USING ELECTRONICS
a book of things to make
USING
ELECTRONICS
a book of things to make
HARRY ZARCHY
illustrated by the author
THOMAS Y. CROWELL COMPANY
NEW YORK
by the author
WHEEL OF TIME
USING ELECTRONICS
a book of things to make
Copyright 1958 by Harry Zarchy
All rights reserved. No part of this book may
be reproduced in any form, except by a reviewer,
without the permission of the publisher.
Manufactured in the United States of America
by the H. Wolff Book Manufacturing Company, Inc.
Library or Congress Catalog Card No. 58-0721
First Printing
For Bill and Steve,
who are off to a good start
in electronics
CONTENTS
The Nature of Electricity
Basic Procedures
Antennas and Grounds
Radio Components
Construction Hints
Electronics Experiments
(1) Simple Crystal-Detector Receiver
(2) Crystal-Detector Receiver with Hand Wound Coil
(3) "Emergency" Crystal-Detector Receiver
(4) Crystal-Detector Receiver with Tuned Circuit
(5-9) Simple Transistor Hookups
(10) Single-Transistor Receiver with Hand-Wound Coil
(11) Crystal-Detector Receiver with Transistor Amplifier
(12) Two-Transistor Receiver
(13) Two-Transistor Receiver
(14) Two-Transistor Receiver
(15) Single-Transistor Regenerative Detector Receiver
(16) Two-Transistor Regenerative Detector Receiver
(17) Solar Battery Power Supplies
(18) Sound Amplifier
(19) Room-to-Room Communicator
(20) Wireless Broadcast Oscillator
How a Vacuum Tube Works
(21) Vacuum-Tube Grid-Leak Detector Receiver
(22) One-Tube Regenerative Receiver
How to Learn the Code
(23) Code Practice Oscillator
Index
USING ELECTRONICS
a book of things to make
THE NATURE OF ELECTRICITY
This is a book for young people who would like to
work with electronics but have had no previous experience. Anyone who
can follow the simple directions in this book will be able to build
radio receivers and other experimental devices described later.
Parts used in radio construction will be identified
and their uses explained, so that you will know how they function in
each circuit. You will also learn how to use and care for tools
necessary in our electronics experiments.
Electricity is a form of energy that travels through
wires. Electronics is concerned with electrical energy that travels
through space. In order to understand electronics it is first necessary
to understand electricity.
The ancient Greeks knew that rubbing a piece of
amber with a cloth would cause it to attract bits of feathers and other
matter. For hundreds of years this attraction remained unexplained and
was simply regarded as one of nature's curiosities.
In the sixteenth century William Gilbert, one of
Queen Elizabeth's physicians, became interested in the behavior of
amber. As a result of his experiments he determined that many
substances, such as sulfur, sealing wax, rock salt, and glass, had the
same properties as amber: when rubbed with a cloth they attracted other
substances. This phenomenon became known as electricity, after
elektron, the Greek word for amber.
Gilbert published a book, in which he described his work. Other
scientists became interested in the problem. Bit by bit they unlocked
nature's secrets to arrive at a bet ter understanding of the nature of
electricity. Some discoveries were made accidentally, but most were the
result of careful, painstaking investigation. Theories were developed
and laws were formulated concerning the behavior of electricity; the
study of electricity became a recognized science.
We may be certain that none of these early experimenters had the
faintest notion that he was contributing toward the development of a
force that would one day change the face of the earth. Could any one of
them have predicted the electric light, radio, or television? Could
Gilbert possibly have known that his interest in the curious behavior
of amber would one day furnish the world with undreamed-of power?
Yet that is exactly what has happened. The twentieth century has become
the age of electricity. We have become the masters of a mighty force
which furnishes us with power on such a vast scale that many phases of
modern life depend upon it. Electricity has become so important to our
civilization that we cannot possibly do without it. It does our heavy
work in mines and factories. It runs machines which eliminate much of
the drudgery of farm life. Electrical appliances in our homes perform
chores by the dozen; among other things they make our toast, give us
light, condition our air, refrigerate our food, and wash our clothes.
Electricity is responsible for the development of the transportation
industry. Automobiles, airplanes, trucks, and buses use internal
combustion engines in which power is developed when an electrical spark
explodes a mixture of gasoline and air. Every motorist knows what would
happen if his battery were to break down. It would have to be replaced
before his car could run again.
Perhaps the
most startling developments have been made in the field of
communications. Electricity has made possible the development of the
telegraph, telephone, radio, and television. A flip of the switch can
bring us entertainment, an educational program, or the latest news
bulletin. On-the-spot reporting enables us to hear or see radio or
television broadcasts of events as they are taking place, even though
they may be thousands of miles away! These things have become so
commonplace that we attach no great importance to them. Electricity,
radio, and television are part of our lives; and we seldom think about
the wonderful inventions that have made them possible.
How does electricity work? Until very recently no one had the answer to
this question. Now scientists attempt to answer it with the electron
theory.
According to the electron theory, all
matter (anything that has weight and takes up space ) is composed of
molecules. If you were to break up a crystal of salt into very tiny
particles, then keep on subdividing these particles into still smaller
particles, you would eventually end up with molecules of salt A
molecule is the smallest possible particle of a substance that retains
all the properties of that substance. Needless to say, molecules are
too small to be seen.
Molecules are composed of
still smaller bodies called atoms. The atom is the smallest particle
into which substances known as elements can be divided. Scientists have
found 92 elements occurring in nature; all known natural substances are
composed of combinations of these elements. For example, a molecule of
water contains atoms of hydrogen and oxygen. Other molecules have more
complicated structures; borax is made up of atoms of sodium, boron,
oxygen, and hydrogen.
The structure of the atom
is particularly interesting, for it is electrical in nature. Physicists
have discovered that atoms are not solid bodies but are composed mainly
of empty space. Each atom has a core, or nucleus, which contains
positively charged particles called protons, and neutrons, which are
electrically neutral particles. Revolving around the nucleus are one or
more particles with a negative charge; these are electrons.
The atom has been compared to the solar system, with the electrons
(planets) whirling around the nucleus (sun. This comparison is even
more striking when we learn that the electrons revolve around the
nucleus in definite paths, or orbits. Consequently, they are said to
have a planetary motion.
Atoms are electrically
neutral. Each positively charged proton in the nucleus is usually
balanced by a negatively charged electron. Illustrations of the atoms
of different elements show this clearly. The hydrogen atom has the
simplest structure; its nucleus contains one proton which is balanced
by one planetary electron. The helium atom has two neutrons and two
protons in its nucleus and is encircled by two electrons. Uranium has
the most complex structure of all the elements; it has ninety-two
planetary electrons which revolve around the nucleus in seven different
orbits.
In some elements the electrons in the
outer orbit are held together very loosely. Under certain conditions
one or more are forced free and are able to move from one atom to
another. These are called free electrons. When an atom gains or loses
electrons, it is no longer electrically balanced, or neutral; it has
become ionized. An atom that has lost electrons is left with a slight
positive charge and is known as a positive ion. A negative ion is an
atom that has gained electrons and has a negative charge.
The movement of free electrons in a conductor constitutes an electric
current. An electron moves a very short distance and then collides with
a nearby atom. It forces one or more electrons out of their orbit and
takes their place. The replaced electrons, now free, move on to repeat
the process with other atoms. This movement of free electrons takes
place throughout the substance that is conducting the electric current.
Some substances contain large numbers of free
electrons. These substances are called conductors of electricity
because they permit electrons to flow through them very easily. Metals
such as silver, gold, copper, brass, aluminum, zinc, and iron are
conductors. Although silver is best, copper wire is generally used in
electrical work because it is considerably less expensive.
Other substances such as asbestos, rubber, mica, dry wood, and glass
have very few free electrons and consequently permit practically no
current to flow through them. They are known as insulators. Insulating
materials are used as coatings or wrappings on wires that carry
electric current.
In order for a conductor to
carry current, it must be part of a complete electrical circuit. Try
this experiment: touch one end of a copper wire to the bottom of a
flashlight bulb, and the other end to the side of the bulb. Since the
wire contains an abundance of free electrons, you might expect current
to flow and light the bulb but nothing of the sort will happen. The
bulb will not light because the free electrons in the wire are
vibrating aimlessly in all directions instead of moving in one
direction. A source of electrical energy is needed to start the flow of
current; this will make our circuit complete.
Electrical energy can be furnished by an ordinary
drycell battery, such as is used in a flashlight. It acts as a pump and
starts the current flowing in a circuit. Batteries are made in several
sizes; No. 6 dry cells are often used for electrical experiments. The
battery (strictly, "battery" should be used only for two or more cells)
goes to work as soon as it becomes part of the circuit. Electrons rush
to the negative terminal (-) and excite free electrons in the
conductor. At the same time the positive terminal (+) begins to attract
excess electrons from the other end of the conductor.
Electrons within the circuit no longer move about in a haphazard manner
but commence a steady drift, or flow, in one direction. They move from
the negative terminal, through the conductor and lamp, and back into
the cell through the positive terminal; current flows in the circuit
and the lamp will light. Should any one of the connections be broken,
the circuit will be incomplete and the flow of current will stop.
Electrons migrate through the conductor rather
slowly, but the electrical energy they produce is transmitted at the
speed of lightabout 186,000 miles per second! This can be
explained if you will imagine a row of marbles arranged in a straight
line so that they are all touching. Strike the marble at one end and
the impact will imme diately be transferred to the one at the opposite
end.
Something very similar takes place in a
complete electrical circuit As soon as the cell begins to push
electrons into one end of a conductor, other electrons are forced out
of the opposite end.
Wires can be used to carry
signals. This is really very commonplace, for every time you press a
doorbell push button you are sending a signal along a wire. The push
button is a switch that closes a circuit and enables current to flow
through a set of conductors and a buzzer.
The
telegraph works on the same principle. An operator at one end of the
line creates dots and dashes by pressing a key; this closes and opens
the circuit The flow of current energizes a sounder at the other end of
the line; this reproduces the signals sent by the operator. Telegraphic
signals can be sent wherever there are wires to carry the current.
Magnetism
Electrical energy can be transferred from one
conductor to another even though they are not connected to each other.
This depends upon the fact that electricity and magnetism are closely
related.
Ancient people knew of the existence of naturally magnetic substances
such as lodestone, a form of iron ore. They discovered that a piece of
lodestone would attract bits of iron and cause them to stick to it.
Furthermore, as long as these pieces of iron were in contact with the
lodestone, they would act as magnets and attract other bits of iron. As
soon as contact was broken, they would lose their magnetic properties.
It was also known that pieces of steel which were stroked with
lodestone would become permanently magnetized.
We know considerably more about magnetism. In 1819 Hans Christian
Oersted, a Danish physicist, discovered that every conductor carrying a
current is surrounded by a magnetic field. If the conductor is formed
into a coil, the strength of the magnetic field is concentrated A piece
of soft iron placed within such a field will become magnetized and will
act as a magnet as long as current flows through the coil.
