'+' " " ... ' ')' •• 'l.

Series 724

•

There are now o\cr )50 Ladybird lilies co\'ering a "ide range of subjects and reading age~. Write for a free illustrated catalogue from Ihe publishers LADYBIRD BOOKS LTD LoUl/,hborOUih Le,CC:Slcl'l>hirc. EnaJand

• 15p

o nl4 0'24 7 NET •

a ~ ""'·-r Radio

" , TECHNIQUES

MATERIALS REQUIRED

S oldBr Wire with II COre of flux

Electric Soldering Iron. 25 wett with pencil bit IS ideal

.......-~~ ._-"-

'1 Clsln both surfaces to be joined with sandpaper or (hit edge of 8 kmfe_ The metal surfaces should be clB,n and bright

2 ) Join the wire to the tag to make 8 firm joint that will not move

3) Put the hot bit of the iron firmly upon the j oint and allow a few seconds for the joint to hea t up. Take care. The soldering Ifon bi t and metal shaft

4) Lightly push the end of the solder onto the joint and allow the solder to flow freely over the whole joint. DO NOT apply the solder to the bi t the heat of the joint should do the melling .

5} Allow the solder to cool until it goes solid.

GOOD SOLDER JOI NTS should be bright and shinY. If the joinl looks very dull and grey it may be a bad Jomt called a DRY JO JNT. If this IS so. remake the JOint. Components are usually soldered to copper tags. It is possible 10 replace the screw cups of the radio Wi th small copper nails and solder t he components onto these. W rap the wlfes around the nail before soldering so that the JOint is made firm.

Series 724 How to O1;ll-..e it

With Jup£'rh coloLir diagrams alld all equally eas)'-to~/ollolI' texl, Ihis fascillating book shows hoI\' 10 make a tran'iis/or radio capable of receh'illg a IIIl1uher of stmiom.

First to be cle.\uihed is"a simple crystal sef Ihal IWllly , .. orks. Srage-hy-srage the sel is added 10 (each slage a complete workillg radio) until a powerful/ral/sistor radio is completed.

Easily, illexpellsil'el.v, and leamillg as YOII go, as each sec/ioll is added YOIl II'ill experience the thrill, shared by all radio constructors, of listeniflg to SOUl/cis coming fr011l a radio receil'er that is YOllr 011'1/ , .. ork.

A ladybird ·How 10 make if book

making a Transistor Radio by G. C. DOBBS w ith illustrat ions by B. H. ROB INSON

Publishers: Ladybird Books Ltd. Loughborough © ladybird Books lid (formerly Wills & Hepworlh lid) 1972

P"nr~d In fngl.nd

The w orld of radio

Today, radio is pari of our everyday life, yet only

at the beginning of this century it was a miracle. It

IS still something to marvel at- and a fascinating

subject 10 study.

With this book you will learn, stage by stage. how

to build more than one simple receiver. Everything

you should know is explained simply and illustrated

clearly, and you will have the pleasure of constructing

something marvellous that really works.

All the components used are easily available, usually

from your local radio dealer. Most large towns have

shops specia lising in stocking components, but it is

also very easy to obtain any you need by post. All the

popular electronic construction magazines contain

advertisements for mail order supplies. Many of these

suppliers provide catalogues which list all the com­

ponents you will need.

4

Radio reception

Radio waves cannot be seen o r heard. They are c/e(,lro-lIIagt/l!fic waves, and they now backwards anti forwards (oJcillafl!). Each backward and forwa rd movemcnt is known as a crcle. Thc ra te o f this back­"'ard and fo rward movement (oJcillarion) is kno\\Il as the frequency of the wa\c.

A radio ",a\-e is a lso a carrier wavc. It can 'carry· an electro-magnetic wave of a freq uency exact ly similar to the sound waves of the speech or music which is being tran!.mitted. Th is frequency, which is imposed on the carrier wave, is known as an lll/dio frequency. It is this electro-magnetic lludio frequency that a rad io receiver translates into sound via the loudspeaker. For a more detai led exp lanation of this. you might like to read pages 20-26 of the Ladybird book: · How it Works Telev ision'.

When radio waves carrying an aud io frequency arrive at a receiver, three things happen :

I. TUNI NG. The radio frequency ca rrier wave is picked up by the aerial. A long this same aeria l pass the radio frequency carrier waves from many o ther tran::.mitting sta tions, but the signa l we wa nt from a particular transm itter must be sclected (t uned) from them.

2. DETECTION. The next stage is the detection !.tage, in \I,hieh thc electro-magnetic audio frequency wa\-'e is detected from the carrier .... :ave.

3. I MPLlFlCATlON. The electro-magnetic audio signal \.\;Ive is amplified, or made bigger, and translated from an electrical signal into SOUN D by the loud-speaker. from which we hear a repro­duction of the transmitted sound.

