The
HJW
Electronics Choccy Block Crystal Set Radio Kit CBCSRK-1T




I've sold lots of these since 2011, and people are having plenty
of success with them. If you're thinking of buying one of the
commercial plastic sets without an aerial coupling coil, output
transformer, and with an untested earphone, please consider this
one instead. I can confirm that it is much better, and you won't
risk the 40% chance of getting a dud earphone that are the
current odds. (See The
Problem
With Crystal Earphones)
A
complete kit of all the electronic parts shown is available,
and the details of how to buy are shown below.
Sales
Outlets:
The kit is also available on various online
marketplaces, where overseas shipping options are
available to some countries. Please see Usefulcomponents
/ HJW Electronics Online Marketplace
Buying Options
Introduction
This is a small crystal radio which can
be constructed without soldering using 3A terminal strip or
"Choccy Block." It's best to read right through the page
first before starting out.
How To Build The Radio
Here is the electronic schematic diagram. If you're familiar with
electronics this will be helpful. If not, you can just look
at the pictures to build the kit. In addition to the parts
provided in the picture you will need a screwdriver for the
terminal block, some pliers, some wire cutters and some form of
wire stripper. To make the paper tubes for the coils you will need
some paper, glue, scissors and sticky tape.
Schematic Diagram - Click To
Enlarge

Link to PDF
Schematic Diagram Of Choccy Block Crystal Set Radio
Bill Of Materials (BOM):
Qty Refs Type
Value
1
3
Amp terminal strip
1
LT44 transformer
1 VC1A variable capacitor
210PF total
2 M2.5
4mm screws
3 M2.5
20mm screws
3 M2.5
nuts
5 M2.5
washers
1 D1 silicon
Schottky diode
BAT48
1 C1 ceramic disc
capacitor
470PF
1 Y1 ceramic
/ crystal earpiece
1 30SWG /
0.315mm enamelled copper wire 4m
1 1m
yellow aerial connection wire
1 1m
green earth connection wire
1 1
yellow insulated crocodile clip
1 1
green insulated crocodile clip
1 control knob
for short shaft
1 100mm
long, 9.5mm diameter ferrite rod
1 300mm
single core red hookup wire
1 300mm
single core black hookup wire
1 R1 carbon
film resistor 0.5W
2M2
2 C2,C3 ceramic disc capacitor
100NF
1 Q1 NPN small signal transistor
BC549C
1 J1 Single AA battery holder

The
underlined items in the parts list above are links to more
detailed descriptions. You don't need to look at, or understand
the contents if you just want to make the kit. The last four
items in the BOM are included in the kit, but they are a 2025
addition. You might decide to scroll down to the bottom of the
page to find out how they can be used.
General Connections
Link
to
Large Photo of Choccy Block Crystal Radio Wiring

In the photo above you can see each component and where it is
connected quite easily, using the large wiring picture if
needed. You need to cut a group of two terminals from one
end of the block for the connections to the earpiece. The
red and black link wires, the aerial and earth wires, and the
crystal earpiece wires need to be stripped of plastic insulation
where they connect into the terminal block. The coil wire
ends need to have the enamel insulation scraped off where they
connect in. The LT44 transformer should be connected so that
the outer two wires of the end with three wires are connected to
the earpiece wires. You can leave the middle wire
disconnected. The transformer is best butted right up against the
terminal strip which gives it a little bit of support to stop the
wires bending. The same applies to the variable capacitor.
The Variable Capacitor
Connections
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. The variable
capacitor is actually two capacitors which are connected together
in the terminal strip circuit. As supplied, the connection
strips exit at the side of 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.