The existence of this magnetic force can be shown by
constructing an electromagnet. This requires the use of a dry-cell
battery. Make a coil by winding about 50 turns of insulated copper wire
around a large iron nail. Connect one end of the wire to one of the
battery terminals. Touch a small nail to the large nail and you will
find that they are not attracted to each other. Now touch the other end
of the wire to the other battery terminal, completing the circuit so
that current flows through the coil. The large nail wffl immediately
become magnetized and will attract the small naiL Break the electrical
circuit and it will lose its magnetism. The magnetic force can be
increased by adding turns to the coil, or by increasing the current
flowing through the coil.
This experiment also shows that electrical energy can be converted to magnetic energy.
In 1831 Michael Faraday demonstrated that magnetism
could be converted to electrical energy. He produced electrical current
in a coil by moving a magnet through it. Strangely enough, no current
flowed while the magnet was motionless. Faraday reasoned that the
movement of the magnet or of the magnetic field surrounding it was in
some way responsible for the current produced in the coil.
Faraday's conclusion was correct. The moving magnetic field cut across
the coil, setting free electrons in motion and developing current The
moment the magnetic field stopped moving, current stopped flowing.
Faraday's experiment explains how electrical energy can be transferred
from one conductor to another. We know that we can create a magnetic
field around a coil by passing a current through the coil. If we place
a second coil very close to the first one, the movement of the magnetic
field around the first will develop electrical current in the second.
If you have a galvanometer (an instrument for measuring very small
amounts of current) you can perform an interesting experiment that will
demonstrate this idea very clearly.
Make a
coil by winding about 50 turns of insulated wire around a pencil or
wooden dowel; this is the primary coil Wind another coil of
approximately the same number of turns on top of the primary; this is
the secondary coil A device that transfers energy from the primary to
the secondary coil is known as a transformer.
Attach the ends of the secondary coil to the galvanometer. Current
flowing in this coil will be shown by a movement of the galvanometer
needle. One end of the primary is attached to a 6-volt dry-cell
battery, as shown in the illustration.
Make the connection so that current flows through
the primary coil. The galvanometer needle will move, then return to
zero. Release the connection, and the needle will once more move and
return to zero.
Now let us analyze what has
happened. As soon as current began to flow through the primary coil, a
magnetic field built up around it. Magnetic lines of force cut across
the secondary coil, developing a surge of current. Current stopped
flowing in the secondary the moment the field reached full strength and
became stationary. Releasing the connection stopped the current flow in
the primary. The magnetic field began to collapse, again cutting across
the secondary coil and inducing another surge of current.
Here is another experiment that you can perform to show how energy can
be transferred from the primary to the secondary coils of a transformer.
You will need about 40 nails 1 1/2 or 2 inches long.
Any iron nails will do. Stack them into a neat bundle, then wind about
100 turns of wire around them to make the primary coil. Fasten the last
turn in place with a strip of adhesive tape. Make the secondary coil by
winding the same number of turns directly over the primary coil. Scrape
about one inch off the ends of the wires of both coils so that they are
bright and clean.
You have just
constructed a simple iron-core transformer. The nails form the iron
core, which concentrates the magnetic lines of force and prevents them
from leaking off.
Hook up the primary circuit.
Attach one end of the coil to one of the battery terminals; the other
end should remain free, leaving the primary circuit open. A flashlight
bulb takes the place of the galvanometer in the secondary circuit. Any
bulb will do, but the type used with small penlight batteries is best,
since it uses very little current.
Prepare the bulb by brightening all its metal
surfaces with steel wool or with fine emery cloth or sandpaper. This
will remove film and insure good electrical contact with the wires. One
end of the secondary coil must make firm contact with the bottom of the
bulb, and the other end with the side. It is a good idea to have
someone assist you by holding the wires against the bulb.
Complete the circuit in the primary by touching the free end of the
coil to the unused battery terminal. The bulb will flash, but will not
remain lighted while the circuit is complete. Scratch the end of the
coil against the terminal, and the bulb will show a brighter light.
What you are actually doing is making and breaking the primary circuit
very rapidly.
If we can keep the magnetic field in motion we will
have a constant flow of current in the secondary coil. We have shown
that this can be done by rapidly making and breaking the circuit in the
primary coil, but the same result can be achieved in another manner.
We have been using current supplied by a battery,
which pushes electrons in only one direction; this is known as direct
current. Certain types of generators can supply us with alternating
current, which reverses its direction of flow periodically. Each
complete reversal of current is known as a cycle. The frequency of a
current is determined by the number of cycles that occur each second.
Depending upon their specific use, alternating currents can be
generated at almost any frequency. For example, house current is
commonly supplied at 60 cycles; this represents 120 changes of current
direction each second.
These changes of current direction may be shown as a series of wave
forms. The horizontal line running through the center of the waves
represents a point of zero voltage. Each complete cycle consists of a
surge of positive current (shown above the line), and a surge of
negative current (shown below the line). The positive half of the cycle
starts at zero, builds up to a maximum voltage, drops back to zero, and
immediately changes direction, becoming negative. The negative half
cycle rises to its maximum, drops back to zero, and changes its
direction, becoming positive.
Alternating current is in constant motion. When it
flows through the primary coil of a transformer it creates a constantly
moving magnetic field. Each time the current changes its direction, the
field builds up and collapses, inducing a constant flow of current in
the
secondary.
RADIO WAVES
Under certain conditions energy can be transferred
from one conductor to another even though they are separated by
hundreds of miles. This fact is responsible for the development of
radio.
Two things are part of every radio system: a transmitter and a receiver.
The transmitter is a device that converts sound into electrical energy;
this energy is broadcast through an antenna in the form of
electromagnetic waves. Electromagnetic waves travel through space.
Scientists do not fully understand how this takes place. Many theories
have been evolved to explain this phenomenon, but none has proved
completely satisfactory. Despite our lack of complete understanding of
these waves, we have gained a tremendous amount of knowledge about them
and are able to put them to use.
The action of
electromagnetic waves has often been compared to the waves that are
formed when a pebble is dropped into the water of a still pond. Waves
radiate evenly on the surface of the water, gradually becoming smaller
as they move away from the center of the disturbance.
The transmitting antenna radiates electromagnetic
waves in much the same manner. They travel through space in all
directions, gradually losing their strength as the distance increases.
Eventually some of these waves are intercepted by a receiving antenna.
As they pass through the antenna, they set electrons in motion and
cause a flow of current. For this reason an antenna must be a good
conductor.
The current picked up by the antenna
is fed into the receiver, which converts it back into sound. Through
the marvels of electronics your radio receiver reproduces the identical
sounds that are picked up by a microphone in the broadcasting studio.
BASIC PROCEDURES
Every craft requires the use of certain tools, and
radio work is no exception. An elaborate workshop is not needed, but a
few tools are essential. Good tools are a wise investment, so buy the
best that you can afford. Never abuse tools by using them improperly.
Keep them clean and dry. Should they show signs of rusting, they should
be rubbed clean with fine emery cloth and coated lightly with machine
oil. If possible, keep your tools in a rack or store them in a drawer
when they are not in use.
Pliers
Long-nosed pliers are used by all radio technicians. They are needed to
hold wires and other parts that are being soldered and to bend wires
into desired shapes. They should not be used for heavy work, such as
tightening large nuts or bending nails and bolts. Think of this tool as
an extension of your fingers that enables you to reach otherwise
inaccessible places.
Diagonal cutting pliers
are used to cut wires. There are many other types of cutting pliers,
but they are mostly used for heavy-duty work. Diagonals have slim jaws
and can be used in tight spots. Use them only on wire, never on nails
or bolts, for the cutting edge can be nicked veryeasily.
Screwdrivers
Screwdrivers used in radio or electrical work
should have insulated handles, usually made of plastic or hard rubber.
They are made in several different sizes. The size you will need for a
particular job will depend upon the size of the slot in the head of the
screw or bolt that is to be tightened. The end of the screwdriver
should just fit into the slot it engages. If it is too small it will
mutilate the screw head, leaving sharp bits of protruding metal that
can be quite hazardous. One that is too large will have to be forced
into the screw slot, with the same results. The experiments in this
book require the use of only one screwdriver; be sure it fits the
screws you are using.
Penknife
Wires and other parts that are to be soldered or are required to make
good electrical contact are usually scraped. An old penknife makes an
ideal scraper; it needn't have a sharp edge. Never use a razor blade
for scraping. Not only is it a dangerous practice, but you may remove
too much metal from the wire, leaving it weakened.
Socket Wrenches
Socket wrenches are used to tighten nuts.
They may be purchased singly or in sets of assorted sizes. As the name
implies, this tool consists of a socket, which is attached to a handle.
The socket is slipped over the nut and the handle is turned like that
of a screwdriver. Some sets consist of one handle with interchangeable
sockets. You simply select the socket that fits the nut to be
tight ened, then fasten it to the handle by pressing it into
place.
Socket wrenches are not essential, but they will
make your work easier. If you do not have any, you can still tighten
nuts and bolts very simply. Slip the nut over the bolt and turn it as
far as you can with your fingers. Grasp the nut with a large pair of
pliers, then use a screwdriver to turn the bolt the rest of the way.
HOW TO SOLDER
Solder is an alloy of lead and tin. It melts very readily and is
used to fasten together wires and other metal radio parts. Do not
attempt to do this with any type of cement, glue, or paste. These
materials are insulators, and will prevent current from flowing through
the joint you make.
Soldering requires the application of heat In the old days people used
a soldering copper, which was heated over a fire. When the copper
became hot enough to melt solder, it was put to use. When it cooled it
was reheated. Tinsmiths still use large soldering coppers, heating them
with portable torches.
Radio men use
electrically heated soldering irons. Although they are called "irons",
they actually have copper tips. They are made in many different types
and sizes and are rated according to their wattage, or electrical power
needed to bring them to their operating temperature.
Very small electric irons are known as soldering pencils. These have
fine tips and are excellent for delicate work. They consume very little
power; a 15-watt pencil will do most soldering jobs. Many soldering
pencils have interchangeable tips. When one wears out, it can be
removed with a pair of pliers and a new one can be substituted. Some
pencils consist of a handle into which heating units of different sizes
can be screwed; these can be purchased separately.
Soldering is also done with an electric soldering
gun. Such guns are usually made with a pistol grip so that they can be
handled easily. Pressing a switch causes current to flow through a
built-in transformer, and the gun is brought to operating temperature
in a matter of seconds.
As soon as the joint is soldered, pressure on the switch is released and the gun is allowed to cool.
Not all metals can be soldered. Radio work usually
calls for soldering copper or tinned surfaces, and this can be done
very easily. Do not try to solder aluminum, as this cannot be done with
ordinary soldering equipment; special types of solder must be used.