6

AERIAL The aerial picks up electro.magnetic, radio frequency, carrier waves

TUNING

DETECTION

I A,MP'LIFICA TlON

LOUD SPEAKER

A camer wave of II particulM frequency is selected

The electro ·magnetic. audio frequency wave IS detected from the camer wave

The electfo-magnetfc, audio frequency wave IS amplified

The electro-magnetic audio frequency wave is converted into a sound wave

Reading circuit diagrams

Before building a piece of electronic eq uipment. a

constructor needs to know how to wire up the various

components. Thi s can be done wi th the help of a

diagram showing how the components are joined.

Electronic engineers have developed a ·shorthand·

method of showing the various components and their

joining wires. This is the circuir diagram.

A circuit Jiagram is like a map. All the electronic

components have symbols. and the connecting wires

are shown as straight lines. The positions of the com­

ponents on the circuit diagram do not normally corres­

pond to their aC/lial positions in the completed equip­

ment. Sometimes a layolI! diagram is a lso used to show

where to position the components on a base board.

Many symbols are used in circuit diagrams and we

will learn them as we meet them in the course of our

construction. To make construction easier we will use

both circuit and layout diagrams.

8

f Bulb

Switch

-Battery --

CIRCUIT DIAGRAM

LAYOUT DIAGRAM

A crystal set - the tuned circuit

A crystal SCt. the simplest form of radio recei\er. 1~

the first stage of our radio. It can opera te entirely on

its 0\\ n as a rad io recci\cr, e\en if no funher stage ...

are added.

As v.e have read on page 6. an aerial picks up all the

radio signals (the radio frequency carrier \\a\es) in il.'>

path. BUI we need only signals of a certain frequency

those scnt o ut by a particular transmitter. We thcrefore

need some sort of filter to allow only one radio station

to be received al once. A /IIf/cd circuif is this filter.

because it can be tuned to any frequcncy we want. and

can select one of the many rad io frequency carrier

waves reach ing the aerial.

A tuned circuit consi ... ts of a coil of wIre and a f!mill.f:

capacifor. On the orposite page you can see a circuit

d iagram for a tuned circUlI. and beneath are shown the

actua l componerlls a coil and fllllill!! capacifor.

T he panicular radiO frequency wa\es \\ hich a tuned

circuit selecb derend.., Ull the number of turn ... 01 \\lre

on the coil and the ·va1ue· of the capacitor. Since 11 is

inconvement 10 keep changing the number of turns of

\\lre on the cod. the capacitor is made with movable

plates so that the tuning can be \:.trled and the sli:maJ..

we \\an t sck!,:tcd.

10

A TUNED CIRCUIT

Tunmg Capac/tor

A c ryst al set - the cry st a l d iod e

After the tuned circuit has selected the particular

carrier wave frequency required, the audio frequency

signal which it carries must be extracted from it. The

CTyst(JI (sometimes referred to as a diode) is a simple

device that does this by allowing current to pass only

one way. It is used in the crystal set as a 'detector'.

The action is a comple\ one, but we can simply say that

it detects from the radio frequency carrier wave the

electrical waves of the same frequency as sound (a/ldio)

waves and wh ich a loudspeaker or headphone can

convert into sOllnd.

On the opposite page you will see a circuit diagram

showing the tuned circuit mentioned on the previous

page but with the symbols for the diode and the ear·

piece included. With the addition of the aerial and earth

symbols. we now have a circuit diagram for a complete

working radio set.

Also illustrated are a typical earpiece and a crystal

diode.

12

Aeri8f

,..-" Ferrite ",,-,Coil

- Eanh

Diode

O·OOO5".1F Tuning CapacltOT

THE CRYSTAL SET CIRCUIT DIAGRAM

1 Tools f or radio construction

Simrk radio construction. of the sort described in

this book. requires fc\\ tools. Mo<,! of these can be found

111 a normal household toolbox or arc ine-.;pcmi\c to

buy.

During the building of the radio. \vires have to be

bent into place and sometimes cut to the correct length.

Some components are joined by P. V.c. covered copper

\virc \\ hich ha!> to be cut to size and Its covering removed

from the ends to make the connection. T he mo~t w.eful

1001 for this is a pair of pointed-nose pliers. The:>c \\'111

abo strip the plastic from the wire before the connce­

tions are made. A pair of '" Ire cutters may also be used

to cut and !.lrip wire.

It is possible to buy an inexpensive little tool called a

',,,.ire-cutter and stripper' to do both jobs. A pocket

knife is useful for scraping the ends of the \\ire to

dean them before the connections are made. Sandpaper

or the rough edgc of a matchbox can also bc used to

clean ",ires before they are connected. A medium-S ized

screwdriver ", ill be needed for making the connections.

