present, the electric light is turned off.

Using this circuit requires that the sensor be
mounted near a window where it can detect out-
side light.

Whatever you do, don't mount the sensor be-
neath the electric light you're controlling.

Multiplexer Circuit

A multiplexer circuit lets your computer monitor
many digital input signals using its limited num-
ber of I/O lines. To make an eight-input multi-
plexer, you'll have to have these components:

Part

Quantity Part

Number

2 9-pin D-subminiature female

276-1538

(only one necessary for VIC

and Atari)

1 74LS151 IC

None

1 IK ohm resistor 271-8023

(not required for Atari)

1 Solderless breadboard

276-175

140

More Ideas

CHAPTER 12

Follow these steps to put together your
multiplexer:

1. Insert the 74LS151 IC so that its pin 1 goes into
point F2 of the solderless breadboard, and its
pin 9 into point E9.

2. Solder a 4-inch-long wire to pin 1 of one of the
9-pin plugs (for the Atari and VIC-20 versions,
solder this wire to pin 6 of the single 9-pin plug
required). This wire carries the selected data
line's signal to the computer.

3. Connect the other end of this wire to point J12
of the breadboard.

4. The IK ohm resistor bridges the points HI 2 to
H6 of the board. (When building the Atari ver-
sion, use a jumper wire in place of the resistor.)

5. Solder wires about four inches long to pins 1, 2,
3, 7, and 8 of the second 9-pin plug (in the case
of the Atari and VIC versions, solder these
wires to the same pins of the single 9-pin plug).

6. The wires from pins 7 and 8 provide the power
to the circuit. Connect the wire from pin 7 to
point XI, and the wire from pin 8 to location Yl
of the board.

7. The wires from pins 1, 2, and 3 insert into plug
points D7, D8, and D9 respectively. These last
three pins are used to select the data line signal
"seen" by the computer.

8. Connect jumper wires as follows:

Jumper From To

1 Y9 J9

2 X2 A2

3 G8 G9

Pins 1-4 and 12-15 of the 74LS151 IC are the
input lines of the multiplexer. They can be con-
nected to any sensor which provides an on/off
signal. The sensors may be electronic (like the dig-
ital light sensor) or simply switches between the
pins and ground.

142

More Ideas

Figure 1 2-5. Multiplexer Still Life

• • » » - * >>•'*'•'<'' '

, , « » ■ ■ S - . .

A congregation of wires from the multiplexer lead off to various
digital sensors.

If you've built a multiplexer for the Commodore
64 or Commodore 128, the 9-pin plug with one
wire attached is inserted into control port 2. The
other 9-pin connector plugs into control port 1.

Since you used only one 9-pin plug for the
Atari version, insert it into control port 2.

Program 12-3 allows the circuit to monitor eight
digital inputs. It outputs three digital signals to the
multiplexer circuit, which tell it to route one of the
input signals to a pin of the computer's I/O port.
The program sequentially selects each of the eight
input lines and checks its condition. (This is called
polling.) If any of the eight inputs is at a logic low
state, the computer prints out such a message on
the screen.

Program 1 2-3 — Commodore 64/1 28

PK 10 PRINT "PRESS X TO EXIT PROGRAM "
GM 20 REM SET DATA DIRECTION REGISTER FO

R OUTPUT
QJ 30 POKE 56322,255
RP 40 FOR 1=0 TO 7

143

CHAPTER 12

PE 50 REM SET DATA OUTPUT
SR 60 POKE 56320,1

RG 70 IF (PEEK(56321)AND(2T0) )=0 THEN PR

INT " SENSOR ";I
RK 80 NEXT I

EP 90 REM RESET DATA LINES FOR INPUT

RQ 100 POKE 56322,0

ES 110 GET B$

BX 120 IF B$="X" THEN END

CP 130 GOTO30

Program 12-3— VIC-20

CM 10 PRINT " MONITORING SENSORS"

HR 20 PRINT "PRESS X TO EXIT"

DP 30 REM SET DATA DIRECTION REGISTER FO

R OUTPUT
QH 40 POKE 37139, ( 2 T 2+2 T 3 + 2 T 4 )
GM 50 FOR 1=0 TO 7
SP 60 REM SET DATA OUTPUT
GH 70 POKE 37137,1*4

QC 80 IF (PEEK( 37137 )AND( 2T5) )=0 THEN PR

INT " SENSOR ";I
JM 90 NEXT I
GJ 100 POKE 37139,128
ES 110 GET B$
BX 120 IF B$="X" THEN END
MQ 130 GOTO40

GM 140 PRINT "TURN SWITCH OFF"

Program 12-3 — Atari

ME 100 REM MULTIPLE INPUTS

JG110 PRINT "MONITORINB SENSORS "