Making the Coils
To wind the coils you need the wire supplied, some sellotape,
scissors, ordinary paper and some glue. The process in shown in
the pictures. You first need to make two paper tubes by cutting a
piece of paper 25mm wide and about 100mm long and another about
20mm wide by 100mm long. The tubes are formed by applying glue to
two thirds of the length of the paper and wrapping it loosely
around the ferrite rod. The tubes need to be able to slide
up and down on the rod, so don't wrap it too tightly. It's best to
leave the glue to dry before winding the wire on so that the tube
doesn't crush and bind on the rod. The wire is secured at one end
of the paper former with a thin strip of tape and then for the
larger main coil, wind 50 turns onto the tube. The aerial coupling
coil requires just 30 turns. It is better to turn the tube rather
than winding the wire around by hand, because winding it around
puts a twist in the wire. Again, you need to wind the turns just
tightly enough to stay in place so that the coil and tube can
still move on the rod afterwards. At the end of each coil, another
thin strip of tape holds the far end of the wire in place. If you
lose exact count of the number of turns don't worry. It need only
be approximate.
Update: The
ferrite rod provided is now 100mm long, so it's a bit easier to
handle and you can make the paper tubes a bit wider. You still
want to be able to slide the antenna coil close up to the tuning
coil though.




Connecting to the Crocodile Clips
The crocodile clips are much more convenient for connecting to
aerials and earth connections than bare wires. You need to strip
the ends of the yellow and green wires first, then remove the
insulating boot from the clips as shown. The metal flaps on the
ends of the clips can then be crushed down on the folded-back
stripped wire ends to make a firm connection before putting the
boot back on from the other end of the wire. To remove and replace
the insulating boot, hold the jaws open by clipping them onto some
small object.



Finishing Off
It is worth checking that
all of the wires are trapped under each screw correctly and that
none of them will pull out. When everything is wired as in the main picture, the
control knob can be fitted to the shaft of the variable capacitor
and you are ready to test.
Testing and Operation
Connect the green earth wire to your earth connection and the
yellow aerial wire to your aerial. With the coils in about
the position shown, turn the tuning knob until a station is heard.
You may find that you can increase the loudness of a particular
station a bit by moving the coils around on the ferrite rod, but
the adjustment is not critical. There's very little to go
wrong, so troubleshooting should be confined to checking that the
connections are made as shown. That concludes the building
instructions and should get you going. Once it's working you can
fix the terminal strips down to a wooden board by carefully
banging in nails down through the holes or using the M2.5 screws
provided. If you decide to fix the variable capacitor to a front
panel, only use the very short 4mm screws provided. These are
guaranteed short enough not to foul the insides of the
capacitor.
If you want to get on and build it, that's all you need to know
but there's more information below which might be useful and some
technical discussion.
More Information
Aerials and Earths
I have successfully used this to pick up the main MW station in
the UK just by connecting to a central heating radiator for the
earth and to the outer casing of a television antenna cable,
disconnected from the TV, for the aerial. This is not ideal
because the TV cable is nailed to the house which both blocks and
drains away a lot of the MW radio signal. It is much better if you
can erect a long wire high up in a garden and there are many
places online which will show you how to do that, taking the usual
precautions against lightning strikes and going near to power
cables. Not everyone has a garden though, and you don't want
the TV aerial being disconnected on a regular basis, so there are
other interesting ways to get an aerial and earth.
Farm and Other Fences
While out walking I wondered if there were any useful MW signals
being picked up on farm fences. They are not good aerials for a
number of reasons, but they can be quite long which can make up
for that. You need to connect the earth clip to the lowest wire
and the aerial clip to the highest wire. They will always be a bit
corroded so you have to wiggle the clips to get a good
connection. If you find a fence in good condition, outside
of a deep valley which has dry posts and isn't covered in wet
bushes, you will almost always get reception of one or more
station. Station pickup depends on the direction of the fence as
well, so it's worth trying a few if you are out walking.
Some fences are electrified and it will be pretty clear which
those are, those being a single wire with insulated posts.
Obviously, I have to say it: Don't use those.
Update September 2011: If you
get a high-ish spot and a decent dry fence this can work even
better than I expected. I walked up Clee Hill in Shropshire
recently and attached the set to an ordinary fence with the
ground wire clipped to the standard square mesh below and the
barbed wire on top. I wasn't expecting much because the mesh and
wire sections have little separation, only a few inches. Signals
from Absolute radio, Five Live and another station were good and
loud. I needed to move the aerial coupling coil away from the
main coil on the rod to help separate the stations. The air
traffic radar station in the background is coincidental.
Update October 2011: The New
Forest campsite at Holmsley is neither particular high, nor is the
fence all that uncluttered; Still I picked up three stations from
there and found that it was best using either myself holding the
green clip for the ground or the very bottom fence wire which was
partially buried.

