The Breadboard Six Transistor Radio Kit

Whole Breadboard AM Radio
            Kit Built Up

A complete kit of electronic parts is available on Ebay here: Useful Components Ebay Shop

parts
            included in the breadboard AM radio kit

The original choccy-block version of this radio kit that uses 3A screw-down terminal strips is shown here:  Choccy Block Transistor Radio.  It uses the same circuit but the kit is a bit more expensive and it takes longer to build that way.  This version also uses smaller 0.25W resistors and a plug-in volume control which suits this construction method.

Paper documentation is not provided.

Here is a link to a YouTube Video Demonstration of the Breadboard Radio Kit.

This is a MW AM  transistor radio which can be constructed without soldering using a standard small plug-in electronic breadboard. It is a modern response to the 1971 Ladybird Book Radio and works almost as well as some AM radios that you could buy in a shop. It's best to read right through the construction notes first before starting out. 

Link to PDF Schematic Of Breadboard Transistor Radio

Schematic Diagram of Breadboard Transistor Radio

Choccy Block TRF
            Radio Schematic Diagram

Bill Of Materials

Electronics Parts Included in the Kit

Qty  Refs             Type                         Value

1    VC1              variable capacitor            200PF approx
1    L1               ferrite rod d=9.5mm l=100mm   see text

2    R1,R3            resistor                      1K
1    R11              resistor                      470R
1    R12              resistor                      1.8K
3    R2,R4,R8         resistor                      6.8K
1    R7               resistor                      12K
1    R9               resistor                      27K
3    R5,R14,R15       resistor                      47K
1    R10              resistor                      100K
1    R6               resistor                      330K
2    R13, R16         resistor                      220R

1    VR1              volume pot pcb                10K
1    C9               ceramic capacitor             68pF
5    C1,C2,C3,C4,C5   ceramic capacitor             10NF
1    C6               ceramic capacitor             100NF
1    C7               electrolytic capacitor        10UF
1    C8               NP elec capacitor             4.7UF
3    C10,C11,C12,C13  electrolytic capacitor        100UF

2    D1,D2            diode                         1N4148

1    Q4               NPN audio transistor          BC549C
1    Q3               PNP audio transistor          BC560C
1    Q5               NPN driver transistor         BD139
1    Q6               PNP driver transistor         BD140
2    Q1,Q2            NPN RF transistor             BF199

Miscellaneous

1    LS1              65mm 8R loudspeaker       
1    J1               PP3 battery clip
1    S1               slide switch pcb
1                     plug-in breadboard
1                     3A 3-way terminal strip
2                     M2.5 bolts 4mm
4                     M2.5 washers
2                     short shaft control knob
1                     single core wire 4m

Extras Not Supplied

Paper glue
Paper
Small screwdriver
Scissors
Sticky tape
Small wire cutters
Craft Knife
PP3 9V Battery


Link to PDF of BOM for Choccy Block Transistor Radio


Pre-Requisites


Tools

I supply a kit of all the electronic parts, wire and breadboard. I've assumed that you have a small screwdriver for the terminal screws, some small wire cutters for trimming the component leads and some means of stripping the insulation from the wire.  A craft knife is good for that.  You will also need some paper, glue, scissors and sticky tape. 

What do you Need to Know Before You Start?

The components list has links to photographs that should allow you to make sense of the different values and to identify which leg is which.  You need to know about resistor colour codes and you can find that information in many places.  It should be possible to build the radio just by looking at the pictures, but later on I've included full electronic explanations.  If those don't make any sense, don't let them put you off just building the circuit.

The schematic diagram can look a bit daunting to the novice.  Don't Panic.  You don't need to understand it all to make it work.  The idea of this design is that if you follow the plan it should work without too much fiddling.  Experienced builders will find the schematic diagram helpful as well. 

How To Build It: Follow the Pictures and Schematic Diagram


The main pictures taken from directly above in plan view will show where everything goes most of the time.  There are more pictures taken from different angles later on, if the main ones are not clear.  This might happen where one wire may have covered up the exact plug-in position of one underneath it.  Where the plug-in hole is obscured by the component, it is marked on the picture by an "X"
 
Assemble the Resistors

Breadboard TRF Radio with
          Resistors Inserted

Start by plugging the resistors into the correct holes in the breadboard as shown.  They should be cut to length so that they sit flat against the board when plugged in.  This stops them wobbling about and avoids having long legs shorting out against other wires.  The resistors can be plugged in either way around, but it makes it easier to check the colour codes if they are inserted the same way around as shown in the picture.  You can click on the pictures to see a bigger version.