This radio is to be bui lt on a wooden base. so a fe\\

simple woodworking tools hal1d'><.l\\. gimlet. brace

and bit- are also requlr('d.

,.

.,p

*

Screw to adjust Stripper lor diHerMl wire SIZes

Stdpping

..... ~~ t ,

\

-.-CLEANING WIRES

.\

Fixing the components

In electronic construction it IS usual to solder the

components into their places in the circuit. This radio

is designed ror the beginner, so a non-soldering method

or connection is used. For those who wish to solder the

conne<.:tions, the te<.:hnique is explained on the inside

back coyers or this book.

A simple met hod or connecting the components

uses woodscrews and washers. The screws are No. 6

brass coun tersunk. haIr inch screws and the washers

are No. 6 brass screwcups. T wo dozen or each are

required. All are obtainable rrom your local iron­

monger.

The method is explained in the diagram opposite. but

remember the rollowing lips: (I) Make sure the wire

to be connected is scraped clean until it shines. (2) Where

severa l wires are trapped under one wa~her, do not

finally tigh ten the screw until all the "ires are in place.

It is possible to test a connection with the simple

battery and bulb arrangement in the diagram. Touch

the two leads on to some or the bare wire either side or

the connection. Ir the connection is good. the bulb wi ll

light up.

16

NO.6 erass Screwcup

•

Making the c ircu it board

In the early days of radio. receivers were made on a

baseboard sometimes called a 'bread board'. We are

using a form of bread board mounting.

Softwood is used for the base. Mount the screws and

screwcups, used for connections, at one inch intervals

in two rowS of eight. The rows should be an inch·and­

a-half apart. This is shown in the drawing facing this

page. The two large holes are made with a brace and

bit and are for mounting the control components.

The accuracy of the measurements is not vitally

important. but it would be helpful to follow those given

as closely as possible. Not all the screws are used in

bui lding the first part of the radio; the additional

screws are needed later as the radio is completed.

Although this board is very simple to make, it is

important to try to get the screws in straight so that the

screwcups press firmly onto the wires.

18

•

H l ' " H'

HOW TO MARK OUT THE BASEBOARD

\I

~::~::.:~"1,"r.":~o~m:~;tM m.rJced sidfj With a t-bit drill/rom tIM otMr bit just .pptllJlS on sithl, haN way through the bOMd drill right through)

MAKINO THE HOLES

Section of thtt counter· --. U.tt. ginWt or btMMwl to mne hoh. /or t~ SCftIW$. (Do not fully tlghttlfl tIM $CfttW • ., this It.".)

Winding the coi l

Before the crysHll set can be built. \\e mm,! wind a

tuning coil for the tuned ci rcuit. T Ills is done on a piece

of ~ inch diameter ferrlle rod . Ferrite is iron dust

st uck together. Usua lly fernte rods a re 6 inches long.

so cut a notch all round the centre a nd smartly snap the

rod in half (Figs. I and 2).

Wrap about I ~ inches of stick v tape around the centre

of the rod on \.\hich to \\ ind the cOi l (Fig. 3).

Obtain some 36 s.v".g. (stn ndard \\ ire gauge) enamel·

led copper wire frol11 yo ur radio or electr ical dea ler.

Leav ing abou t 3 inc hes of wire. secure one end o f the

wire wit h st icky tape (Fig. 4) and wind 5 turns. ~ide

by side. o nto the rod (Fig:. 5). Pu ll o ut a 3 inch loo p

a nd 1\' 1st the wiTe unti l it is t igh t up to the rod (Fig. 6).

T hen wind a furt her 45 (Urns. side by side. securing the

final end " .. ith tape. T he \\ ho le coil can be co .... ered "ith

tape to keep the windi ngs secure.

Remember to Ica\c about 3 inches of spare "i re at

both ends and at the t\\ isted \\ ire (the 'tapping' point).

Later the enamel mu<;( he scraped off $0 that connections

can be made.

The illustra t ions opposite " ill guide you thro ugh

each stage.