Solder is manufactured in different forms. Bar solder and solid wire
solder are used by plumbers and tinsmiths. Neither one of these is
suitable for our purpose. Rosin-core solder is the only type
recommended for electrical connections. This is made in the form of a
hollow wire, the inside of which contains rosin. The rosin serves an
important purpose: it acts as a flux which keeps the joint clean as it
is being soldered. The heat of the soldering iron causes metals to
tarnish, or form an oxide very quickly. This coating or discoloration
will prevent the solder from flowing and forming a good joint. Flux
dissolves the oxides and makes soldering possible.
Never
use acid-core solder. This is a type of hollow wire solder in which
acid is used as a flux. It will enable you to make a good soldered
joint, but the acid will eventually cause corrosion.
Soldering involves more than just applying heat. Follow these steps for good results:
(1) All
surfaces that are to be soldered must be clean. Tarnish and dirt must
be removed, otherwise the solder will not adhere. Brighten the metal by
scraping it with a knife blade or rubbing it with sandpaper, emery
cloth, or steel wool.
(2) Parts to
be joined by soldering must make good contact Wires should be twisted
together. When soldering a wire to a larger surface, hold it in place
with longnosed pliers. Do not try to fill gaps between wires and other
parts with solder.
(3) A new soldering iron must be
tinned. This consists of coating the tip with a layer of melted solder.
First, scrape or sandpaper the tip until it is clean and shiny. Plug in
the iron and allow it to become hot enough to melt a bit of solder that
is touched to it. Run melted solder all over the tip of the iron, then
wipe it on a clean rag; this will remove any excess solder, leaving a
thin, even coating. After an iron has been tinned, it is simply heated
each time it is to be used.
(4) Touch the
hot, tinned iron to the joint that is to be soldered. Then bring the
end of the wire solder into contact with both the iron and the joint.
The solder will melt at once. Quickly remove the solder, but keep the
hot iron in place for a moment; this allows the solder to flow around
the joint. It also gives the flux a chance to boil off.
Never make a soldered joint by
touching the solder only to the hot iron; this may result in a "cold"
joint. The solder will melt but may not adhere properly to the metal
parts that are to be joined.
(5) After you have removed the iron from the
soldered joint, allow the solder to "set," or harden, before you touch
or disturb it in any way. Use as little solder as possible. A joint
that has been properly soldered has a bright, shiny appearance. If it
looks dull or slightly rough, reheat it until the solder flows freely
again.
Some radio parts have wire leads (pronounced leeds),
or "pigtails" by means of which they are soldered into circuits.
Excessive heat applied to these leads may be conducted to the part
itself, causing damage. When soldering such parts, grip the wire lead
with long-nosed pliers, just above the point you are soldering. Heat
from the soldering iron will be absorbed by the plier tips, and will be
prevented from passing along the wire to the part.
WIRE
Wire is manufactured in different sizes, which
are identified by gage numbers. The number of a wire refers to its
diameter, or thickness; the higher the number, the thinner the wire.
Insulated wire is always used in radio construction.
This is important, otherwise wires might touch at the wrong places and
cause short circuits. Some commonly used insulating materials are
rubber, plastic, and braided cotton covering.
Most people use hookup wire for circuit wiring. This wire may consist
of a single, solid wire or it may be made up of separate wire strands
twisted together. Stranded wire is more flexible and can be twisted and
bent many times before breaking. On the other hand, solid-conductor
wire is less expensive and easier to handle. Hook up wire is always
tinned, and it need not be scraped before it can be soldered. The end
of an insulated wire must be bared before it is fastened in place in a
circuit. Use your penknife to cut
lightly through the insulation all around the wire. Hold the blade at
an angle, so that the sharp edge faces the end of the wire. If you hold
the knife blade at a right angle to the wire, you may cut through the
insulation and nick the wire: this will cause it to break the first
time it is bent. Using diagonal pliers, grasp the insulation below the
point you have cut, then pull it free.
The easiest type of hookup wire to use is push-back wire. No cutting is
necessary. In order to bare the end of the wire, simply push the
insulation back with your fingers.
Magnet wire
is used for winding coils. It may be purchased in spools weighing 1/2
pound or 1 pound. Sizes 24 to 32 magnet wire may be used for all the
experiments described in this book. You can also use it instead of
hookup wire in constructing our low-powered circuits.
Magnet wire is usually insulated with enamel or plastic coating or
cotton wrapping. Coated wires take up less winding space than wrapped
wires and make smaller coils. A penknife, sandpaper, emery cloth, or
coarse steel wool can be used to remove coated insulation from wire.
Chapter 3
ANTENNAS AND GROUNDS
An antenna, or aerial, is a conductor that
intercepts broadcast electromagnetic waves. Every radio receiver must
have an antenna. Commercially built receivers usually have small loop
or coil antennas concealed within the cabinet.
A
good antenna will improve the performance of any radio. This is
particularly true of the low-powered receivers we are going to build.
Any substance that is a good conductor will make a good antenna. The
simplest antenna is a long length of wire connected to the receiver.
Although any wire can be used, most antennas are made of stranded
copper wire. Your antenna should be erected as high as possible and
kept away from power lines and from touching trees, buildings, or tin
roofs. The longer it is, the more efficient it will be. If possible,
make your antenna at least 100 feet long. If that is not feasible, a
shorter wire (at least 25 feet long) will have to do.
Antenna wire may be bare or insulated. Fasten each end of the antenna
wire to a porcelain insulator. Pass the wire twice through one of the
ends in the insulator, then wind it back on the antenna itself, using
at least 10 turns. Suspend the antenna between your building and
an other building, a tree, or post. This can be done by run
ning a wire from the free end of each insulator to a firmly anchored
screw eye or screw hook, as shown in the illustration.
The end of the antenna near your house should be
connected to a lead-in wire, which then goes to the antenna terminal of
your receiver. Make the connection by scraping both ends, then twisting
them tightly together. A few turns of electrician's tape over the joint
will prevent it from weathering. Any insulated wire makes a good lead.
The illustration shows a typical long-wire antenna system which makes
use of a device known as a lightning arrester. This arrester protects
the receiver in case the antenna is struck by lightning, and it should
be part of every outdoor antenna installation. It will sidetrack a
discharge of lightning directly to ground instead of permitting it to
reach the receiver.
The arrester has two terminals. The lead-in wire is
connected to one of them, from which it goes directly to the receiver.
Another length of wire is connected to the other terminal; the other
end of this wire is clamped to a ground rod. This is a copper-plated
steel rod from four to eight feet long that is driven into the ground
next to the building. Ground rod and clamp sets are sold by radio
supply stores.
Any type of metal pipe four to
eight feet long driven into the ground makes a satisfactory ground rod.
The ground wire can be fastened to it with a ground clamp which may be
bought at any electrical supply store.
Many
experimenters, particularly those who live in apartment houses, cannot
erect an outside antenna. In this case one can be improvised by
connecting a wire to a metal window screen, a bedspring, or any other
large metal object that is not in contact with the ground. Reception
can often be improved by using just a few feet of wire as an antenna. A
short length of wire can even be dangled from a window in order to get
it out of the way. Make a hidden indoor antenna by tacking a length of
wire on top of the picture molding in your room. Or, try running some
fine magnet wire around the baseboard; it is so thin as to be
practically invisible.
A ground is a connection
made to the earth, either directly or indirectly. Most of the receivers
described in this book require ground connections. The best ground is
made by attaching a wire to a cold water pipe. Scrape a small section of
pipe until all paint is removed and the metal is bared. Pass a few
turns of wire around the clean area and twist it tight. A ground can
also be made by driving a metal rod or pipe into the earth; it should
be at least 3 or 4 feet long.
If you cannot hook up to either a cold-water pipe or
an outside ground, try attaching a wire to a radiator pipe. This is
definitely a second choice, to be used only when a proper ground
connection is not available.
Chapter 4
RADIO COMPONENTS
Many different parts are used in radio
construction. In order to build a piece of electronic equipment it is
necessary to know exactly where each part fits into the circuit.
Circuit Diagram
This information can be found in a circuit diagram,
which is actually a radio construction plan. Instead of being
represented by pictures, parts are shown as symbols. Symbols have been
standardized so that they have the same meaning for everyone.
All wires in circuit diagrams are shown as lines. A wire connection is
indicated by a heavy dot where lines meet or cross. If no connection is
to be made at that point, one line is drawn looped over the other. This
system is used in all the circuits shown in this book In other types of
circuit diagrams connections may be shown by lines that meet or cross;
no dots are used.
Capacitors
Capacitors (also known as condensers) are devices that can store an
electrical charge. When used in an electronic circuit, a capacitor will
charge and discharge current at the same frequency as the current that
is applied to it. The amount of current it can handle depends upon its
capacity, or capacitance. It will not pass direct current although it
will permit the flow of alternating current.
Capacitors are rated in units called farads. Since a farad is much too
large for use in radio circuits, capacitance is usually measured in
smaller units. A microfarad (abbreviated uF is one-millionth of a
farad; a micro- microfarad (abbreviated uuF) is one-millionth of a
micro farad.
Both fixed and variable
capacitors are used in radio work Values of fixed capacitors axe either
marked on each piece or are shown by color-coded dots. Since there are
several different color codes, a confusing situation exists. It is
recommended that beginners buy capacitors on which values are clearly
shown. Any type of fixed capacitor may be used to construcfthe circuits
described in this book.
Variable capacitors are
used in tuning circuits. They contain two sets of metal plates; one set
is fixed and the other is movable. Turning the plates varies the
capacitance of the unit. This makes it possible to receive stations at
different frequencies. You will need only one capacitor in order to
construct the projects in this book; it should be rated at about 365
mmF.
Resistors
Every conductor offers some opposition or resistance
to a flow of current. Copper wire used in radio circuits permits
current to flow readily and for our purposes is considered as having no
important resistance.
Insulators permit no current to flow through them
and are used to block the flow of current completely. Other substances
permit a partial flow of current; they may be thought of as partial
conductors. Depending upon their composition, they can be made with a
specific degree of resistance, so that a certain amount of current will
flow through. These are known as resistors. They are used to restrict
the flow of current in circuits.
The unit of resistance is an ohm, shown by the Greek
letter omega (Q. One ohm is written as lQ. One thousand ohms can be
shown as either 1000Q or as IK (K is the symbol for 1000). Thus 15,000Q
and 15K represent the same value of resistance. One megohm (commonly
called a meg and abbreviated as 1M) is equal to 1 million ohms.
The more resistance a circuit contains, the less
current it will pass. It is therefore important to use resistors of the
same value as those specified in the circuit you are following.
However, the circuits in this book can be constructed by using
resistors of approximately the same value as those indicated.
Variations of 25 per cent in resistor values will not affect their
performance. Very few resistors are marked with numerical values; most
are color coded. The code consists of bands of color encircling the
resistor. Each color represents a digit. If you memorize the color
code, you will be able to identify resistor values at once.
