A complete kit of all the electronic parts shown is available,
and the details of how to buy are shown right at the bottom of
this page.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
The smaller of the two coils connects into the breadboard as shown
in the picture. That’s 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
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
Alternatively,
for dispatch via first class post to UK addresses, you can
pay using your usual credit / debit cards or Paypal balance
via the following button where at £15.95, the total is
slightly cheaper than the ebay shop:
Your order
will usually be sent the next working day. If it
is particularly urgent, please check with
me first at vila_nestor@hotmail.com in case I am on
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