Other Structures
If you see any two metal structures which are not obviously
connected together you can see if there is reception available
from them. For example at various times in the past, crystal
radios have been used in prisons connected to a water pipes for an
earth and a bed frame for an aerial. Always stay away from using
the electrical mains earth and avoid connecting to household
appliances.
Boxing it Up
If you want to carry this around on walks, it's a bit inconvenient
in the basic form which was intended to be the easiest way to
build it up. If you want to put it in a small box, the first thing
to do is to change the variable capacitor connection strips back
around so that they once more exit at the same side as the shaft.
You can then fit the variable capacitor into the terminal strip
upside down so that the shaft faces downwards, or outwards from
your box, and the terminal strip and transformer will all be flush
with the same surface. I've shown an example below where I have
used an old cassette case. This isn't very good because the
transformer is too big so you have to cut a hole for it in the
back. It doesn't matter which terminal strip connection along the
block each circuit node connects into, so you can shuffle the
parts along towards the left to save space, and cut off the unused
terminals. The tuning knob is specially selected as one which
works well with this tuning capacitor: The grub screw is quite
near to the bottom of the knob and it has a recess underneath which clears
the mounting screws. This allows it to fit well on the short
tuning shaft.



Soldering
If you're happy soldering the wires directly together it is
possible to squeeze things up even further. If you shorten
the ferrite rod and wind the coils in a small pile you can just
about fit the whole thing into an old 35mm film canister. The
tuning knob sits neatly at one end. Something a bit like
this was used in the Maze prison in the early 1980s. If you want
to shorten the ferrite rod to fit it in a box, first use a hacksaw
to cut a shallow notch in it. Then wrap the rod in a tea towel and
snap it into two pieces at the notch by hand.