ON 120 PRINT :PRINT " PRESS X TO EXIT

FN 130 REM SET DATA DIRECTION RESIST

ER FOR OUTPUT
FO 140 POKE 54018, 48
CI 150 POKE 54016, 7
FL 160 POKE 54018, 52

DB 170 REM CYCLE THROUGH OUTPUT COMB

I NAT I ON
Bfl 180 FOR 1=0 TO 7
DO 190 POKE 54016, I
AF 195 FOR J=l TO 10:NEXT J
EH 200 IF (STRie(0)=0> THEN PRINT " S

ENSOR " ; I

144

More Ideas

LL210 REM CHECK IF X KEY PRESSED

KB 220 IF PEEK (764) =22 THEN 250

BN 230 NEXT I

BH 235 GOTO 180

NG 240 REM CLEAR LAST KEYSTROKE

CP 250 POKE 764,255

AL 260 REM RESET DATA LINES FOR INPU
T

GC 270 POKE 54018,48

CF 280 POKE 54016,0

FP 290 POKE 54018,52

A Demultiplexer Circuit
A demultiplexing circuit can be made using a
74LS154 IC. This chip takes four input signals
from the computer to output a signal to one of 16
lines. The output line corresponding to the input
signal combination goes logic low, while the other
output lines of this IC remain high.

Quantity Part

1 9-pin D-subminiature female

1 74LS154 IC

1 Solderless breadboard

Part

Number

276-1538

None

276-175

You'll also need some solid copper wire for
jumpers and some stranded copper wire for other
connections.

1. Insert the 74LS154 IC so that its pin 1 goes into
point G6 of the solderless breadboard, and its
pin 24 into point C6.

2. Solder wires about four inches long to pins 1, 2,
3, and 4 of the 9-pin plug.

145

CHAPTER 12

These wires connect to the breadboard at these
locations:

Wire

from Pin To Point

1 A7

2 A8

3 A9

4 A10

3. Solder two wires to pins 7 and 8 of the 9-pin
plug. Connect the wire from pin 7 to point XI,
and the wire from pin 8 to location Yl of the
board.

4. Solder wires for the digital output lines to these
points on the breadboard:

Digital

Output Line To Point

1 J7

2 J8

3 J9

4 J10

5 Jll

6 J12

7 J13

Digital

Output Line To Point

8 J14

9 J15

10 J16

11 A17

12 A16

13 A15

14 A14

15 A13

5. Connect jumper wires as follows:

Jumper From To

1 Y17 J17

2 X6 A6

3 A12 Y20

4 Bll B12

146

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CHAPTER 12

When you've finished, plug the 9-pin plug into
control port 2.

The 74LS154 is interfaced to the computer using
four lines from the control port. These four lines
output the signals that select which of the output
lines goes logic low. The outputs from the de-
multiplexer can be used to turn on electronic
switches or other devices which require digital sig-
nals. The power for the demultiplexer circuit, as
for the multiplexer, is obtained from pins 7 and 8
of the computer's control port.

Sending signals to set an output line of the
demultiplexer is very similar to sending the output
required from the computer to select an input line
of the multiplexer. The only difference is that four
output lines from the computer are used instead of
three. Try writing a program which will sequen-
tially set each of the demultiplexer's output lines
to a logic low state. You can verify your program
using your logic probe.

148

CHAPTER 13

Robotics

One of the most exciting applications of digital
electronics and computers is robotics. While a ro-
botics control system is beyond the scope of this
book, what you'll find here are some ideas to get
you started.

The actions of most robots are carried out by
motors. To move, for instance, a mobile robot's
wheels are turned; when a robot's arm goes up,
the arm's joints are moved by motors. These mo-
tors, in turn, are controlled by a computer. The
computer is programmed to turn the motors on
and off so that the robot moves in a coordinated
fashion to perform a task.

You already have enough expertise to control a
dc motor with your computer. You can simply
switch it on and off by using an electronic switch
circuit. If you use a second electronic switch cir-
cuit, with a DPDT relay in place of the SPDT, you
can control a dc motor to turn in both directions.

Though you've already built an electronic
switch circuit in Chapter 8, it will be easier if you
just begin from scratch when putting together a dc
motor control circuit. Here's what you'll need:

Part

Quantity Part

Number

9-pin D-subminiature female

276-1538

2N2222 NPN transistors

276-1617

2.2K ohm resistors

271-8027

IN914 diodes

276-1620

5 -volt SPDT DIP relay

275-243

DPDT DIP relay 5 -volt coil

275-215

Solderless breadboard

276-175

151

CHAPTER 13

c
o

o
E

E
o
O

b
o
n
~o
o
o

o
O

* *, 60
n c S 3
H 3 D- &.

Robotics

Part

Quantity Part Number

1 7400 IC (Atari) 276-1801
1 DC battery-powered motor None

You'll also need some solid copper wire for con-
nections to the 9-pin plug, motor, and battery, as
well as for jumpers on the solderless breadboard.