In order to determine the value of a resistor hold it as shown in the
illustration and refer to the table here.
Consider only the first three bands marked on
the resistor; disregard the fourth. Let us assume that the resistor we
wish to identify is coded as follows: first band, red; second band, green; third band, orange.
According to the table, red has a value of 2, which is the first digit
of the resistor value. The second band is green, so the second digit is
5. The third band, orange, shows that three zeros must be added to the
two digits we already have; this gives us 25,000.
Here are some other resistor color combinations you can check just for practice:
Brown, orange, yellow: 130,000
Yellow, blue, orange: 46,000
Brown, black, green: 1,000,000, or 1 meg
Orange, green, green: 3,500,000, or 3.5 meg
Potentiometer
A potentiometer is a variable resistor. The
amount of resistance in the circuit is changed by turning a control
shaft, which moves a sliding arm along a resistance element.
Potentiometers, often called pots, are used mostly as volume controls.
Transistors
Most of the circuits in this book make use of
transistors. Transistors serve the same purpose as vacuum tubes and
have several characteristics that make them ideal for our experiments.
They are more efficient than tubes, considerably smaller, less
expensive, and they will last indefinitely. Furthermore, they consume
practically no current, and the batteries you use will last a long
time.
Transistors have three leads which are
identified in the illustration as c (collector), b (base), and e
(emitter). Notice the unequal spacing of the leads; the collector is
always the one that is farthest away from the other two.
There are two types of transistors: P-N-P and N-P-N. You can tell which
type is indicated on a circuit diagram by carefully observing the
arrowhead that represents the emitter. In a P-N-P transistor, the
arrowhead points toward the inside of the symbol In the N-P-N type it
points toward the outside. Do not use these types interchangeably.
Their elements are connected to the plus and minus battery terminals in
different ways. Even a momentary surge of current flowing in the wrong
direction can ruin a transistor.
Batteries
The symbol for a single dry cell is two vertical
lines. The short line represents the negative terminal, and the long
line the positive. The actual voltage used in a circuit can be shown by
symbol if it is not too great. Since each cell delivers 1 1/2 volts, a
power supply consisting of 6 volts would be indicated by four cell
symbols. However, it would be impractical to show a 45-volt or 90-volt
power supply in this way. For this reason, battery power supplies are
generally shown by drawing the symbol for two or three cells; the
actual voltage needed is marked on the circuit diagram.
Detector
The detector, or rectifier, is an important part
of every radio receiver. In order to understand what a detector does we
must first consider what happens at the broadcasting studio. When
speech or music is picked up by a microphone, varying alternating
currents are generated up to about 15,000 cycles per second; these are
known as audio frequencies (abbreviated AF). These frequencies are too
low to be broadcast directly.
Frequencies of
more than 20,000 cycles per second are known as radio frequencies, or
RF; high-frequency RF currents can be broadcast easily. The transmitter
at the broadcasting station generates
a steady RF current, or carrier. The exact frequency generated depends
upon the frequency at which the station is broadcasting. Stations in
the broadcast band operate between 550,000 and 1,650,000 cycles per
second. For convenience, we shorten these numbers by referring to them
as 550 and 1,650 kilocycles; a kilocycle is equal to 1,000 cycles.
The audio and carrier signals are now mixed. The carrier no longer
consists of a steady RF current but has the varying audio signal
impressed upon it. It is now known as a modulated carrier. This is the
electromagnetic wave that is sent out by the broadcasting station. The
modulated carrier is picked up by the antenna of your radio receiver. A
tuning circuit separates the desired signal from others, so that it is
heard without interference. Now the detector goes to work: it separates
the audio from the carrier. The carrier is by-passed, or shunted to one
side; it is no longer needed. The audio is amplified and sent along to
the headphones or loud speaker, from which it emerges the same as
the one that was picked up by the microphone at the studio.
The separation process is known as detection, or
demodulation; and the device that does this job is the detector.
Detectors used in the receivers we shall build are made of a substance
known as a semiconductor. A semiconductor permits current to pass
through it very easily in one direction but offers high resistance to
current flowing in the other direction. It acts as a one-way gate,
which transforms alternating current into a form of direct current.
(Alternating current reverses its direction periodically, while direct
current flows in only one direction).
There are two types of detectors that we can use. One,a type that has
been in use for many years, uses a chunk of galena, a lead ore. This is
mounted in a holder. A fine wire "cat's whisker" comprises the other
part of the detector. The whisker is touched to different places on the
surface of the galena. Some spots are more sensitive than others; it is
allowed to rest at that point which produces the loudest signal.
Cat's-whisker detectors are not very satisfactory. The slightest
movement will disturb the setting of the wire, and a sensitive spot
will once again have to be found. Dirt will impair the sensitivity of
the galena, and you will have to be careful not to touch its surface. A
fingerprint can leave a film of oil which may put you out of business.
The best type of detector to use is a crystal diode. This is a sealed
unit that contains a bit of germanium. It also has a cat's whisker,
which is permanently fixed in place so that no tuning is necessary. All
you need do is hook the diode into a circuit and it will work
automatically. One end of each diode is marked with a K, a band, or a
large dot which indicates the positive or cathode side.
In some circuits the manner in which the diode is connected does not
affect its efficiency. In others, reversing the diode connections will
improve reception.
Headphones
Headphones are used by all electronics
experimenters. They are more sensitive than loudspeakers and will
enable you to hear very weak signals. There are several types of
headphones on the market. The most familiar is the one that has two
earpieces, connected by a band that slips over your head. There are
also single-ear units. These may be the type that fit over your ear, or
they may resemble hearing-aid earpieces which fit into your ear.
Some phones make use of crystals as receiving units. These will not
work in our transistor circuits. Use only magnetic phones.
Impedance is opposition to the flow of alternating
current, and is expressed in ohms. The best headphones to use are those
that have an impedance of at least 2,000 ohms. These are known as
high-impedance phones. The higher the impedance, the more sensitive the
phones will
Chaper 5
CONSTRUCTION HINTS
All projects in this book are assembled on a base.
The simplest base is a piece of soft pine about 1 inch thick, 8 inches
wide, and 12 inches long. These dimensions are approximate and need not
be followed exactly.
Circuits can be assembled
with little or no soldering by fastening all wires to terminals, which
can be made of 5/8-inch or 3/4-inch round-headed wood screws and washers.
To make a terminal, slip a screw through a washer,
then turn it part way into the wooden base. The hole in the washer must
be smaller than the head of the screw. In order to fasten two or more
wires to a terminal, wrap them once around the screw, under the washer.
Turn the screw down to make firm contact against the base.
Bases can also be made of either hard Masonite or 1/4-inch plywood.
Since both of these materials are too hard to take screws readily, a
hole must be drilled where ever a terminal is to be placed, and a small
machine screw is passed through the hole. Machine screws used as
terminals should be 1/2 inch long; you should also have two
hexagonal, or hex, nuts to fit each screw. For example, if you are
using 1/2-inch x 5-40 machine screws, you must match them with
5-40 hex nuts. Number 5 refers to the thickness of the screw; 40
indicates the number of threads per inch.
To
make a machine-screw terminal, first drill a 3/16-inch hole through the
base; this will permit a No. 5 screw to go through. Push a screw
through the hole from the bottom and thread a nut on it from the other
side of the base; tighten the nut. Wires are wrapped once around the
screw, after which a second nut is tightened down to hold them firm.
Machine-screw terminals are best.
You can save yourself the chore of drilling holes by
using a piece of Masonite that comes with holes already drilled in it
This perforated Masonite can be purchased at lumber yards and hardware
stores.
Circuit construction can be simplified
to a great degree if you are willing to do some preparatory work. Parts
with wire leads can be fastened to terminals very quickly and easily if
they are first soldered to soldering lugs. Then, instead of wrapping a
wire lead around a terminal, simply place the lug over it Lugs are made
of tinned sheet brass and are inexpensive. Remember to grip all wire
leads with long-nosed pliers when soldering them to lugs; this will
prevent delicate parts from becoming damaged.
Transistors are particularly sensitive to excessive heat. Do not solder
lugs to transistor leads. The safest way to fasten them into a circuit
directly is by wrapping them around terminals.
The best way to install transistors is by using special sockets, as
illustrated. The transistor leads are plugged in, making tight contact.
Solder a 3-inch length of wire to each socket connection, then solder
lugs to the ends of the wires.
Regardless of
its size, a single dry cell will deliver 1 1/2 volts. A large cell can
furnish more current than a small one, but that is the only difference
between them; their voltage output is the same. Ordinary flashlight
battery cells can be used to supply power for transistor circuits. They
can be connected in series, in which case their total voltage is the
sum of the voltages of all the cells used. For example, two series-
connected cells will furnish 3 volts; four cells will supply 6 volts,
etc.
To connect two cells in series, first fasten them
to gether with a couple of rubber bands or wrap them with
cellulose tape. Solder a long lead to the positive terminal of the
first battery; this represents +3 volts. Solder one end of a short wire
to the bottom (negative terminal) of the same battery, and the other
end to the positive terminal of the second battery. Another long wire
soldered to the bottom of the second battery represents -3 volts.
Any number of cells may be connected in the same way; join the positive
of one to the negative of the next, and so on. Manufacturers stack
small cells to make batteries capable of delivering high voltage. A
45-volt battery contains 30 cells; batteries with outputs of 67 1/2 and
90 volts contain 45 and 60 cells each.
Assemblies of flashlight cells should be strapped to the base by
attaching a piece of wire to terminals on both sides, as illustrated.
These are not electrical con nections; the terminals are simply used as
anchor points for the wire strap.
All large or heavy parts should be secured to the
base. In order to mount the variable capacitor you will need a small
piece of sheet metal about 1 inch wide and 2 inches longer than the
capacitor. The bottom of the capacitor has two threaded holes which
usually take 6-32 machine screws. Do not use screws more than 1/4 inch
long or they may interfere with the movement of the capacitor plates.
Drill a 3/16-inch hole near each projecting end of the metal strip and
use either wood screws or machine screws to fasten it to the base. The
illustration shows how variable capacitors are wired into circuits, and
also how they are mounted. The movable plates of a variable capacitor
are always connected, or grounded, to the frame. Since the frame is
fastened to the metal plate, all capacitor ground connections are made
to the plate. Connections to the stationary plates are made to a small
lug on the side of the capacitor.
Coils consist of turns of wire. Any size magnet wire
between No. 24 and No. 32 will do for the projects in this book. It may
have either coated or wrapped insulation. Coils with primary and
secondary windings are called transformers. Doorbell transformers and
output transformers are wound around laminated iron cores, and are
known as iron-core transformers. The iron core of a transformer is
shown on circuit diagrams by vertical lines between the primary and
secondary windings. The primary is usually on the left side of the
symbol, and the secondary is on the right. If the position of the
symbol is
changed, the primary and secondary will be clearly marked. Both
doorbell and output transformers have perforated feet, so that they can
be mounted on a base with screws.
