The
Choccy
Block Crystal Set Radio Kit


I've sold quite a few of these now and people are having 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 right at the bottom
of this page.
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 a pdf electronic schematic diagram. If you're
familiar with electronics this will be helpful. If not, you
can just look at the pictures. 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 Schematic Diagram
of Choccy Block Crystal Set Radio
Bill Of Materials:
* 3 Amp terminal strip
* LT44 transformer L2
* variable capacitor VC1
* 2 M2.5 4mm screws
* 3 M2.5 20mm screws
* 3 M2.5 nuts
* 5 M2.5 washers
* BAT48 schottky diode D1
* 470pF ceramic disc capacitor C1
* ceramic / crystal earpiece Y1
* 30SWG / 0.315mm enameled copper wire 4m
* 1m yellow aerial connection
wire
* 1m green earth connection
wire
* 1 yellow insulated crocodile
clip
* 1 green insulated crocodile
clip
* control knob for short shaft
* 100mm long, 9.5mm diameter
ferrite rod
* 100mm single core red hookup
wire
* 100mm single core black
hookup wire

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.
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.
A complete kit of electronic
parts is available on Etsy at the following link, where
overseas shipping options are available to some countries:
Crystal Set Radio Kit of Electronic
Parts, Solderless Choccy Block / Terminal
Strip: Etsy
Listing Number 881740008
Etsy shop top level for other kits and
components:
UsefulComponents Etsy Shop
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 2014.
Edit History
26-AUG-2022: Removed references to my ebay shop, having
been "binned-off" from that site for telling the truth
about their business practices to the
UK vice-president.