If you're building the Atari version of the con-
trol circuit, you'll need a longer than normal
solderless breadboard.

The Commodore Version

Here's how to build the Commodore version of
the dc motor control circuit.

1. Wire the 9-pin plug. Cut four pieces of wire,
each about four inches long, and remove
about 1/4 inch of the insulation from the
wires' ends.

2. Solder wires to pins 1, 2, 7, and 8 of the 9-pin
plug.

3. Connect the four wires as follows: The wire
from pin 8 connects to point Yl on the solder-
less breadboard. The wire from pin 7 connects
to point XI of the solderless breadboard. The
last two lines, from pins 1 and 2, connect to
locations A3 and A6 respectively.

4. Connect the two 2.2K ohm resistors between
points C3 and F3, and points C6 and F6 on
the solderless breadboard.

5. Mount one 2N2222 transistor so that its base
inserts into point H3, its emitter into point H2,
and its collector into point H3.

6. The second 2N2222 transistor should be
mounted so that its base inserts into location
H6, its emitter into point H7, and its collector
into H5.

153

CHAPTER 13

7. The first diode is connected so that its cathode
lead inserts into point X4 of the solderless
breadboard and its anode lead connects to
point A4. (A band around one end of the di-
ode is generally used to identify the cathode
lead.)

8. The second diode should be placed so that its
cathode lead goes into location X7, while its
anode lead connects to point A7.

9. The SPDT relay is inserted so that its normally
open and normally closed pins attach to points
F9 and E9 respectively. The common pin con-
nects to point E14. Its coil terminal should be
connected to points E10 and F10.

10. Insert the DPDT relay so that its normally
open pins 1 and 2 attach to points E23 and
F23 respectively. Its normally closed pins 1
and 2 should be connected to points E21 and
F21 respectively. The two common pins, 1 and
2, are attached to locations E19 and F19. Fi-
nally, the coil terminal is connected to points
E16 and F16.

11. The following jumper connections are required:

Wire

From

X10

A10

X16

A16

A21

J23

A23

J21

H19

110

J16

12. Solder two wires to the battery you'll use, one
wire to the positive pole, the other to the neg-
ative pole.

154

Robotics

13. Connect the positive pole wire to point 123 of
the solderless breadboard. Connect the nega-
tive pole wire to point B23.

14. Solder two wires to the dc motor, one to its
positive pole, the other to its negative pole.

15. Connect the positive pole wire to point A19 of
the breadboard and the negative pole wire to
location A14.

Figure 1 3-2. The Finished Commodore Board

This solderless breadboard contains the most complex circuit
project in this book.

Changes When Constructing the Atari Version
Refer to Figure 13-3 for the layout of the Atari
version.

Substitute the following steps in the Atari version:

3. The wires from pins 1 and 2 connect to points
J25 and A26 respectively.

155

CHAPTER 13

Robotics

11. These are the jumper connections for the Atari
version:

147' -

Wire

From

vi n
X10

A10

Xl6

A16

fc.7

prj

G30

B31

D26

C27

D28

D29

C29

D30

H25

G26

G27

G28

H28

G29

A21

J23

A23

J21

J31

Y31

H19

110

J16

13. Connect the positive pole wire from the bat-
tery to point 123 of the solderless breadboard.
Connect the negative pole wire to point 121.

16. Insert the 7400 IC into the solderless bread-
board so that its pins 1 and 8 go into plug
points F25 and E31 respectively.

Warning: If you use a larger motor, remember to
mind the ratings of the solderless breadboard and
its relays.

The circuit uses two signals from the com-
puter — one to switch on the motor and the other
to reverse the battery polarity to the motor. Re-
versing the battery polarity to a dc motor causes
its shaft to rotate in the opposite direction.

The motor can be used to turn the wheels of a
small mobile "turtle" robot. If you use a dc gear
motor with a reasonably low rpm speed and
enough torque, you can even control a joint of a

157

CHAPTER 13

robot, such as the elbow or shoulder of a mechan-
ical arm. You can use several of these motor con-
trol circuits to operate a multijoint robot. Consider
using a demultiplexer to send the digital signals to
different motor control circuits.

Some robots simply turn on their motors for a
certain amount of time, assuming that the parts
manipulated by the motors reach particular points.
But if, for example, something obstructs their
movements, these robots continue working, oblivi-
ous to their situations. More intelligent robots use
sensors to inform them about their environments.
If you build a mobile robot, you can use micro-
switches to check whether your robot has bumped
into a wall. If it requires several such sensors, you
may have to use a multiplexing circuit. You can
also mount potentiometers to turn with the joints
of a mechanical-arm robot. The values returned by
the A/D converter of your computer allow you to
keep track of the arms' positions.