20

Twist (he wire

\..

~~~ I': '\

~T~ '\ \~f.·" , '..' ) . j , ;

" '\. r

2) Snap the rod by holding close to the notch with forefingers lind thumbs onl'iJ.otherwise it may splinter)

\

5) Form the coil, either by winding the wire 01 rotating the rod. whichever you find ".$ier

" Twist the tlJPPlflg point with IJ (inger in the loop. holding til« rod still with the wire tt8PP~ undet (InglU MId thumb

ANilJ'

Coil

SYMBOLS

Building the crystal set

Having made the coil, the ferrite rod can be mounted

on the baseboard by twisting a loop of stiff wire around

each end and screwing the wire under two screwcups,

as shown opposite.

Next the tuned circuit can be completed by the addi­

tion of the tuning capacitor. You can buy one from

your local radio dealer and you should ask for a

Q'OOO5I-lF solid dielectric type. The illustration opposite

shows how it is mounted on the baseboard, the capacitor

spindle passing through the i inch hole. The wiring

connections are made to the IWO terminal nuts on the

back of the capacitor. The connecting wire used should

be PVC covered copper wire. the ends being bared

before they are trapped under the screwcups.

The illustration also shows how the diode is included

in the circuit by means of the screwcups. The earpiece

is also shown inserted. When purchasing this. you

should ask for a crystal earpiece. A pair of high

resistance headphones could be used in place of an ear­

piece. and may give even better results. Resisrance is

discussed on page 32, in the section headed 'Resistors'.

On the following two pages there is some useful

information about the aerial and earth.

22

The crystal set - its aerial and earth

The aerial is the part of the receivcr \\hich 'picks up'

the radio signals. Unlike our crystal sct, many modern

transistor rad ios are sufficien t ly sensitive to pick up

enough signal from the ferrite core of the tuning coil.

Although the final radio which we will make will not

need an acrial and earth. the crystal set relies on the

electrical current set up between a good aerial and earth

to work well.

A suitable aerial can be made from inexpensive PVC

covered copper wire. The idea is to suspend as much

of this wire as possible. as high as possible. The far end

can be attached to a tree. the top of a pole or even the

eaves of a friendly neighbour's house. but this can be

dangerous and is best done by an adult or under the

supervision of an adult. The wire shou ld be fastened

onto a large screw eye or hook. The near end of the wire

may be taken in the top of an upstairs window and

connected to the aerial connection on our radio.

For the aerial to be effective. the other side of the

tuned circuit has to be connected to earth. The simplest

method is to attach the earth lead to the nearest house­

hold water pipe. These pipes lead to the ground. If

your water pipes arc plastic. make your own carth by

driving about a yard of copper pipe in to soft earth and

connecting thc eanh wire to thc top.

24

AtH'i4J Wire

Trell or Pole

E."h Wire

An experimental P.O.W . radio

During the last war. some prisoners in prison camps

used home-made radios to keep in to uch with events

in their native lands. Often, a ll the components had to

be made from items picked up within the camp. Diodes

were almost impossible to obtain , so severa l ingenio us

home-made alternatives were tried.

The diagram shows one simple method of improvising

a diode. Take a sma ll piece of wood, about I inch thick.

and a piece of washed coke about the size of a pea.

Mount the coke on the wood as shown. using a screw

and screwcup, and taking a lead from beneath the

screwcup. Wind about a foo t of steel wire- a length of

piano or guitar wire is idea l- around a number 4 or 6

knitting needle, leaving about I inch on each end. The

\\jre should now be slightly springy. Fasten one end 10

the board with a screw and screwcup, taking ano ther

lead from this screwcup. Stretch the wire unti l the free

end lightly touches the coke.

Remove the diode from the crystal set and screw the

wires in its place. It should be possible to hear some

local stations by adjusting the tuning capacitor. If

nothing is heard. keep moving the position of the wire

on the coke until a signal is heard.

26

+ 3"

-I

Pencil ~Lmes

I I

The transistor

To most people the word TRANSISTOR means a

small radio sel. but a transistor is really a single electronic

component. The transistor is one of the most important

inventions of the last 25 years. It has helped to make