Assemble the Capacitors and Diodes

Breadboard TRF Radio with
          Resistors, Capacitors and Diodes Inserted

Identify the ceramic capacitors, the electrolytic capacitors and the diodes and fit them as shown.  The diodes have to be plugged in the right way around, as identified by a black band at one end of the glass case. 
With the exception of the bipolar one, the electrolytic capacitors have to be connected the correct way round as indicated by a white stripe with a "-" negative symbol on the negative side.
 
Assemble the Transistors. 

Breadbaodr AM radio
          project with Resistors, Capacitors, Diodes and Transistors
          Fitted

Fit the six transistors as shown.  They all need to be the correct way round as identified by the flat side on the small ones and the plastic side with the type number on for Q5 and Q6.  The leads of Q5 and Q6 are quite large for the breadboard holes so ease them in gently to avoid damaging the internal spring contacts.

Assemble the Loudspeaker, Wire Links, Battery Clip, VR1, SW1.

MW Radio on Prototype
          Board Complete Components Detail

The on-board wire links are made by cutting approximate lengths of the single core wire supplied and stripping about 5mm of insulation from each end using a conventional wire stripper or by scoring around the insulation with a craft knife and pulling the small piece of insulation off.  Use an appropriate level of care with craft knives;  They are razor sharp.  The volume control VR1 needs to have the pins bent as shown so that it will plug into the board pointing out sideways. 

Spekaer wires passed
          through the tah hole and crimped with pliers

You need to connect two lengths of wire to the loudspeaker by passing it through the terminal hole one or twice and crimping it down firmly with pliers to make a good connection.  The other ends can then be plugged into the board as usual.

Wind and Connect the Antenna Coils

The input coupling coil is the small one which takes the signal from the ferrite rod and feeds it into the radio.  This is constructed as follows.  First make a paper tube about 2.5cm long.  Use a strip of some normal printer or magazine paper about 2.5cm X 10cm.  Put stick glue on about 8 of the 10cm length then wrap this around the ferrite rod so that the glue sticks the paper and forms the tube.

Coupling Coil Former
          Glue PosistionCoupling Coil
          Former Roll StartComplete Coupling
          Coil Paper Former

After this has dried, using single core PVC insulated copper wire, wrap 6 turns onto the tube.  Secure the start end with sticky tape first and then the far end when it is complete.  You can follow the photographs which show this process.  Don't wrap the paper, the wire or the tape too tightly or you won't be able to slide the coil up and down the rod. 
I recommend winding the wire by rotating the rod and former rather than wrapping the wire around.  Wrapping it around tends to put a twist in the length of the wire which makes it tangle up more easily.

Coupling coil wire
          taped and winding startedTRF Radio Coupling
          Coil Complete


The main MW tuning coil is the large one which is connected to the tuning capacitor.
  The paper tube for this is made from paper 6cm X 10cm.  First wind the coil in one direction across the tube for about 25 turns and then 20 turns over the original turns coming back in the other direction

Main TRF Tuning Coil
          Winding StartMain MW Radio Coil
          Winding StartedMain Tuning Coil Halfway
Main Coil
          Overwinding StartMain MW AM Coil Almost
          CompleteCompleted MW Tuning
          Coil on Ferrite Loopstick

Connect the Variable Capacitor VC1 to the Main Coil

Looking at the variable capacitor from the bottom side, the side without the control shaft, you will see two trimmer screws and the three strip connections.  The two trimmer screws add extra fixed capacitance to the variable elements.  They should be adjusted as shown in the third photograph for minimum extra capacitance.  This is the position where the semi-circular moving plates attached to the screws are clear of the lower metal plates.

Variable capacitor trimmers
              set to maximumtuning capacitor trimmers
              set to halftuning capacitor trimmers set
              to minimum

The middle strip is the common ground for the two capacitors and the outer strips are the remaining connections.  As supplied, the connection strips exit at the side nearest to the top face of the capacitor.  To fit it into the terminal strip you need to remove the transparent plastic case of the variable capacitor, and unfold the strips so that they go out of the holes in the bottom face, as shown in the picture sequence.


Plastic AM Capacitor As
            SuppliedVariable Capacitor With the Back Cover
            Removed To Move the LeadsCapacitor Connection Strips As Required
            for connection into terminal strip


VC1 connects only to the main MW coil using the three-way terminal strip provided.  The link wire between the two outer terminals connects the two variable capacitors inside the case together to work in parallel. 

VC1 Wiring
                pre-assemblyVariable Capacitor Connected to Main Coil Using
                Terminal StripTRF Radio Coil Connection Detail

The smaller of the two coils connects into the breadboard as shown in the picture.