Once you build the motor control circuit, you
can control it from your computer in much the
same way as the following demonstration program
does. Plug the circuit into control port 1 of your
computer. (The Commodore 64/128 version of the
program requires that you insert a joystick into
control port 2.)

Program 1 3-1 — Commodore 64/1 28

AK 100 REM MOTOR CONTROL DEMONSTRATION
SS 110 REM REQUIRES A JOYSTICK
HC 115 PRINT "JOYSTICK CONTROLS " : PRINT
SA 120 PRINT "UP - MOTOR TURNS IN ONE DIR
ECTION "

CH 130 PRINT "DOWN - MOTOR TURNS IN OPPOS

ITE DIRECTION"
CE 132 PRINT : PRINT "PRESS FIRE BUTTON TO

I SPACE] EX IT DEMO"
DB 135 B=0
HK 140 A=PEEK( 56320)

CG 150 IF (A AND 2T0)=0 THEN GOSUB 1000:
GOTO 140

158

Robotics

EP 160 IF (A AND 2TD=0 THEN GOSUB 2000:

GOTO 140
CQ 170 GOSUB 3000

PM 180 IF (A AND 2T4)=0 THEN END
JX 190 GOTO 140

CB 1000 REM TURN MOTOR ON IN ONE DI RECTI
ON

CM 1010 REM CHECK IF THIS HAS ALREADY BE

EN DONE
FK 1020 IF B=l THEN 1100

MA 1030 REM SET FIRST DATA LINE FOR OUTP
UT

SM 1040 POKE 56323, PEEK ( 56323 ) OR ( 2 T0 )
GD 1050 REM SET FIRST DATA LINE LOGIC LO

W TO TURN ON MOTOR
EG 1060 POKE 56321, PEEK( 56321 )ANDNOT(2T0

)

KD 1070 B=l
RF 1100 RETURN

JJ 2000 REM TURN ON MOTOR IN OTHER DIREC
TION

KJ 2010 REM CHECK IF THIS HAS ALREADY BE

EN DONE
KK 2020 IF B=2 THEN 2100

CR 2030 REM SET BOTH DATA LINES FOR OUTP
UT

QX 2040 POKE 56323, PEEK( 56323 )OR( 2T0+2T1
)

CM 2050 REM SET BOTH DATA LINES LOGIC LO
W TO TURN ON MOTOR AND REVERSE P
OLARITY

MQ 2060 POKE 56321, PEEK ( 563 21 ) ANDNOT ( 2 T0

+ 2U)
RC 2070 B=2
MC 2100 RETURN
MC 3000 REM TURN MOTOR OFF

EC 3010 REM CHECK AND SEE IF THIS HAS BE

EN DONE
JP 3020 IF B=0 THEN 3100

KM 3030 REM TURN BOTH DATA LINES LOGIC H
IGH

BM 3040 POKE 56321, PEEK(56321)OR(2T0+2Tl
)

SG 3050 REM RESET BOTH DATA LINES FOR IN
PUT

PX 3060 POKE 56323, PEEK( 56323 ) ANDNOT ( 210

+ 2TD
XS 3070 B=0

HA 3100 RETURN

159

CHAPTER 13

Program 13-1— VIC-20

XS 10 PRINT "KEYBOARD CONTROLS ": PRINT
SB 20 PRINT "< - MOTOR TURNS IN ONE DIREC
TION"

KR 30 PRINT "> - MOTOR TURNS IN OPPOSITE

[SPACE } DIRECTION"
DE 40 PRINT: PRINT "PRESS SPACE BAR TO EXI

T DEMO"
CH 50 B=0
EH 60 A=PEEK(197)

MS 70 IF A=29 THEN GOSUB1 20 : GOTO60
PS 80 IF A=37 THEN GOSUB210 :GOTO60
MK 90 GOSUB300

XD 100 IF A=32 THEN POKE 198,0:END
CP 110 GOTO60

FJ 120 REM TURN MOTOR ON IN ONE DIRECTIO
N

PP 130 REM CHECK IF THIS HAS ALREADY BEE

N DONE
SE 140 IF B=l THEN200

XK 150 REM SET FIRST DATA LINE FOR OUTPU
T

FJ 160 POKE 37139, PEEK ( 37 1 39 ) OR ( 2 T 2 )