electronic equipment smaller and less dependent upon

high voltages.

A transistor is li ke two diodes working together inside

one case. Most transistors have three Icads. emitter.

collector and base. A transistor can be damaged unless

the co rrect leads ::Ire used when connecting it into a

circuit. The base lead is usually the middle wire and the

collector lead is ~e t slight ly apart from the ot her two

wires. The transistors we are to use a lso have a red

spot painted on the c::Ising to indicate the collector lead.

We sha ll be using transistors as amplifiers: that is.

10 m::lke electrical signals stronger. Transistors are

designed so that a small. changing current between the

basc and emitter produces a large current change

belween the collector and the emitter. So small signals

applied to the basc will appear ampli fied (or made

bigger) al the collector. Our first transistor will be used

to make louder the sound coming from the crysta l set.

28

s .... --...

THE SYMBOl FOR A lRANSlSTOR

OC7f

I:

------TRANSISTORS

3~~-~~ OC11 TRANSISTOR (.Actwl sin)

Adding a transistor

T he circuit dia gram shows how a simple transistor

amplifier is inserted \\ here p reviously we had thc ear­

piece. Compare this diagram with the circuit diagram

of the crysta l set on page 23.

T he transistor used is an OC71. and other extra

components for this stage are a 2·7K resistor and a 9-

volt battery. The battery is the small t ransistor radio

type PP3, for which can be obtained an inexpensive

snap connector. Resistors are discussed on the next page.

T his resistor forms an output ' Ioad ' ror the collector,

and the earp iece is connected across the resistor.

Take care to connect up the circuit as shown in the

drawing. The connections of the transistor arc im­

portant. Join the emitter to screw 5, the base to E and

the co llector to F. The earpiece is removed and rc­

connectcd to F and G. Connect the rcd battery con­

nector lead to screw 5, and the black lead to screw G.

T he radio is switched on by pushing the connector into

place on top of the battery.

T his one transistor should make a big improvemen t

to the crystal set ; the same stations will be heard but

they will be much louder.

30

Resisto rs

Resistors arc very common components in electronic

construction. Look at most pieces of equipment and

you will see several small, wi re-ended tubes with brightly

coloured bands. These are called resistors because t hey

'resist' the flow of electrica l current in the circuit.

This is done to adjust the vol tages and flow of current

at various places in a circuit.

T he little tubes are filled with a carbon composit ion.

and the coloured bands give the mille of the resistor.

This val ue is ca lled the resisrance and is measured

in ohms. When one knows the co lou r code opposite.

it is easy to work out the resistance from the coloured

bands. T he to lerance of the resisto r. that is. how close

one can expect it to be to the va lue. is sometimes in­

dicated with another coloured band. Our radio can use

resisto rs of any tolerance. so this band may be ignored.

The symbol fo r resistance is the Greek leiter omega o.

Quite often, very high resistances are used- sometimes

thousands or millions of ohms. To reduce the nu mber

of noughts. we call thousands K 0 (Kilo ohms) and

millions M n (or Mega ohms). T he ohm sign is usually

omitted from circu it diagrams when the Ki lo ( K) and

Mega (M) signs are used.

32

THE SYM BOL FOR A RES ISTOR

RE SI STOR COLOUR SCOPE

o (or 0 noughts)

GREEN 5 (or 5 noughts)

1 (or 1 nought) BLUE 6

(or 6 noughts)

7 (or 7 noughts) VIOLET

2 (or 2 noughts)

3 8 (or 3 noughts) (or 8 noughts)

9 (or 9 noughts)

GOLDS·,,, SIl.VER 10·'t. SA LMON 20'1..

= 47000 or

Biasing the transistor

The transistor amplifier we have buill is the simplest

arrangemenl possible. If a transi stor is to be used effi·

ciently. onc has to supply thc correct voltages to its

various parts; this is called hiasing. In our simple

armngement Ihe signal from the diode provided a

voltage to hias Ihe base of the transistor. Resislors can

be added 10 make the biasing correct.

The circuit sho\\ s how the transistor is biased \\ ith

two rcsistor~. (JJK O-orange. orange. orange. and

150K 11 brown. green. yellow.) They give the correct

voltages between the collector. base and emitter. Take

care to connect the resistors in the r laees shown or the

effect will be ruined.

Thl:!. addition to the radio will not increase the volume