In order to wind a coil you must have a form.
Suitable forms can be found very easily. The hollow cardboard cores
from either toilet tissue or paper towels make ex cellent coil forms.
You can make a better coil by winding it around a piece of wood about 1
inch thick, 2 inches wide, and 5 inches long; this coil will be used in
most of the receivers described in this book. The first step is to give
the form a coating of Duco cement. This applies to both cardboard and
wooden forms. The cement prevents them from absorbing moisture and also
serves as insulation. When the cement has dried thoroughly you are
ready to start the windings.
Make the first
winding 1 inch from the end of the form, and anchor it in place with a
strip of adhesive or cellulose tape. Wind about 3 inches of coil,
leaving 1 inch of the form exposed; anchor the last turn in place.
Leave about 12 inches of wire at each end of the coil, and solder each
to a lug. Windings must be tight and close together.
To mount a cardboard-core coil, bore a hole near
each end of the form where it will not interfere with the windings. The
coil can be fastened to a wooden base with Wood screws. If the holes
are made to coincide with those in perforated Masonite, the form can be
mounted with machine screws.
Wooden coil forms
can be mounted in two ways. Drill holes through each end of the form
and fasten it to your wooden base with long wood screws. Or, make
simple brackets of thin sheet metal, as shown in the illustration.
Screw one side of each bracket to the end of the form, and the other
side to the wooden base. Make the brackets long enough to enable you to
fasten them to perforated Masonite.
A ferrite
loopstick is a commercially made coil which is used in some of the
circuits in this book. It is very sensitive, and very selective when
used in a tuned circuit. This coil is not tapped by a sliding arm, as
is the wound coil. Instead, it is tuned by varying the position of the
f errite slug (a mineral composition), around which the coil is wound.
The coil windings are brought to two terminals. A short wire antenna
may be attached to one of them, in which case the other terminal is
connected to ground. The wire antenna may be removed, and either
connection used as antenna or ground; it makes no
difference. Loopsticks are usually furnished with small
mounting brackets, together with instructions for installation.
Vacuum-tube sockets should be firmly mounted to the base. When using
perforated Masonite or thin plywood, insert two long machine screws
from the bottom. Fasten these in place with a hex nut. Screw a second
nut to each screw, about 1/4 inch from the top, as illustrated. The
screws must be the same distance apart as the mounting holes in the
socket so the socket can be slipped over them. The socket rests on the
two hex nuts; a third set of nuts holds it in place.
Potentiometers should be mounted firmly, too. Make a bracket from a
piece of wire coat hanger, as shown in the illustration.
The operation of a loudspeaker depends upon the
fluctuation of a magnetic field. There are two types of speakers:
electrodynamic and permanent magnet. Do not attempt to use an
electrodynamic speaker. In the first place, a large supply voltage is
required to create the magnetic field needed to operate it; this is
impractical. The permanent-magnet type (abbreviated PM) has a powerful
alnico magnet built into it. A magnetic field is always present, and
high voltage is not needed to energize it. To tell whether your speaker
is a dynamic or PM type, hold a screwdriver against the center of its
back. If it is a PM speaker, the screwdriver will be strongly attracted
to the magnet An electrodynamic speaker will not attract metal.
Most modern radios use permanent-magnet speakers. They are inexpensive
and can be purchased locally at almost all radio shops. If you have a
small radio that is no longer needed at home, you can "cannibalize" it
by removing parts you need to construct circuits. Almost any speaker
can be used, provided it has not been mutilated.
An output transformer is a device that matches the output of an
electronic circuit to a loudspeaker. In small radios the output
transformer is usually mounted on the speaker frame. Do not try to
remove it, or you may damage the speaker. Loudspeakers must be handled
with care, as their paper cones are quite delicate and can be punctured
easily.
You will find four wire leads coming
from the output transformer. Two of these are soldered to terminals on
the loudspeaker. TTiese are the secondary transformer leads, which are
attached to the voice coil, a coil of very fine wire positioned around
the magnetic core of the speaker. Electrical energy causes the magnetic
field in the voice coil to fluctuate; this makes it move. The movement
of the voice coil is transmitted to the entire cone, producing sound.
Do not disconnect these leads. The two leads coming from the other side
of the output transformer are the primary leads. One is soldered to a
tube socket and the other to the power supply of the radio. Both of
these leads can be cut at these points, leaving as much wire as
possible attached to the transformer. It is a good idea to extend these
wires by soldering about 2 feet of hookup wire to each one. You can
then leave the speaker attached to the radio chassis, and even replace
it in its cabinet; this will improve its tone quality.
You may not have an old radio that can be taken apart, in which case
you will not have a speaker with an output transformer mounted on it.
The speaker and transformer will then be bought separately. It will be
necessary to identify the primary and secondary leads of the
transformer. These are usually color-coded by the manufacturer; primary
leads are red and blue, while secondary leads are black and green. It
is not necessary to mount the output transformer directly on the
speaker. This is done commercially in order to save space. Mount your
output transformer in any convenient location on the
baseboard and run leads to the speaker, which can be kept some distance away.
In all of our transistor circuits a doorbell transformer may be used as
an output transformer, with excellent results. The heavy wire
transformer leads are always connected to the primary winding. Two
terminals on the other side of the transformer are the secondary-coil
connections.
Transformers designed for use with
model trains make fine output transformers for transistor circuits. The
end that is usually plugged into the house current receptacle is the
primary. The secondary winding is the one that is attached to the train
tracks. Some train transformers have a control which can be used to
change their voltage output. Try the control in different positions to
see whether there is any difference in the tone produced in the
loudpeaker.
Chapter 6
ELECTRONICS EXPERIMENTS
Circuit No. 1:
Simple Crystal Detector Receiver
parts needed
type 1N34 crystal diode detector, or equivalent
headphones
antenna
ground
The simplest radio receiver you can make
consists of a crystal detector and a pair of headphones. These should
be connected to an antenna and a ground, as shown in the circuit
diagram. The acompanying pictorial illustration shows how the actual
connections are made.
If you hook up the parts properly you will get radio reception. Several
stations may come in at the same time, but this must be expected, since
the circuit contains no provision for separating signals. This simply
demonstrates that you can build a working radio with very few parts.
Circuit No. 2
Crystal-Detector Receiver with Hand-Wound Coil
parts needed:
crystal diode detector
headphones
hand-wound coil (see p. 57)
antenna
ground
Let's add a coil to our elementary receiver.
There will be an immediate increase in volume. Current developed in the
coil is also induced in the windings of the headphones, since they are
both connected in parallel. However, we still cannot separate incoming
signals; the receiver lacks selectivity.
Circuit No. 3:
Emergency" Crystal-Detector Receiver
parts needed
safety pin
hand-wound coil
pencil graphite
antenna
coin
ground
headphones
This experimental circuit shows how a radio
receiver can be quickly put together in an emergency. The only
commercially made part you need is a pair of headphones.
The detector, the most important part of the receiver, can easily be
improvised. A "cat's whisker" is made from a safety pin, and any coin
can be used instead of galena. Use pliers to bend the pin so that it
barely makes contact with the coin. The illustration shows how the
detector is mounted on a baseboard. If you are patient and make the
right contact in exactly the right spot, it will work.
You can improve the cat's whisker very easily. Sharpen a pencil, then
carefully cut away enough wood to expose 1 inch of lead. Break it off
and fasten it to the end of the safety pin with about a dozen turns of
bare wire.
Remove the coin and try several steel objects such
as razor and knife blades. You will find that most other metals also
make good detectors. Programs can be heard through scraps of aluminum,
galvanized iron, brass, copper, and silver. Do not brighten or polish
them in any way. The best results will be obtained when the pencil tip
rests against a tarnished surface. The exact pressure exerted by the
pencil-tip cat's whisker must be determined by experiment. Bend the
safety pin until it just barely makes contact with the metal detector.
Circuit No. 4
Crystal Detector Receiver with Tuned Circuit
parts needed:
crystal diode
headphones
hand-wound coil
variable capacitor (about 365uuF)
antenna
sliding arm for coil ground
We have improved the selectivity of circuit No.
3 by adding a variable capacitor. The combination of the capacitor and
coil makes a tuned circuit. Varying the capacitance changes the
frequency to which the tuned circuit is particularly sensitive. If we
vary the coil's characteristics, or inductance, we can make the
receiver even more selective. This can be accomplished by making a
sliding arm which taps the coil as the arm is moved across the top
surface of the coiL The arm is made from a piece of wire coat hanger or
other stiff wire, shaped as shown in the illustration. Bend the wire
with a pair of pliers, then
scrape all the paint from the ends of the shaped piece, exposing bright
metal underneath. Mount one end of the arm on the baseboard. The other
end should be adjusted so that it scrapes the top of the coil. Wipe it
across the coil a few times, leaving a mark on the insulated wire.
Scrape all the insulation from the coil windings at those places that
have been touched by the arm. Use a knife blade, sandpaper, or emery
cloth. Move the arm across the coil: it must make firm contact with the
bare wire. If it seems loose, remove it from the coil and reshape it
with your pliers.
This simple tuned circuit will enable you to separate stations to
some degree, but you must not expect perfect selectivity. Nearby
stations may create strong interference.
Circuits Nos. 5, 6, 7, 8, 9:
Simple Transistor Hookups
parts needed
1 transistor, any type
antenna
headphones
ground
Transistors are somewhat like crystal diodes;
they are also made of germanium, a semiconductor (see page 47). Instead
of having two leads like a diode, a transistor has three leads. These
are identified as the collector (c), base (b), and emitter (e). Since
the transistor is a semiconductor, it will also function as a detector.
Simple hookups can be made using a transistor and a pair of headphones;
they must be connected to an antenna and ground. Any transistor may be
used; both P-N-P and N-P-N types will work. These circuits will deliver
varying degrees of volume; try them all.
Circuit No. 10
Single Transistor Receiver with Hand Wound Coil
parts needed
type 2N107 P-N-P transistor,
or equivalent
headphones
hand-wound coil
antenna
ground
This is the next logical development in the
construction of a transistor receiver. Current picked up by the antenna
is fed into the coil, which forms a simple, nonvariable tuned circuit
The transistor acts as a detector and also amplifies the signal
slightly. There should be an increase in volume over circuits Nos. 5 to
9.
None of the circuits described up to this point requires power
supplies. No batteries or other source of current has been needed. We
have been able to pick up electromagnetic waves with the antenna and
convert them into sound. Battery power supplies are used in the
circuits that follow.
Circuit No. 11
Crystal-Detector Receiver with Transistor Amplifier
parts needed
type 2N107 P-N-P transistor, or equivalent
1N34 Crystal diode
variable capacitor
.02uF fixed capacitor
220,000-ohm resistor
SPST (single-pole single throw toggle switch)
3-volt power supply
headphones
antenna
ground
In this circuit a single-transistor amplifier
has been added to the crystal-detector receiver shown in circuit No. 4.