CS 170 REM SET FIRST DATA LINE LOGIC LOW

TO TURN ON MOTOR
RF 180 POKE 37137, PEEK(37137)ANDNOT(2T2)
FF 190 B=l
MC 200 RETURN

JQ 210 REM TURN ON MOTOR IN OTHER DIRECT
ION

QR 220 REM CHECK IF THIS HAS ALREADY BEE

N DONE
QC 230 IF B=2 THEN290

PR 240 REM SET BOTH DATA LINES FOR OUTPU
T

PR 250 POKE 37139, PEEK ( 37 1 39 ) OR ( 2 T 2+ 2 T 3 )
PX 260 REM SET BOTH DATA LINES LOGIC LOW

TO TURN ON MOTOR AND REVERSE POL

ARITY

AG 270 POKE 3 7137 , PEEK ( 37137 ) ANDNOT ( 2 T 2+

2T3)
MM 280 B=2
SK 290 RETURN
RP 300 REM TURN MOTOR OFF

CR 310 REM CHECK AND SEE IF THIS HAS BEE

N DONE
EG 320 IF B=0 THEN380

BM 330 REM TURN BOTH DATA LINES LOGIC HI
GH

160

Robotics

QQ 340 POKE 37137 f PEEK(37137)OR(2T2+2T3)
FH 3 50 REM RESET BOTH DATA LINES FOR INP
UT

CQ 360 POKE 37139,PEEK(37139)ANDNOT(2T2+

2T3)
KX 370 B=0
HS 380 RETURN

Program 13-1 — Atari

AO 100 REM MOTOR CONTROL DEMONSTRAT I
ON

NF110 PRINT "KEYBOARD CONTROL"
Cft 120 PRINT

BF130 PRINT "< - MOTOR TURNS IN ONE

DIRECTION"
KJ 140 PRINT "> - MOTOR TURNS IN OPP

OSITE DIRECTION"
LO 150 PRINT "S - STOP MOTOR "
AK 155 PRINT :PRINT "X - EXIT PROBRA

EB 160 B =

CN 170 A = PEEK (764)

IK 180 IF A = 54 THEN GOSUB 1000:BOTO
170

IN 190 IF A = 55 THEN BOSUB 2000:BOTO
170

NF 200 GOSUB 3000

AK210 IF A = 22 THEN END

6F 220 SOTO 170

06 1000 REM TURN MOTOR ON IN ONE DIR
ECTION

OJ 1010 REM CHECK IF THIS HAS ALREAD

Y BEEN DONE

PD 1020 IF B=l THEN 1100

AB 1030 REM SET DATA LINES FOR OUTPU
T

IP 1032 POKE 54018, 48
FG 1034 POKE 54016, 3
10 1036 POKE 54018,52

NJ 1040 REM SET FIRST DATA LINE LOGI

C LOW TO TURN ON MOTOR
FD 1050 POKE 54016, 2
HH 1060 B=l
KC 1 100 RETURN

NH2000 REM TURN ON MOTOR IN OTHER D
IRECTION

OK 2010 REM CHECK IF THIS HAS ALREAD

Y BEEN DONE

161

CHAPTER 13

PS 2020 IF B = 2 THEN 2100

CP 2030 REM SET BOTH DATA LINES FOR

OUTPUT
JA 2032 POKE 54018,48
FH 2034 POKE 54016,3
IP 2036 POKE 54018,52

DF 2040 REM SET BOTH DATA LINES L06I
C LOW TO TURN ON MOTOR AND R
E VERSE POLARITY

FC 2050 POKE 54016,0

HJ 2060 B = 2

KD2100 RETURN

FN 3000 REM TURN MOTOR OFF

JJ3010 REM CHECK AND SEE IF THIS HA

S BEEN DONE
PS 3020 IF B = THEN 3100
JD 3030 REM SET BOTH DATA LINE LOGIC
HIGH

FF 3040 POKE 54016,3

DL 3050 REM RESET DATA LINES FOR INP
UT

JC 3060 POKE 54018,48
FF 3070 POKE 54016,0
JA 3090 POKE 54018,52
IA 3095 B =
KE3100 RETURN

162

Appendices

COMPUTE !'s Guide to

Typir iy ill r i uyivji i lo

Computers are precise — type each program in this
book exactly as it's listed, including the necessary
punctuation and symbols, except for special char-
acters noted below. We've provided a special list-
ing convention as well as a program to check your
typing — "The Automatic Proofreader."

Programs for Commodore and Atari 800/XL/XE
computers may contain some hard-to-read special
characters, so we have a listing system which indi-
cates these control characters. You'll find these
Commodore and Atari characters in curly braces;
do not type the braces. For example, {CLEAR} or
{CLR} instructs you to insert the symbol which
clears the screen on the Atari or Commodore ma-
chine. A complete list of these symbols is found in
the table below. For Commodore and Atari com-
puters, a single symbol by itself within curly
braces is usually a control key or graphics key. If
you see {A}, hold down the CONTROL key and
press A. This will produce a reverse video charac-
ter on the Commodore (in quote mode) or a
graphics character on the Atari.