Further Technical
Discussion
Circuit Operation - Earpieces
Anyone who's played with crystal sets a bit will see that
everything in the circuit is pretty standard. There are some
small points to note. The beige coloured ceramic / crystal
earpieces are quite sensitive and in circuit model terms appear
much like a 26nF capacitor in series with some lossy resistance.
26nF is about 6K Ohms impedance at 1kHz. The LT44 is used in
reverse to provide a higher impedance audio match to the top of
the tuning coil via the diode. Looking into the diode
side of the transformer, the impedance actually increases when the
the earpiece is connected due to a fortunate broad resonance with
the earpiece and transformer leakage inductances. The result is
audio which is a bit restricted on bandwidth but which has a
fairly good match and efficiency. Some of the ceramic earpieces in the
supply chain are very poor at the moment and I recently sent back
67 out of an order of 100 as completely dead on arrival. The ones
supplied with the kit are 100% tested.
Diodes
You can use a 1965 Mullard OA81 or OA91 germanium point contact
diode if you really want to. In this circuit I've found that the
modern low Vf silicon Schottkys work as well as the best germanium
diode out of the junk box, and a lot better than a poor one of
which there were plenty.
Coils
You can go to great lengths to improve the Q of crystal radio
coils with modest results. If you wanted to eek a bit more out of
this one, separating the turns on the tuning coil by one wire
separation might help a bit if you are really going for it.
FAQs
Q: Can I use some different headphones?
A: Generally the 32 Ohm headphones that come with portable
audio equipment won't work in this circuit. The ceramic earpiece
and transformer combination are intended to get the best loudness
from weak signals. If you have some special high impedance radio
headphones, those might work.
Q: I've read that piezo earphones like those used here have
more than 1 Million Ohms impedance. Surely that is high enough to
connect directly without the transformer?
A: This is a common misconception. These ceramic element
earphones have more than 1 Meg Ohm resistance at D.C. as you would measure with a
multimeter, but they have quite a high capacitance. At audio
sound frequencies like 1kHz, that capacitance has an impedance of
about 6K Ohms. 6K is still quite high but this makes for a very
poor radio if you connect the diode to the top of the tuning coil
and the other end of the diode direct to the earpiece, as it loads
the tuned circuit too heavily giving very sloppy tuning and is
also a poor power match giving low sound level. Unfortunately you
see this statement and circuit configuration all too often.
Q: What does the ferrite rod do in this crystal set radio?
A: That's a good question, worthy of a separate page with an explanation
of using a ferrite rod in a crystal set radio.
Q: Isn't AM radio dead in the UK?
A: As of early 2025, not quite. Absolute Radio dropped off-air
before the end of their license agreement under an Ofcom cloud,
some time ago. My local Classic Gold at Fareham went off last
year. There's still some AM fun to be had, especially if you're
near one of the remaining music stations like Caroline out at
Orford Ness in Kent, Lyca Radio at Brookmans Park in Hertfordshire
for some Bangra, and BBC Five Live and talkSPORT also at Brookmans
park. BBC Radio Wales and Scotland still put out a strong signal
too, and there is some Punjabi weirdness on 558 kHz, where you may
hear religious music for hours at a time completely free from
advertisements. Now that's commitment!
Q: Does it matter which direction you wind the two coils in?
A: No. In this instance, it makes no difference.
Q: Does it matter which way round the diode goes?
A: No. It can go either way round in this instance.
Q: Can I wind the coils on a metal sleeve so that I can
slide them up and down more easily?
A: No. The metal will form a shorted turn and completely
eliminate any signal. You could use some small bore plastic pipe
or some cut-up lemonade bottle, though. Watch it; Cut-up plastic
lemonade bottle can be sharp.
Q: The knob shown in some of the pictures has 0 to 10
markings, and the one delivered has a pointer triangle.
A: The pointer type is more appropriate. The variable
capacitor only rotates through 180 degrees and the numbers cover
300. I'll change the pictures on a major update.
More on Crystal Earpiece
History
"When I were a lad..." OK, but bear with me on this
one. In the distant past I had two or three of these
items. One came with a Japanese "Gakken 150 in 1 electronics
kit" with the spring terminals, and another one or two came a
local shop in Blackburn. I've discussed elsewhere that one of
these worked spectacularly well, and others less so, probably due
to the 'good' one having some leakage resistance. That would make
the standard poor crystal radio circuit without a bleed resistor
across the earpiece or a transformer work OK. But at some stage,
one of them finally broke and I thought that I would take it apart
to see how it worked. What was inside?
The two wires under the screw-down cover go to terminals embedded
in the plastic much like the current items. On the other side
there were two conductors that converged at an apex, where there
was a small lump of something which I assume was a piezo crystal
of some sort. This was quite small, a maximum of 3mm diameter.
This poked through and was glued through a very fine, easily
broken aluminium diaphragm. You could see this glued part from the
other side when you removed the earpiece tube. This contrasts with
the current design which has a flat piezo element of maybe 15mm
diameter on a stiffer diaphragm, much like you'd see in the back
of a digital watch or standard piezo sounder. The question is,
were these old earpieces genuinely higher impedance, i.e. lower
capacitance due to having this smaller area crystal, and with the
lighter diaphragm also possibly higher efficiency? It might
explain why so many current crystal set designs just assume you
can connect these similar-looking modern earphones directly.
Crystal Set Modifications
Including Massive Cheating, The CBCSRK-1T Single
Transistor Version
Astute
readers of the instructions so far may have noted that as of
JUL-2025, there are two unused capacitors, a resistor, a
transistor, and a AA battery holder included in the standard
crystal set kit as an added extra bonus. Please click on the
link above to discover more, especially if you're in a poor
reception area, or can't get much aerial wire aloft. It's
amazing what you can do with one transistor and a single AA
battery, or even a lemon.
Should you have any problems or questions, my main
email address is shown below. This address has been the same
since 1997, and unless I'm on holiday, it is checked daily
including the spam folder.

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© Henry J. Walmsley 2025.
Recent Edit History
24-JUN-2025: CBCSRK-1T
massive cheating added and some minor
formatting