A resistor is also added to limit the current applied to the base of
the transistor. An N-P-N transistor such as 2N170 may be substituted
for the P-N-P type, in which case the battery leads must be reversed.
The transistor acts as a current amplifier and will make reception
considerably louder. There should be enough volume to drive a
loudspeaker. When connecting the loudspeaker, remember that the
secondary of the output transformer is connected to the voice coil, and
the primary leads go to the phone terminals. Not all output
transformers will work equally well. They have different
characteristics, depending on the type of tube for which they were
originally designed. Special transformers have been designed for use
with transistors, but we are using components that are available
everywhere. If you have more than one transformer, use the one that
provides the loudest signal.
I t is important to observe the correct polarity
(connections of + and - leads) when hooking up the power supply, or
the transistor will be damaged. A 3-volt supply (two flashlight cells
in series) will work in this circuit.
The
diagram shows a switch, which is convenient but not essential. Just
disconnect one of the power supply leads when the set is not in use.
Do not expect the volume of this receiver to be
comparable to that of a commercially built radio. Strong local stations
will be heard very well, while distant or weak stations will come
through with decreased volume. This is an experimental circuit, the
purpose of which is to show that a single-transistor amplifier can
develop enough power to drive a loudspeaker.
Circuit No. 12:
Two-Transistor Receiver
parts needed
hand-wound coil
4000-ohm resistor
SPST switch
variable capacitor
2 type 2N107 P-N-P transistors, or their equivalents
headphones
antenna
3-volt battery power supply
ground
In this circuit the first transistor is used as a
detector amplifier. It is coupled directly to the second
transistor which acts as an audio amplifier. In Other words, the
first transistor not only detects the incoming signal but also
amplifies it. The amplified signal is fed from the collector of the
first into the base of the second transistor, where it is amplified
further. From here it goes to the phones, where the electrical impulses
are converted to sound.
Reception will be much
louder than that provided by the preceding circuit. Loudspeaker
connections are indicated on the circuit diagram. Use a single-pole
single-throw (SPST) switch.
Circuits Nos. 1 to 12
have been shown in both pictorial and diagrammatic form so that
beginners can become familiar with construction practices. Experienced
radio men never use pictorial illustrations. Anyone who wants to work
with electronics must learn to interpret diagrams. For this reason,
only circuit diagrams will now be given.
Circuit No, 13:
Two-Transistor Receiver
parts needed:
hand-wound coil
variable capacitor
1N34 crystal diode
type 2N170 N-P-N transistor, or its equivalent
type 2N107 P-N-P transistor, or its equivalent
4700-ohm resistor
100uuf fixed capacitor
25uf fixed capacitor
headphones
6-volt power supply
SPST switch
antenna
ground
The signal is detected by the crystal diode,
amplified by the N-P-N transistor and coupled directly to the P-N-P
transistor, which acts as a second amplifier. Direct coupling was used
in circuit No. 12, but coupling from one P-N-P transistor to another is
not too effective. A much greater gain, or increase, in signal strength
can be obtained by feeding the signal from an N-P-N transistor into a
P-N-P type. Volume should be
increased in this circuit, so that a loudspeaker can be used to good
advantage. If you do not already know them, refer to other circuits for
loudspeaker connections.
Certain precautions
must be observed when using both P-N-P and N-P-N type transistors in
the same circuit. They are not interchangeable and can easily be ruined
if they are accidentally switched. Also, make it a practice to see that
the switch is in the off position before inserting transistors into
their sockets.
Circuit No. 14
Two-Transistor Receiver
Parts Needed:
hand-wound coil
variable capacitor
1N34 crystal diode
5600-ohm resistor
100uuf fixed capacitor
headphones
2N107 P-N-P transistor, or its equivalent
6-volt power supply
2N170 N-P-N transistor, or its equivalent
SPST switch
antenna
ground
This circuit is essentially the same as the
preceding one. The only difference is that in this case a P-N-P
transistor is used in the first (detector-amplifier) stage, feeding the
signal into an N-P-N type, which is the audio amplifier.
The use of the 100uuf capacitor is optional Try the
receiver with and without it; omit it if it doesn't seem to be needed.
Attach a 3- or 4-foot length of wire instead of an
antenna and operate the receiver without an external ground; you may be
surprised at the results. For best reception, you will, of course, need
both an outside antenna and a proper ground.
Circuit No. 15
Single-Transistor Regenerative Detector Receiver
parts needed :
hand-wound coil with tickler winding
variable capacitor
2N107 P-N-P transistor, or its equivalent
5,000- to 500,000-ohm potentiometer
500,000-ohm resistor
.05uf fixed capacitor
.001uf fixed capacitor
100uuf fixed capacitor
headphones
SPST switch
3-volt power supply
antenna
ground
The first step is
to modify the coil by adding a "tickler " winding. Remove the coil from
the baseboard and wind 15 turns of magnet wire over the coil 1/4 inch
from the end that is to be connected to ground. Fasten the winding in
place with a strip of tape. Leave 12-inch leads at the ends of the
tickler. Refasten the coil to the base board.
The signal is intercepted by the antenna and carried to the tuning
circuit, where the desired station is selected. From here it goes to
the transistor. Emerging from the collector, it goes through the
tickler coil, which is magnetically coupled to the main coil.
Transformer action takes place between the tickler and the large coil.
Current flowing in the tickler induces current in the other coil,
adding to the incoming signal strength. The strengthened signal is fed
into the transistor, and again fed back through the tickler circuit.
This feedback cycle is repeated over and over, and is known as
regeneration.
If regeneration is unchecked, too much signal energy
will be developed and it will "spill over" or oscillate. Oscillation
will be heard as a whistle, buzz, hiss, or other sound distortion. The
potentiometer acts as a regeneration control.
Regenerative receivers must be tuned carefully.
First tune in a station by adjusting the coil arm and the variable
capacitor. Move the arm to the point where the loudest signal is heard,
and leave it there. From now on all tuning is done with the capacitor.
Advance the regeneration control until you hear oscillation sounds. If
none are heard, it is a sign that regeneration is not taking place; the
tickler leads must be reversed. Final adjustment is accomplished by
turning the regeneration control so that it is just below the point of
oscillation.
Regenerative receivers are very sensitive to weak
signals and provide greater selectivity than many other types.
Circuit No. 16
Two-Transistor Regenerative Detector Receiver
parts needed:
coil with tickler winding
variable capacitor
1N34 crystal diode
2 type 2N107 P-N-P transistors, or their equivalents
2 .05uf fixed capacitors
5ooooo-ohm potentiometer
headphones
3-volt power supply
SPST switch
antenna
ground
This circuit is essentially the same as the
preceding one, except that two transistors are used. Volume and
selectivity should be increased, so that a speaker can be hooked up
with good results.
Circuit No. 17:
Solar Battery Power Supplies
A photocell is a small metal unit, one side of
which is coated with selenium. When light strikes the selenium layer it
Is converted to electrical energy. One type of inexpensive cell may be
bought which will produce .5 volt of electricity when exposed to
average sunlight. The cells output voltage is decreased under light of
less intensity, so do not expect the same results when it is held under
a 100-watt lamp. Other types of cells use silicon or
cadmium sulfide instead of selenium. Photocells (also called sun cells
or solar cells) can be used instead of batteries to supply power for
any of the transistorized devices shown in this book. They are rugged
and will last indefinitely as they do not wear out. However, more than
one must be used, since .5 volt is not enough to energize a transistor
with any degree of efficiency. At least 1.5 to 2 volts is needed. Cells
can be hooked up in series to produce any desired voltage. Each unit
has a red (positive) lead and a black (negative) lead. Series
connections are made by soldering together the red and black leads of
adjoining cells. Four photocells will furnish 2 volts. When making up a
power supply unit, screw the cells to a small base, with the selenium
surfaces all facing the same way. Several transistorized portable
radios which are powered by solar batteries have appeared on the
market. These will work anywhere provided there is a light source to
which the power pack can be exposed. Since the solar cells have an
indefinite life, the power supply is virtually permanent.
Numerous other uses have been found for solar cells, particularly in
automatic devices. They are used in traffic control systems, automatic
headlight dimming mechanisms, factory-line inspection systems,
amusement devices, burglar alarms, door controls, garage door openers,
and automatic counters.
Photocells are used in
photographic exposure meters. Light is gathered by a lens and
concentrated on the cell, which is connected to a sensitive meter. The
meter is usually calibrated to show lens-stop openings instead of
current.
Circuit No. 18
Sound Amplifier
parts needed:
2 permanent-magnet loudspeakers, about 4 inches
in diameter
type 2N170 N-P-N transistor or its equivalent
50,000-ohm resistor
2 output, doorbell or modeltrain transformers
6-Volt power supply
SPST switch
type 2N107 P-N-P transistor, or its equivalent
In this circuit a small permanent-magnet speaker is used instead of
a microphone. If you talk into the input speaker, your voice will be
heard coming from the output speaker. The amplifier is the same as the
one used in circuit No. 14, where it amplified the broadcast signal
selected by the tuning circuit. In this circuit it will amplify the
sound fed into the input speaker.
This is how the circuit works: When you talk into the input speaker, sound waves generated by your voice
cause the cone to move. As the cone moves, it also moves the voice
coil, which is centered around a powerful permanent magnet. You will
remember that moving a coil through a magnetic field generates electric
current.
This current is transferred from the
voice coil to the secondary winding of the transformer. Current is
induced in the primary and fed into the base of the P-N-P transistor,
where it is amplified. It then passes into the base of the N-P-N
transistor where it is further amplified, and then through the primary
of the output transformer. Current induced in the secondary winding
causes the voice coil to move. Since the voice coil is attached to the
speaker cone, sound is generated.
Notice that the output transformers are connected in
the same way: the secondary windings are connected to the speaker voice
coils in both cases; the primary transformer windings go to the
circuit. In this circuit and the next one amplification will take place
depending upon the transistors used and the efficiency of the input and
output transformers.
Circuit No. 19:
Room-to-Room Communicator
parts needed:
circuit No. 18
DPDT(double-pole double-throw) switch
The amplifier described in circuit No. 18 can be
used as a room-to-room communicator in your home. All you have to do is
add a double-pole double-throw switch. The first step is to connect
switch terminals A and C, and B and D, as shown in the diagram. Use
insulated wire.
Disconnect the primary
transformer lead that goes to the base of the P-N-P transistor, solder
a piece of wire to it. and connect it to switch terminal X. Use another
wire to connect switch terminal B to the base of the P-N-P transistor.
Disconnect the primary transformer winding that is
connected to the collector of the N-P-N transistor, solder a piece of
wire to it, and connect it to switch terminal Y. Connect switch
terminal A to the collector of the N-P-N transistor with another wire.