in any noticeable way. but the biasing is essential be·

fore the next stage is added.

The wire ends of resistors. like any other components.

can become dirty and greasy. ~o !icrare Ihe wires clean

before screwing them into place.

34

A dd ing a second transisto r

Before we can add another transistor to our radio. two new components must be introduced-a pOlen­liomeler and a capacitor.

The potentiometer is a variable resistance. Look at the drawing on the opposite page and you will see that it has three contacts. The outer two arc each end of a circular carbon track. The centre tag is joined to a movable contact which slides over the track when the shaft is rotated. By moving the shaft, which is usually fitted with a p lastic knob. any part of the resistance can be used and tapped o ff through the centre tag. Poten­tiometers are often used, as in this case. as volumc comrols.

A capacitor is a componcnl which allows the signal to pass but not the fixcd voltages uscd to power a circuit. It consists of two plates which are separated by insulating material; the circuit symbol shows this. Electrolytic capacitors must be connected into a circuit the righ t way round. One end is marked either with a red band or a -"- sign, and Ihis side goes to the positive side of the circuit. The negative (- ) \~ire is connected to the metal can which forms the body of the capacitor. and which may be marked with a black band. Sometimes this is not obvious. but follow the circuit and layout diagrams with care to avoid mistakes. Other fixed capacitors can be connected either way round: we will meet these later. The value of capacity is written in microfarads. (IlF or sometimes MFD). We require IO~l F at a work ing voltage of 16-volts.

36

150K

Den

9 volts

CIRCUIT DIAGRAM

Symbol Symbol

ELECTROLY~T:'C:-~~--· CAPACITOR POTENTIOMETER

The second transistor stage

The circuit for the second stage is ~ho\\n on th e

pre\iou~ page. T he signal is tal-en from the collcctor of

our first tramistor. through the capaci tor to the sliding

contact of the \olume control potentiometer. The nearer

the centre contact is adjustcd to thc base side of the

control. the more signal is allo\\ed to go to the bilse

of the second transi~tor. The greater the ~ignal reaching

this transistor. the louder \\ill be its out rut.

The potentiometer is mounted In the spare 9 J11ch hole.

Its tags arc too close together to use the large screwcup~.

so small screws are used. The ... e arc ~cre\\ ed down

through the holes in the potentiometer tags. trapping

the leads under the tags.

Rem ove the ]·7K resistor from F and G and connect

in the ne\\ components carcfully. follO\\ing the layout

diagram facing thi~ page. As you worl.. compare the

layout diagram. connection b) connection. \\ ith the

circuit diagram. This additiona l transi~lor \\ill greatly

increase the volume. enough to drive a loudspeaker,

\\hieh is the next addition.

38

I I I

Bend tags to I~ IIlIt on bolKt!

/ CUI, With a hM:IcSIIW, 10 the same hmgt/! liS tuner spindle

Malee II hol. in th8 bOMd with II SCTtlwdrilltJr to accommodllie this proillClion

10 9

,

/

I

10K I potentiometer

/ Pencil Lines

Adding a loudspeaker

Using our radio with an earpiece means. of course. that only one person at a time can hear the programme. A loudspeaker changes electrica l waves into sound waves in the same way that an earpiece docs. but it has a large cardhoard cone so that se\oeral people can hear the sound . Loudspeakers can have very large cones. but a smatter cone speaker \l.ith a diameter of 3 inches is ideal for ou r purpose.

Loudspeakers arc made with inputs of varying resistances. and o ur radio requires a 3 or 5 n type. Unfortunately the lo udspea ker cannot directly replace the earpiece because its resistance is much 100\er than that of the earpiece. so it has to be matched into the circuit with a trollS/armer. A transformer can change a current o f one voltage to a smaller current of a higher voltage or to a larger current of a smaller voltage. It consists of a 'primary' and 'secondary' winding of fine \\ire round a metal "former'. The transformer we require is the Eagle LT700. Thi s transformer has rather long leads. and these may be shortened by winding them tightly ro und a knitting needle. This makes spiral leads which can then be pulled out to reach the screws.

Scre\\ the transformer onto the base. as shown ; remove the 2·7K resistor across G and H and connect the correct leads (colo ur coded) to the screweups. Two more screwcups are added at the end of the board to connect the output leads to the loudspeaker, using two small crocodile clips.