Graphics characters entered with the Commo-
dore logo key are enclosed in a special bracket:
f<A>]. In this case, you would hold down the
Commodore logo key as you type A. Our Com-
modore listings are in uppercase, so shifted sym-
bols are underlined. A graphics heart symbol
(SHIFT-S) would be listed as S. One exception is
{SHIFT-SPACE}. When you see this, hold down
SHIFT and press the space bar. If a number pre-
cedes a symbol, such as {5 RIGHT}, {6 S}, or
f<8 Q>], you would enter five cursor rights, six
shifted S's, or eight Commodore-Q's. On the

165

APPENDIX A

When You
Read:

{CLR}

{HOME}

{UP}

{DOWN}

{LEFT}

{RIGHT}

{RVS}

{OFF}

{BLK}

{WHT}

{RED}

{CYN}

{PUR}

{GRN}

{BLU}

{YEL}

Atari, inverse characters (white on black) should
be entered with the inverse video key (Atari logo
key on 400/800 models).

Press:

SHIFT

CLR/HOME

CLR HOME

SHIFT

t CRSR J

t CRSR i

SHIFT

«— CRSR —

— CRSR-

CTRL

CTRL
CTRL

CTRL

See:

□

When You
Read:

& 3 2
i 4 §

& *3

g 7§

! Fl }

{ n }

{ F3 }
{ F4 }

{ re }

{ F6 }
{ F7 }
{ F8 }

Press:

COMMODORE

SHIFT

See:

BIB

Bffl

Ell

□
■ ■

■

Whenever more than two spaces appear in a
row, they are listed in a special format. For ex-
ample, {6 SPACES} means press the space bar six
times. Our Commodore listings never leave a sin-
gle space at the end of a line, instead moving it to
the next printed line as {SPACE}.

The Automatic Proofreader
Type in the appropriate program listed below,
then save it for future use. The Commodore
Proofreader works on the Commodore 128, 64,

166

COMPUTE! 's Guide to Typing In Programs

and VIC-20. Don't omit any lines, even if they
contain unfamiliar commands or you think they
don't apply to your computer. When you run the
program, it installs a machine language program
in memory and erases its BASIC portion automati-
cally (so be sure to save several copies before run-
ning the program for the first time). If you're
using a Commodore 128, do not use any
GRAPHIC commands while the Proofreader is ac-
tive. You should disable the Commodore Proof-
reader before running any other program. To do
this, either turn the computer off and on or enter
SYS 64738 (for the 64), SYS 65341 (128), or SYS
64802 (VIC-20). To reenable the Proofreader, re-
load the program and run it as usual. Unlike the
original VIC/64 Proofreader, this version works
the same with disk or tape.

On the Atari, run the Proofreader to activate it
(the Proofreader remains active in memory as a
machine language program); the BASIC loader
automatically erases itself. Pressing SYSTEM RE-
SET deactivates the Atari Proofreader; enter
PRINT USR(1536) to reenable it.

Once the Proofreader is active, try typing in a
line. As soon as you press RETURN, a pair of let-
ters appears. The pair of letters is called a
checksum.

Compare the value displayed on the screen by
the Proofreader with the checksum printed in the
program listing in the book. The checksum is
given to the left of each line number. Just type in
the program a line at a time (without the printed
checksum), press RETURN or Enter, and compare
the checksums. If they match, go on to the next
line. If they don't, check your typing; you've made
a mistake. Because of the checksum method used,
do not type abbreviations, such as ? for PRINT.
On the Atari Proofreader, spaces are not counted

167

APPENDIX A

as part of the checksum, so be sure you type the
right number of spaces between quotation marks.
The Atari Proofreader does not check to see that
you've typed the characters in the right order, so
if characters are transposed, the checksum still
matches the listing. The Commodore Proofreader
catches transposition errors and ignores spaces un-
less they're enclosed in quotation marks.

Program 1 . Commodore Proofreader

10 VEC=PEEK(772)+256*PEEK(773) :LO=43 :HI=44

20 PRINT" { CLR} I WHT J AUTOMATIC PROOFREADER FOR ";:IF

VEC=42364 THEN PRINT "C-64"
30 IF VEC=50556 THEN PRINT "VIC-20 { BLU } "
40 IF VEC=35158 THEN WAIT CLR: PRINT "PLUS / 4 & 16"
50 IF VEC=17165 THEN LO=45 :HI=46 :WAIT CLR:PRINT"12
8 1 WHT j "

60 SA=(PEEK(LO)+256*PEEK(HI ) )+6:ADR=SA

70 FOR J=0 TO 166: READ BYT: POKE ADR, BYT : ADR=ADR+1 :

CHK=CHK+BYT :NEXT
80 IF CHK<> 20570 THEN PRINT "*ERROR* CHECK TYPING

I SPACE j IN DATA STATEMENTS ": END
90 FOR J=l TO 5 : READ RF, LF, HF : RS=SA+RF :HB=INT ( RS/2

56) :LB=RS-(256*HB)
100 CHK=CHK+RF+LF+HF : POKE SA+LF, LB : POKE SA+HF , HB : N

EXT

110 IF CHKO22054 THEN PRINT "* ERROR* RELOAD PROGR

AM AND CHECK FINAL LINE": END
120 POKE SA+149, PEEK (772) : POKE SA+150 , PEEK ( 77 3 )
130 IF VEC=17165 THEN POKE SA+14 , 22 : POKE SA+18,23:

P0KESA+29 , 224 : POKESA+1 39 , 2 24
140 PRINT CHR$ ( 147 ) ;CHRS (17 ); "PROOFREADER ACTIVE":

SYS SA

150 POKE HI , PEEK ( HI ) +1 : POKE ( PEEK ( LO ) +256*PEEK ( HI )
)-l,0:NEW

160 DATA 120,169,73,141,4,3,169,3,141,5,3

170 DATA 88,96,165,20,133,167,165,21,133,168,169

180 DATA 0,141,0,255,162,31,181,199,157,227,3

190 DATA 202,16,248,169,19,32,210,255,169,18,32

200 DATA 210,255,160,0,132,180,132,176,136,230,180

210 DATA 200,185,0,2,240,46,201,34,208,8,72

220 DATA 165,176,73,255,133,176,104,72,201,32,208

230 DATA 7,165,176,208,3,104,208,226,104,166,180

240 DATA 24,165,167,121,0,2,133,167,165,168,105

250 DATA 0,133,168,202,208,239,240,202,165,167,69

260 DATA 168,72,41,15,168,185,211,3,32,210,255

168

COMPUTE! 's Guide to Typing In Programs

270 DATA 104,74,74,74,74,168,185,211,3,32,210
280 DATA 255,162,31,189,227,3,149,199,202,16,248
290 DATA 169,146,32,210,255,76,86,137,65,66,67
300 DATA 68,69,70,71,72,74,75,77,80,81,82,83,88
310 DATA 13,2,7,167,31,32,151,116,117,151,128,129,
167,136,137

Program 2. Atari Proofreader

100 GRAPHICS

110 FOR 1=1536 TO 1700:READ A : POKE I,A:CK=CK
+A: NEXT I

120 IF CKO19072 THEN ? "Error in DATA State

ments. Check Typing. ":END
130 A=USR<1536)

140 ? :? "Automatic Proofreader Now Activate

d. "
150 NEW

1536 DATA 104,160,0,185,26,3
1542 DATA 201,69,240,7,200,200
1548 DATA 192,34,208,243,96,200
1554 DATA 169,74,153,26,3,200
1560 DATA 169,6,153,26,3,162
1566 DATA 0,189,0,228,157,74
1572 DATA 6,232,224,16,208,245
1578 DATA 169,93,141,78,6,169
1584 DATA 6,141,79,6,24,173
1590 DATA 4,228,105,1,141,95
1596 DATA 6,173,5,228,105,0
1602 DATA 141,96,6,169,0,133
1608 DATA 203,96,247,238,125,241
1614 DATA 93,6,244,241,115,241
1620 DATA 124,241,76,205,238,0
1626 DATA 0,0,0,0,32,62
1632 DATA 246,8,201,155,240,13
1638 DATA 201,32,240,7,72,24
1644 DATA 101,203,133,203,104,40
1650 DATA 96,72,152,72,138,72
1656 DATA 160,0,169,128,145,88
1662 DATA 200,192,40,208,249,165
1668 DATA 203,74,74,74,74,24
1674 DATA 105,161,160,3,145,88
1680 DATA 165,203,41,15,24,105
1686 DATA 161,200,145,88,169,0
1692 DATA 133,203,104,170,104,168
1698 DATA 104,40,96

169

Integrated Circuits

7400/74LS00
Quad NAND Gate

Ground 7

14 +5 volts

171

APPENDIX B

7402/74LS02
Quad NOR Gate

—zj— ^

— TL_n]

14 +5 volts

172

ated Circuits

7404/74LS04
Hex Inverter

Ground 7 I 8

173

APPENDIX B

7408/74LS08
Quad AND Gate

Ground 7

+ 5 volts

174

integrated Circuits

175

APPENDIX B

74151/74LS151

Eight Line to One Line Mutliplexer

Input 3

+ 5 volts

Input 2

Input 4

Input 1

Input 5

Input

Input 6

Output

Input 7

Inverted output

Enable

B > In P ut
f select

Ground

176

Integrated Circuits

74154/74LS154

Four Line to 16 Line Demultiplexer

Output

+ 5 volts

Output 1

Output 2

V Output

/ select

Output 3

Output 4

Output 5

Enable 2

Output 6

Enable 1

Output 7

Output 15

Output 8

Output 14

Output 9

Output 13

Output 10

Output 12

Ground

Output 11

177

APPENDIX B

74367/74LS367

Hex Three-State Buffers

Enable input

Ground 8

-1-5 volts

Enable input

Truth Table

Enable Input

Input

Output

H or L

High impedance

178

ntegrated Circuits

179

Index

actuators vii 97
A/D converter 33
analog circuit 3
analog devices 33

analog-to-digital converter. See A/D

converter
applications vii
Atari BASIC 83

"Automatic Proofreader, The" 166-69
binary number system 3-6
bit 5

Boolean algebra 109
breadboarding viii. See also wiring,

methods of
burglar alarm 97-106

Atari program 105

Commodore program 102

constructing 99-100

VIC-20 program 103
byte 6

circles, small 110
circuit, analog 3
circuit, building a viii
circuit, digital 3
cold joint xii
control port pin 82, 109
data vii