To use the amplifier as a communicator, one speaker and transformer are
placed in a remote position. This may be another room or another floor
of your house. Two long wires will have to extend from the speaker
position to the amplifier. One wire runs between the transformer
primary and a terminal on the DPDT switch. The other wire connects the
other end of the transformer primary to the positive power supply
terminal.
Either speaker may be placed at a
distance; it makes no difference. Any wire may be used. However, the
switch should be mounted on the baseboard so that it is easily
accessible.
When the switch is in one position,
one of the speakers is the microphone input and the other is the sound
reproducer. When the switch position is reversed, the functions of the
speakers will be reversed. The one that was formerly the input speaker
will now be in the output circuit. You cannot talk and receive at the
same time.
Mark the switch positions so that one is labeled "talk" and the other "receive".
Cigar boxes make good cabinets for both the basic unit and the remote speaker.
Circuit No. 20:
Wireless Broadcast Oscillator
parts needed:
home-wound coil, or ferrite loopstick
variable capacitor
P-N-P transistor
500,000-ohm potentiometer
RF choke
25 to 50uuf fixed capacitor
2. 005uf fixed capacitors
PM speaker or headphone(s)
SPST switch
6-volt power supply
6-foot wire antenna
This oscillator is a miniature transmitter. It
sends out a signal, such as your voice, that can be picked up by any
nearby broadcast receiver. Audio (AF) signals are those which have a
frequency of from 15 to about 20,000 cycles. Above 20,000 cycles they
are known as radio frequencies, or RF. The oscillator generates an RF
signal which falls somewhere between 550 and 1650 kilocycles, the range
of the average radio receiver. This is the carrier wave, and may he
heard in the receiver as a loud hiss.
A
permanent-magnet speaker is used as a microphone. It is connected to
the circuit through whatever type of transformer you happen to have at
hand. Single or double headphones can also be used as a microphone.
Their leads are connected directly to the input of the oscillator. No
transformer is needed.
When you speak into the microphone, an audio signal
is generated. This is mixed with the carrier wave and broadcast
through the antenna. The transmitted electro magnetic wave now has two
components: the RF carrier and the low-frequency audio current.
If you examine the circuit, you will see the familiar combination of a
coil and a tuning capacitor, which make up a tuned circuit. This is
adjusted to vary the frequency of the transmitted signal so that it may
be picked up at different points on the receiver dial
You will also see that a 6-foot piece of wire is used as a transmitting
antenna. Do not connect this oscillator to an outside or other
long-wire antenna, or your broad casts may be picked up by your
neighbors. Even though we are using a flea-powered transmitter, its
range can be extended when it is coupled to an efficient antenna. This
would be a violation of Federal Communications Commission regulations.
Almost any transistor can be used in this circuit. However, some may
oscillate better than others. If you have more than one, try them all.
Remember to reverse the battery leads when using N-P-N types.
Although you can use your home-wound coil, results will be far better
if you use a ferrite loopstick, already described.
Construct the circuit and hook up the transmitting
antenna. Place the oscillator within a few feet of a radio receiver,
and turn them both on. Tune the receiver carefully, listening for the
hiss that indicates you are receiving the transmitted signal. Now turn
off the oscillator; if the hiss can no longer be heard in the receiver,
you may be certain you have picked up the proper signal.
If you cannot pick up the oscillator's signal, try it another way. Set
the receiver at an unused portion of the band where there is no
station, somewhere near the center of the dial. With the oscillator on,
adjust the tuning capacitor until the signal is brought in. If you are
using a home-wound coil and you cannot tune in the signal, it may be
necessary for you to substitute a ferrite loop-
stick. This can be tuned by screwing the ferrite slug in and out of
position. The sharp tuning circuit formed by the loopstick and variable
capacitor should bring in the signal at almost any point on the
receiver dial.
Now talk into the microphone; your voice should be heard coming out of
the receiver. Adjust the potentiometer to control the volume and clear
up any distortion that may be present.
It is most important that you select an unused
portion of the receiver band. If there is any incoming signal at that
point the oscillator's output will combine with it to produce a third
signal, or heterodyne. This will distort your voice; the only way to
get rid of a heterodyne is to shift the frequencies of both the
oscillator and the receiver.
The circuit contains a radio-frequency choke, marked RFC in the circuit
diagram. This is a small coil with fine windings. Its purpose is to
restrict the flow of alternating RF currents, while permitting direct
current to pass through.
The oscillator can
also be used to broadcast phonograph music. You can play records
without using an amplifier and have the sound come out of the radio.
However, the arm of your record player must have either a crystal or
ceramic pickup cartridge. Magnetic pickup cartridges do not generate
enough current. A shielded wire usually runs from the pickup arm to the
amplifier of the record player. This must be disconnected at the
amplifier. The inner wire is then connected to the base of the
transistor. Solder a wire to the flexible metal wire shield, and
connect it to a .05/ capacitor. Connect the other end of the capacitor
to the "B" plus supply. Turn on the oscillator, and place the pickup
arm on a record; the music will be broadcast.
Chapter 7
HOW A VACUUM TUBE WORKS
Vacuum tubes perform many functions in
electronic circuits. They are used as detectors, amplifiers, and
generators of high-frequency currents. We know that the flow of current
in a conductor is caused by the movement of free electrons. At ordinary
temperatures the electrons remain within the conductor. However, as the
temperature of the conductor is raised, the velocity, or speed, at
which the electrons travel is also increased. If the conductor is
heated to incandescence, electrons can actually fly off into space.
This is called thermionic emission.
Within some types of vacuum tubes electrons are given off by a thin
wire called the filament; these are directly heated tubes. If a
filament is heated in air. it will soon disintegrate. It has been found
that when emission takes place within a vacuum, the filament will not
burn up because no air is present. Consequently, the air is removed
from all vacuum tubes.
In other types of tubes the filament heats another
metallic surface known as the cathode, which becomes the
electron-emitting element; these are indirectly heated tubes. Any
surface that gives off electrons within a tube is called the cathode.
The cathode is connected to a source of current, either directly or
indirectly. As current flows through it, it becomes heated so that it
glows. Some of the free electrons gain enough energy to fly off into
surrounding space. However, they tend to cluster around the cathode in
a cloud. These electrons in the vicinity of the cathode form a negative
charge; this is called a space charge.
Particles with similar charges repel each other, and oppositely charged
particles attract each other. As new electrons are given off by the
cathode, the negative space charge tends to repel the new arrivals,
pushing them back on to the surface of the cathode.
The simplest form of vacuum tube contains two elements. It is known as
a diode. In addition to the cathode it has a plate, which is actually a
small metal plate. When a positive charge is placed on the plate, it
attracts negatively charged electrons within the tube. Electrons move
from the cathode to the plate, causing current to flow.
The moment the positive voltage is removed from the plate, the flow of
current stops. If a negative charge is placed on the plate, it will
repel electrons and no current will flow.
The
flow of current within the tube can be increased by raising the plate
voltage. The greater the positive charge on the plate, the more
electrons it will attract. However, the flow of electrons can be
controlled more easily by inserting a third element between the cathode
and the plate. This is the grid, an open winding of fine wire. A tube
that contains a cathode, grid, and plate is called a triode. The grid
acts as an electronic valve that controls the flow of current between
the cathode and the plate. In order to function, it must be supplied
with either positive or negative voltage. When the grid has a high
negative charge it repels all the electrons emitted by the cathode and
does not permit any to get through to the plate; there is no flow of
current. As the negative voltage or bias is decreased, more electrons
reach the plate and there is an increased flow of current When the grid
is at zero potential (no voltage applied) it has no effect whatsoever;
the flow of current between cathode and plate is uninterrupted. When
the grid is positive it helps the plate to attract electrons from the
cathode; the current flow is increased. Up to a certain point an
increase in positive grid voltage will result in an increase in plate
current.
Thus, the polarity (positive or
negative condition) of the grid voltage controls the flow of current
within the tube. For this reason it is often called the control grid.
A small variation in grid bias causes a much larger variation in plate
current. If we apply a weak signal to the grid, it will be
strengthened, appearing as a strong signal at the plate. This makes it
possible to use a vacuum tube as an amplifier in a receiver. The
alternating-current RF signal intercepted by the antenna is fed into
the grid. It then appears as a greatly amplified signal at the plate.
A tetrode is a tube that contains four elements. It has a cathode, a
control grid, a plate, and an additional grid called the screen grid,
which is located between the control grid and the plate. The screen
grid acts as a shield around the control grid. Since it is supplied
with positive voltage, it helps draw electrons to the plate.
Electrons emitted by the cathode travel at great speed. Some of them
strike the plate with enough force to dislodge other electrons, which
are thrown into space. Since the screen is positively charged, these
secondary electrons are attracted to it This causes secondary emission,
a condition in which a reverse flow of current takes place from plate
to screen.
The undesirable effects of secondary emission are
overcome by inserting a third grid, the suppressor grid, between the
screen and the plate. This suppressor, which is connected to the
cathode, acts as a shield and prevents the screen from attracting
secondary electrons. These electrons are now forced back to the plate.
Tubes that contain five elements (cathode, control grid, screen grid,
suppressor grid, and plate) are called pentodes.
A wire is brought from each tube element to a
terminal pin in the base. Tubes are made to fit special sockets. Each
type of socket accommodates tubes with a different number of pins. The
socket is wired into the circuit, so that tubes can be inserted or
removed whenever necessary.
Tube diagrams must be interpreted carefully. The
grid nearest the cathode is always the control grid. If a tetrode is
shown, the screen is always indicated as a second grid, placed between
the control grid and the plate. In a pentode the suppressor is shown as
a third grid placed between the screen and the plate.
Circuit diagrams sometimes show numbers next to the
symbol for each element. These numbers refer to the pins at the bottom
of each tube. To interpret tube-pin numbers, look at the bottom of the
tube. Miniature glass tubes, the type used in this book, do nof have
bases. Instead, the terminal pins are brought directly out of the glass
tube envelope. Two pins are spaced wider apart than the others; the one
at the left is always pin No. 1. The other pins are numbered clockwise,
from left to right.
Tube socket terminals are numbered in the same way. Be sure to look at the bottom of the socket.
Circuit No. 21:
Vacuum-Tube Grid-Leak Detector Receiver
parts needed:
type 3V4 tube
.05uf fixed capacitor
7-pin miniature tube socket
SPST switch
f errite loopstick
variable capacitor
headphones
1- to 5-megohm resistor
100uuf fixed capacitor
.00luf fixed capacitor
l 1/2 volt "A" battery (filament supply)
22 1/2volt to 67 1/2-volt "B" battery
antenna
ground
Although transistors in many respects are
superior to vacuum tubes, this circuit is included as an example of a
simple, easy-to-build vacuum-tube receiver and to show how a tube
works. You will notice that this circuit does not make use of a
crystal-diode detector. Instead, detection takes place in the grid of
the tube. The resistor connected to the grid is known as a grid leak.