40

od HF=------... TRANSFORMER

-f

CIRCUIT OIAGRAM

Regene rat io n

\Ve ... h~lInO\I fcplacl' pur 'Impk uy,tal "L:l ~tagc Ilith

a trall"'l.~hlr "tag". u ... lng /"t".:('I/('/"tilioli . Kq!l'lleratll1n II III

e l1 ahk \lur fir,,! tr'lI1"i' ( ~lr in the ra di O III he lh ed Ilm.:-:

lilT1e~,

The ~,:If(:uit ~lw\I' Ihe regl'lll.'ratlon .. tag.e, T ill' "'gnal

from the aerial I, tUIll.:d h~ thL: cIli l a nd ,ari,lble

Glp:H:ih . till' orange l11:1r"l'o ,ign:1' then p~:. ... e, thrllUgh

capa(ttl'f CI h' bt.' amplifit.'d h~ th" Iran~i~(lJt' and

arpear dl a rudio fj"('I/lie/le\' ('huke ~ H. I ,l ). Pan o f t im

"ignal. mar ked 111 hlue. P""'I..', thrl1ugh C~ to a pair of

(lEode, II hith chang\: lill' r.ldill :'Ignal 11110 an audio

~Ignal. II hich I'" amplJfkd ag;11Jl hy tilL: tra n si~ tor. I h-:

'iignal. 1ll'\1 marl-.I..·d red, pa~'L:' thruugh C3 hal'l-. 10 the

('Oil. Tht: part or the cotl tapped hL:IIIl'Cn Ihe tralN,lOr

colledor (Ihrl,ugh ell and earlh. a r k)\\~ the 'I!tnal 10

reach the coliect(lr again.

T hl' means Ihal .. \lllle tlflhl' , ig.nal;ll H., I ,C IH\'" bec-Il

amplified Ihrl'\' tlllle", ~u Ihl" i ... a ,-cry l.'l'!lllUmical u,e

of one lran:-.hhlr Pas"lI1g ... . gnab hacl-. "g..un [II-.e II1\'. I ...

ca lled fced"llc/..., If lhl' feedb;ll'k I .. IUtl great. the circuit

II til prodlK(, a IW\I ling ~llund. so C J IS a n adju .... tab[c

capacitor 10 conlr\'! Ih(' anl(lunl of f('l..,uhad. .. I hI.' hn~1

~Ignal I .. taken from R.r.C Ihmugh capacl tllr C-I.

Aellal

C4

~ pu'

HOW ERATION WORKS In CIfCUIt diagrams. Wires which cross one another. (but do NO r touch iJnd afe Nor connected), are sometimes show" M,e this + (as in (he diagram) or /Ike thiS + Shown like thiS -+- always means they are cOllnected

--~~~------------------~~

Adding regeneration

The circuit 5ho\\5 the new stage \\ jlh the componcnh

added. The adju~table cl pac itor is a 10 pF (p icafar:td)

lrillllllCT. This con:.i~ls of two copper plate~ moun ted

on a pot ba~e: the d istance bcl\\ccn the plates can be

adjusted \\ ilh the small sere\\.

Fi\ed capacitors may be flat or cylindrical \\jih IWO

leads: the Iype we requi re. ce/'Omic capacitor!:,. may look

li ke a disc.

The rodio /requelle.\' choJ..f! is really a coi l \\jlh a lot

of turns of \\ ire on a small fOflner. The choke aels like

a resIstance to f:ldio·frequency waves.

Two diodes arc needed to provide the fcedbud. path

from the 500pF capacitor. If the top diode \,ere omitlcd.

the signa l \\Quld nOI be changed into audiO frequencies

~rore it reachcd the tran~i"tor ba<;c. and if tbe botwm

diode \\ ere om itted, the signal \\ould go !>traigbt to

earlb a nd be 1m\.

The I K resistor lo\\ers the ballcry voltage nceded \0

iXmer this stage, It is possible for "Dme of the audiO

~ignalto feed baek to Ihis stage through th is I K resisto r

and distort the signal. T his is prc\cn!ed by the 100111-"

capacitor \\hich tai.;es any stray llignllb 10 earlh \\ ithout

afrccting the battery \oltagc

\II

Y

·OwF

10pF TRIMMER postage stamp type'

DC45

I

I

IK

10«JF

~~; l00-vF

500pF

FREQUENCY CHOKE

E

Wiring the regeneration stage

The dra\\ing ~ho\\s ho\\ thl~ :.tage is \\ired 01110 the

board. Ikfore ""inng the stage. all the component:;

from the cry:o.tal :o.tage. up to ... cre"., E and 5. arc

remmcd C\ccpt for the coil and tuning capacitor. T he

dillde I" remmcd to become one of the 1\\0 diodes in

the new stage. Follo\\ the dra\\ ing ca refully. adding one

component at a lime and checking its posi tion on the

circuit diagram. Therc arc many bare \\Ire., which come

clo,>e 10 cach other. and great care must be ta)..cn to

m~ure they tlo nol touch each other. T he [K re~l!.tor

must not touch ~cre\\~ E :.Ind F.

The trimmer i~ trapped by ib tag~ under screv.s A

nnd B: if it i~ too long or too ... hort. the scre\\s may be

mmed. All thn.'c connections to the coil arc no\\ used.

and If the \\Ire" arc not placed JI1 the correct positions.

regeneration \\111 not occur. The dra\\lng shO\\s hO\\ to

place Ihe~e three \\irc~. The IOJ.li capacitor tal..es the

:,ignal frorn Ihe nc\\ "tage to the ha~c of the ne\! OC71

tranSI!.tor. The ad\alHage of thl':) ne\\ ~tage is that our

radio \\ ill h:ne .1 greater output and no longer need a

large aerial and an earth. Re.generatlon b tricky to gel

u ... ed 10. but once mastered. Ihe results are surprising.

Using the radio

Orcrating a regenerative radio requires care at first

because the trimmer has to be adjusted to suit the