Thatís It, but Have a Check Around

If you've gone stage-by-stage and everything is in the right place, it should be ready to go.  It's worth double checking the direction of the two diodes, the transistors and the electrolytic capacitors. 

Starting Up and Initial Testing
 
Connect the battery, move the slider of SW1 to the end nearest to the two connected wires and rotate the volume control up to about halfway. Turning the tuning control should immediately allow you to hear the main MW stations.
 

The Completed Radio

The Completed Breadboard MW AM
          TRF Radio

Now that it's all together, you can count how many AM stations you can pick up, bearing in mind that the antenna is directional so you have to rotate the radio for the best signal.  If you find that you are missing the high frequency end of MW you can move the main tuning coil closer to the end of the rod, shuffling up the coupling coil to meet it.  How good is it compared to something bought from the shops?  The three models that I have made up are all of identical performance and I think that it compares well with a small shop radio.  The tuning is sharp enough to distinguish the three Absolute Radio frequencies of 1215, 1242 and 1260 kHz in Farnborough, Hampshire UK which is good going for this simpler design.  There's no need for external aerial wires as with some home constructed designs.  The audio amplifier drives the small loudspeaker to a good volume.

My Radio Isn't Working.

Most probably, something is in the wrong hole.  It can be useful to make a colour print-out, place it by the side of the real thing and play spot-the-difference.  Have another look for shorts between components where they are where they are close to other wires.  Check the resistor colour codes as it's easy to neglect those once you've decided that you've chosen the right part and plugged it in.  My favourite mistake is getting the output driver transistors Q5 and Q6 swapped over and one or both of the diodes the wrong way round.  If you've got a multimeter, use the 10V voltage measurement range to check around the d.c. voltages when tuned away from a station.  With a fresh 9V battery you should see voltages that are very close to those marked on the schematic diagram.  Very close in this instance means within plus or minus 0.3V of the figure in the diagram.  The voltages marked on the schematic diagram are measured values for direct connection of 9.0V power with low signal input, all with the negative meter lead connected to the negative battery terminal.  If you see a voltage that is more than about 0.3V out either way, then there is probably something wrong in the circuit around that transistor.  If all voltages look low, the battery rail is probably shorted out in some way.  If you have intermittent operation that comes and goes with vibration or handling, gently prod the various parts of the circuit until you find the sensitive part.  There will usually be a wire loose or shorted in that area.

The area around Q2 and C5 is quite sensitive to interference from nearby wiring.  If you find that the radio is working but is very hissy or squeals, try moving the coupling coil up the ferrite rod a small distance away from the main coil.

End Of Construction


That's the end of the construction notes, and is as far as you have to read if you just want to build the radio.  What follows is some more in-depth technical description and design philosophy discussion for the electronics engineers.  After that there is a FAQ which may be useful if you're still having problems getting going, or if you've bought all your own components.  If you fancy trying some further modifications, there's a page with some ideas on too.




Design Philosophy

As noted at the top, this is intended to be a modern replacement for the 1971 Ladybird Book "Learnabout: How to Make a Transistor Radio" radio.  I've described elsewhere how I didn't have much success with this as a kid, mostly due to the problems with 1960s transistors and their substitutes in a simple design.  I have made up kits of parts for the 1971 G.C.Dobbs design but I didn't want to encourage the use of germanium transistors and LT700 output transformers.  I propose this design as an alternative.  It is more complex and has twice as many transistors, yes;  But with the additional complexity it works better, it is more consistent, you can get all the parts, and it is cheaper. 

Why do we need another design for a simple AM radio when there are already so many? 

Although there are many designs around, none of them seemed to bring together all of the points below.

Uses entirely cheap modern components.
Is very insensitive to any component variation.
Has no integrated circuit black box components.
Can be built with a solderless assembly scheme.
Has no fiddly adjustments.
Uses design principles which can be fully explained.
Uses no funny electronic tricks.
Works as near as possible to something that you might buy from a shop.

Breadboard Construction

I've started from the assumption that soldering puts many people off electronic projects.  A good soldering iron isn't cheap, burns the inexperienced quite readily and will set fire to the house if it doesn't have a proper stand and falls off the desk if left switched-on unattended.  It requires a small amount of skill too.  The plug-in breadboards are not perfect but they are very quick and easy to use.  The availability of these small, cheaper plug-in boards makes this construction method a quicker alternative to building on terminal strips as shown in the choccy-block radio.  If you want to try plugging in different transistors, coils, speakers etc, it's much quicker on the breadboard.

Breadboard Transistor Radio Detailed Circuit Description


Breadboard Transistor Radio FAQ

Breadboard Radio Additional Pictures.

Breadboard Radio Modifications and Extras

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© Henry J. Walmsley 2014.