data direction register 81, 82

Atari 82-83

Commodore 81-82
data port 81

dc motor control circuit 151
Atari program 161-62
Commodore program 158-59
constructing (Atari version) 155-57
constructing (Commodore version)
153-55

VIC-20 program 160-61
decoders 115-18
demultiplexer 115-18, 158
demultiplexer circuit 145-48

constructing 145-46
digital circuitry 109
digital light beam timer 74-78

Atari program 78

Commodore program 76

VIC-20 program 77
digital light sensor 97, 98, 129

variable 67
digital logic 109-20
digital signal 67, 109

high 67

low 67

D-subminiature female connectors 9

electromagnet 90

electromechanical switch 90

electron beam 59-61

electron gun 53

electronic signals 81

electronic switch 81-94, 98, 136

Atari program 93

Commodore program 92

constructing 84-88

using 91, 94

VIC-20 program 92
electrons 53
encoders 115-18
game paddles 33-41

Atari program 39

Commodore program 38

constructing 34-35

"ML Paddle Reader" program 41

reading 39-40

VIC-20 program 39
gates 109-15

AND 111

combining 143

exclusive NOR 113

exclusive OR 112

NAND 111

nonin verting buffer 110

NOR 112

NOT 110

OR 112
H (logic high) 110
IC chip. See integrated circuit chip
icons xiii
input 18

input/output ports 9
integrated circuit chip xi, 118

pins 54
integrated circuits 171-79

demultiplexer, four line to 16 line

177

hex inverter 173

hex three-state buffers 1 78

multiplexer, eight line to one line 176

quad AND gate 174

quad NAND gate 171

quad NOR gate 172

quad operational amplifier 179

quad OR gate 175
interrupt request (IRQ) 39
inverters 110
I/O pin 82
I/O ports 9

181

IRQ. See interrupt request
joystick 97

constructing 23-29

gravity 28-29

port 9-11

push button 23-27

push button, constructing 24-27
joystick port connector 16-20

Atari program 19

Commodore program 18

constructing 16-17

VIC-20 program 19
L (logic low) 100
light-emitting diode (LED) 12, 91
light pen 53-64, 67

Atari program 64

Commodore program 62

constructing 54-58

programming with 61-62

VIC-20 program 63
light sensor, analog 45-50, 136

Atari program 49

Commodore program 49

constructing (Atari version) 47-48

constructing (Commodore version)

45-47

light sensor, digital 67-78

Atari program 73

Commodore program 73

constructing 68-71

VIC-20 program 73
light switch 136-40

Atari program 139

Commodore program 137

VIC-20 program 138
location 83

logic probe 10-13, 123-26

constructing 10-13, 123-25

using 126
logic states

high 15

low 15, 20
low-power Schotkey (LS) 120
"ML Paddle Reader" program 41
monitor 53

multiplexer circuit 140-45, 158

Atari program 144

Commodore program 143

constructing 142

VIC-20 program
multiplexers 115-18
multiplier 13
NAND gates 60
noninverting buffer 60, 90
number systems 4-6

binary 3-6

decimal 4

ohms 13
output 18
output signal 109
PADDLE(X) 34, 50
phototransistor 60, 72
polling 143
ports 9, 10
data 81

input/output 9

joystick 9-11
potentiometer 33
power ratings 14
registers 81
resistor color codes 14
resistors 13, 14, 33, 126
resistor variable 45
robotics 151-62
sensors vii, 81, 97, 109
software vii
soldering 55

soldering iron, heating ix-xii
STICK 18, 74, 82
STRIG 18
switch 3, 72

Atari version 169

Commodore version 168

tilt 28
switch sensor 98, 100

infrared 98

magnetic switch 98

metallic tape 98

ultrasonic 98

vibration detectors 98
tilt switches 28
timer circuit 74
tin xii

tolerance 13

transistor-transistor logic signal level.

See TTL
truth table 109
TTL 3, 120

two-beam digital timer circuit 129-36
Atari program 135
Commodore program 134
constructing 130
using 134

VIC-20 program 135
typing in programs 165-69
voltage 3, 33
watts 14

wiring, methods of viii
circuit boards viii
mounting viii
plug points ix
wire wrapping viii

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