Incoming signal voltage must
pass through the grid leak; as it does, some of it is consumed as it
forces its way through the high resistance it encounters. This is known
as a voltage drop. As the signal varies, so will the voltage drop
across the resistor. This causes the grid voltage to vary. In turn, the
plate current varies in step with the changing grid voltage. Since the
plate current flows through the headphones where it is converted to
sound, it can be seen that the voltage drop across the grid leak is
responsible for the receiver's output.
Grid-leak detectors are very sensitive and are good at bringing in weak
signals. They ordinarily use resistors of from 1 to 5 megohms. Their
chief disadvantage is that they are easily overloaded, or distorted by
strong signals. This condition can be improved to some extent by
reducing the size of the grid-leak resistor. While this will cut down
overloading, it will also reduce the sensitivity of the circuit.
This receiver should provide fine reception with head phones, but
it may not operate a loudspeaker satisfactorily.
A type 3V4 pentode is used as a detector-amplifier. A triode can be
used just as well, but the pentode provides an increased gain in signal
strength. If you do not have a 3V4 tube, try a 1AE4, 1AF4, 1AJ4, 1L4,
1U4, 1T4, or a 3E5. The 3E5 may be substituted for the 3V4 without any
circuit changes. When using the other tubes, a change must be made in
the filament wiring. Pin No. 1 will be connected to "A" +; pin No. 7
will go to ground; pin No. 5 is not used.
The
tube filament is heated by a 1 1/2-volt power supply. You can use
either 1 flashlight cell or a large 1 1/2-volt battery. The filament
battery is always known as the "A" battery. Plate voltage is supplied
to the tube by a "B" battery, of the type used in portable radios. It
may be rated at anywhere from 22 1/2 to 67 1/2 volts.
The negative terminals of the "A" and "B" batteries are connected to
one of the switch terminals. The other side of the switch is grounded.
When the switch is closed, current will flow in both the A and B
(filament and plate) circuits.
Circuit No. 22:
One-Tube Regenerative Receiver
parts needed:
type 3V4 tube
7-pin miniature socket
ferrite loopstick with tickler winding
variable capacitor
loudspeaker and output transformer, or headphones
1- to 5-megohm resistor
100,000-ohm resistor
.05uf fixed capacitor
.001uf fixed capacitor
100 uuf fixed capacitor
RF choke
50,000-ohm potentiometer
SPST switch
lX-volt "A" battery
(filament supply)
22 1/2-volt to 67 1/2-volt "B" supply
antenna
ground
Make a tickler by winding 15 turns of magnet
wire directly over the loopstick coil. Fasten the winding in place with
a strip of tape. Leave about 12 inches of wire extending from each end
of the tickler.
The construction of this
circuit is almost identical to that of the preceding one. The only
difference is that a tickler coil and a few parts have been added so
that the receiver will operate as a regenerative detector.
As in circuits Nos. 15 and 16, regeneration takes place when some of
the output voltage is fed back into the input circuit so that it
reinforces the input voltage.
The RF choke in the tickler-plate circuit prevents
the high-frequency RF alternating current from flowing through the
headphones. This current is by-passed to ground through the .001uf
capacitor, which provides an easy path.
A
100,000-ohm resistor is shown connected to one end of the
potentiometer. Construct your circuit without it If you cannot control
regeneration, put it in.
Hook up the circuit. Place the tube in its socket
and close the switch. Tune the variable capacitor with the regeneration
control (potentiometer) in different positions. You should hear
hissing, squealing, or other signs of oscillation as you tune through
stations. If these sounds are not heard, reverse the ticlder leads.
Bring in a station with the variable capacitor. Adjust the f errite
slug in the loopstick so that best volume is obtained; leave the slug
in this position. Advance the regeneration control until oscillation is
heard. Readjust the variable capacitor for greatest volume, then back
off the regeneration control until the oscillation just disappears.
This is the position for best reception.
Regenerative-detector circuits are very sensitive
and are capable of receiving signals over extremely long distances
under good conditions.
Chapter 8
HOW TO LEARN THE CODE
Circuit No. 23:
Code Practice Oscillator
parts needed:
doorbell, output or model
train transformer
1.5- to 3-volt power supply
transistor, any type
500,000-ohm potentiometer
elegraph key
This is an audio oscillator. It is not a receiver,
as it has no provisions for receiving, detecting, and amplifying a
broadcast signal. Its sole purpose is to provide an audible tone in a
pair of headphones or a loudspeaker.
Construct the circuit as shown. Almost any
transistor can be used. The key acts as a switch, permitting current to
flow in the circuit only when it is closed. If you do not have a key,
you can make one very easily from a strip of sheet metal, as
illustrated.
When the key is depressed, a tone should be heard in
the headphones. The pitch of the tone can be varied by adjusting the
potentiometer. If no tone is heard, turn the potentiometer all the way.
If there is still no tone, reverse the secondary transformer leads;
oscillation may not be taking place. Check all connections; there may
be a break in the circuit.
Some output transformers will work only when they
are connected in reverse. Hook up the primary leads where the secondary
leads should be, and vice versa.
This oscillator will help you to become proficient
in sending and receiving code. Radio amateurs use the Continental
(International Morse) Code. You can practice in privacy at any time
without disturbing anyone by using headphones.
It's easy to learn to send code; receiving code is
more difficult. If you hook up a loudspeaker instead of phones (through
a transformer, of course), a group of people can practice at the same
time. Take turns sending and receiving.
The best way to learn the code is by thinking of
each letter as a combination of sounds, rather than dots and dashes.
Think of each dot as a dit, or short tone, and each dash as a dah, or
longer tone. Here are the letters arranged in groups; learn one group
before going on to the next:
E dit
I dit-dit
S dit-dit-dit
H dit-dit-dit-dit
5 dit-dit-dit-dit-dit
A dit-dah
U dit dit dah
V dit dit dit dah
3 dit dit dit dah dah
4 dit dit dit dit dah
R dit-dah-dit
W dit-dah-dah
L dit-dah-dit-dit
P dit-dah-dah-dit
J dit-dah-dah-dah
1 dit-dah-dah-dah-dah
T dah
N dah-dit
D dah-dit-dit
B dah-dit-dit-dit
6 dah-dit-dit-dit-dit
F dit-dit-dah-dit
2 dit-dit-dah-dah-dah
? dit-dit-dah-dah-dit-dit
M dah-dah
G dah-dah-dit
Z dah-dah-dit-dit
Q dah-dah-dit-dah
, dah-dah-dit-dit-dah-dah
7 dah-dah-dit-dit-dit
K dah-dit-dah
C dah-dit-dah-dit
X dahdit-dit-dah
8 dah-dah-dah-dit-dit
9 dah-dah-dah-dah-dit
0 dah-dah-dah-dah-dah
End Of Message (EOM) dit-dah-dit-dah-dit
Period (.) dit-dah-dit-dah-dit-dah
INDEX
aerial, see antenna
amplifier
amplifiers, how to build:
room-to-room communicator,
sound amplifier
antenna
indoor
arrestor, lightning
atoms:
structure of
audio frequencies
audio signal
base
battery:
solar, 85-87
bias, 100
capacitance, of electronic circuit
capacitors
carrier wave
modulated
cathode
cat s whisker detector
charge, space
choke, radio-frequency
circuit, electrical
circuit, radio
diagrams of
transistor
tuned
Code. Continental
code practice oscillator, how to build
coding, color:
of resistors
of transformer leads
coils
inductance of,
voice coil
communicator, room-to-room, how to build
components, radio
conductor
semiconductor
connection
Continental Code
crystal diode
current, electric
alternating
direct
frequency of
cycle, of electric current
demodulation
detection
detector
diagram, circuit
diode
electricity (see also current)
electromagnet
electromagnetic waves
electron theory
electrons
elements
emission:
secondary
farad
field, magnetic
filament
fixed capacitor
flux
free electrons
frequencies, radio
galvanometer
grid
grid leak
screen grid
suppressor grid
ground
ground clamp
ground rod
headphones
heterodyne
hexagonal nuts
high-impedance headphones
hookup wire
impedance
inductance
induction
insulators
International Morse Code
ionization
ions
irons, soldering
key, telegraph
lamp
lead-in wire
leads, wire
of transformers
of transistors
lightning arrestor
lodestone
loopstick, f errite
loudspeaker
lugs, soldering
machine screws
magnet wire
magnetic field
magnetism
megohm
microfarad
modulated carrier
molecules
Morse Code
nucleus, atomic
ohm
oscillators, how to build:
code practice oscillator
wireless broadcast oscillator
output transformer
pencils, soldering
penknife
pentode
permanent-magnet loud speaker
phones
photocell
"pigtails" see leads, wire
pliers
PM loudspeaker
potentiometer
mounting of
push-back wire
radio frequencies
radio-frequency choke
receiver
receivers, how to build:
crystal-detector receiver with hand-wound coil
crystal-detector receiver with transistor amplifier
crystal-detector receiver with tuned circuit
"emergency" crystal-detector receiver
one-tube regenerative receiver
simple crystal-detector receiver
single-transistor receiver with hand-wound coil
single-transistor regenerative detector receiver
two-transistor receiver
two-transistor regenerative detector receiver
vacuum-tube grid-leak detector receiver
rectifier
resistance
resistor
grid leak resistor
RF, see radio frequencies
screen grid
screwdrivers
screws, machine
secondary emission
selectivity
selenium
semiconductor
series, battery cells in
socket, vacuum-tube
socket wrenches
solar cell
solder
soldering
soldering gun, electric
soldering iron
soldering lug
soldering pencil
space charge
speaker
suppressor grid
switch
terminal
tetrode
thermionic emission
tickler
tinning
tools
transformer
output transformer
transistor
leads of, 52-53
transistor receivers, how to build
crystal-detector receiver
simple transistor hookup
single-transistor receiver with hand-wound coil
single-transistor regenerative detector receiver
two-transistor receivers
two-transistor regenerative receiver
transmitter
broadcast oscillator
Mode
tube, vacuum
tuning
vacuum tube
variable capacitor
voice coil
voltage drop
waves, radio
wire
hookup, 30
lead-in, 34
wrenches, socket
zero potential
About the Author
Harry Zarchy has lived in and around New York all his life. For
many years he has taught fine arts, ceramics, and crafts in New York
City high schools and to adult groups as well. He is a man with
an amazing number of interests. Fishing, model trains, and music are
some of them. He has played the violin, the banjo, guitar, cello,
trumpet, and is a free-lance bass player. Working once as a watch
maker, he developed a curiosity about old timepieces, which led to a
book, Wheel of Time, He has also worked as a house painter, carpenter,
cabinetmaker, counter man in a restaurant, and waiter. His house
is full of electronic gadgets, such as an intercom system and an
amateur radio station. He is a licensed "ham" operator and likes to
relax by chatting with other amateurs. Mr. Zarchy makes his home on
Long Island with his wife and two children.