streng.th of each signal. Begin by connecting about 3

yards of \\ ire to the radio as an aerial: no earth is

required. Set the tuning capacitor about mid\\ay and

sere\\ in the trimmer until the radio begins to howl.

Unscrew the trimmer until the no ise juSt stops. and try

to tunc in a station. The trimmer should then be adjusted

until it is just short of the point when the radio howls.

The general method for recei\ing any stat ion is to tune

it in \\ilh the tuni ng. caracitor and to adj ust the feed­

back !Timmer until the howling just disarpears.

The feedback not only makes the radio more sensitive

bu t also the tuning becomes sharper. so the tuning knob

must be rotaled vcry slowly. T his sensitivity makes the

rad io ideal for the medium wave band. \\ hich is rather

overcrO\vded. The tuning shou ld be sharp enough to

tune in indi\idual stations \\ilhoul the background

sounds of other stations. I n areas of good sigmll strength.

it will be pos~ib le to use the radio without an aeria l at

all. merely re lying upon the signal picked up by the

ferrite rod coi l.

48

The completed radio

Now that the radio is completed, you may \\ish to make the circuit board into a "imple case. The circuit board is used as a front panel. and end and bouom panels are added.

Thc"e can be made from similar wood to the circuit board (4 inch by ~ inch planed sortllood). The boltom panel is the same :.ize as the circuit board and the

measurements for the end paneLs arc ShO\l11 oppo:.ite. The end panel next to the output tram.rortner may be used to hold the loudspeaker. A series of i inch holes arc drilled \Iith a brace and bit to allo\, the "ound to Jcal'e the casco and the "peakcr is :.ccured II ith screW5 and cup:.. The pancl~ are joined \1 ith panel pins and glue.

T he ba!lery may be firmly mounted abo\<c screws 0

and Eon thc circuit board. using 1~ inches of linch elastic and 1\\0 screws and cups. T\\o "mart pla~tic

pointer knobs and a tunJllg scale complete the front panel. The tuning scale i~ a paper gummed label which

can he marked lIith the positions of the main station".

If you hale follO\\ed thesc pages carefully, you \\ill no\\ have a u~eful I\<orking radio. You \Iill also have shared the thrill of thousand" of other radio construc­!or~. listening to sound~ coming from a radio receIver \\llIch is your Ol\n lIar".

50

Contents

The \\MIJ of r~IIJIII

R:UJll) n'u'plilln

R~'adlng ~'ir~'ull JI~lgr;111l\

.-\ cr~ ~t~IJ ,..:t the hilled urnll!

\ cr~~lal -.el Ihe ....r~\tal diode

Tool" fllr r;ldlO c(1lhlrlldion

Fi\lIlg Ih..: (IJlllr,Jllellh

\blll1g the ..:in.:Ult I">l';lfd

\\ indlOg the wd

BudJlllg Ihe (r~\t;d ~et

T he l'r~~lal -.el 11\ a..:nal and earth

All e,penment;11 PO, \\ radio

The tral1\i,lur

AddlOg a tran~i~lor

R":,l\t(1r,

B iasing Ihe tr31hlstllr

.-\ddlllg ;1 , .... '.;nnd tr;lIhl~lor

The ,e":{l1ld tralNslOr <,tagl'

-\dding a loud'peal-...:r

Rcgl'nCrallllll

Adding rcgcnerallOIl

Wirmg the regeneLHi\111 ,tage

l \mg Ihe r.ldiu .

The ":llllll'lcted f:adll'

10

I<

16

IS

20

"

30

.'12

Components list At each stage you have a complete wOlklng ladlO

Stage 1 . Crystal set Ferme lod i (10mm) diameter 2 yds (2 metres) 36 swg enamelled coppel Wlle. 'OOO5tJF (500pF) variable capacitor (solid dleleclflc type) / Crystal diode DAB1 Alternatives OA70. GA71 , DA79 or any general purpose diode .., Crystal earpiece

Stage 2. Adding a transistor DC7l transistor - Alternatives: NKT214, AC122. AC125 or any general purpose PNP audiO transistor 2'7K reSistor 9 volt battery (PP3) and connector

Stage 3. Biasing the transistor 33K resis tor 150K resistor

Stage 4 . Adding the second transistor GC71 Alternat,ves NKT214, AC122, AC125. ACY35 4'7K and 150K resistors. 10K potentiometer 1 OuF (l6 volts working) electrolytiC capacitor

Stage 5. Addmg a loudspeaker Eagle L noo transformer 3 ohm loudspeaker (about 3 (76 mm) diameter)

Stage 6. Adding regeneration DC45 tran~,stor - Alternatives OC44 25A.12, AF116 . AF126, AF127 or anyguneral pUipose PNP RF uarisistor Crystal diode (as stage 1) RadiO frequency choke (!ranSlstor type) 10pF trimmer (postage stamp type) 1 OuF electrolytic capacitor (16 volts wOIkmg) l00uF electrolytiC capacitor (16 volts wOIkmg) 500pF ceramiC capaCitor 0l.uF diSC ceramiC cill)acltor

390K, 4 7K and 1 K reSI<,\or$,

COMPONENTS FOR THE BASE BOARD 4 ~ \102 mm 1 3 nun} planed ,lIwood 12 {305 nml} long 2 dOlen No 6 brass 'iClewcups 2 dozen No 6 brass counter,unk f (13 mm) SUt~WS 1 'I'd (1 melle) sing I. pvc covell·d cOl-lpcr lotlll Wilt'

nga ~ ,.::.T,-..r Radio