In January 2011, we were trying to think of ideas to attract newcomers into the hobby of amateur radio and decided to aim at the youngsters by giving away a crystal set construction kit at the local Charity Gala. We would promote it as green technology – not needing any external power supply – and encourage people to assemble it there and then, test it and go away with a working radio receiver.
Although a crystal set is simple, we envisaged problems constructing it as we would be working in a park and offering the kit to people of all ages who didn't know one end of a soldering iron from another. We found the ideal construction method in the RSGB book “Practical Receivers for Beginners” by John Case GW4HWR (published in 1996) which included a crystal set constructed around a screw connector block.
After a lot of discussion amongst club members we decided to use a variable inductance in parallel with a fixed capacitor to form the tuned circuit and a crystal earpiece as the output.
Using a variable inductance meant that we avoided the cost of a variable capacitor and knob.
We were fortunate in obtaining some short pieces of ferrite rod about two inches long - enough to just fit into the single-layer coil - and then glued cocktail sticks to the end to form a handle. This allowed the users to easily move the ferrite in and out of the coil.
If you don't have any ferrite rod lying around, Maplin sell a ferrite rod with a couple of coils (code LB12N) but a) it is expensive (£2.29 plus delivery) and b) the rod is far too long and needs to be cut down - after all, you only need sufficient ferrite to completely fill the coil. Any excess beyond that doesn't add inductance to the circuit. However, you do gain the coil ready-made and get a spare pile-wound coil intended for long wave reception!
Germanium diodes are now quite expensive but BAT42 Schottky diodes are just as good and a lot cheaper!
Eventually, we found a source of cheap crystal earpieces on the Amazon.co.uk web-site.
Peter G4NTA designed a wallet and kit - most of the following information has been taken from the "pages" which Peter printed on the wallet but the text has been expanded in some places.
Introduction
This crystal radio is the simplest and most basic type of radio receiver that one can assemble and use to listen to medium wave broadcast stations.
Although some relatively modern components have been used in assembling this receiver, the theory and circuitry is exactly the same as those early crystal sets used, perhaps by your Great Grandparents, to listen to programmes transmitted by the infant BBC inthe1920's.
If you wish to know more about radio in general or Amateur radio in particular, please do not hesitate in putting your questions to any members of H.A.D.A.R.S. who will be in attendance here all day
Amateur radio is a fascinating hobby where amateurs can contact each other all over the world and sometimes in Space! Again, for more information please contact any of our club members here today.
Aerial and Earth connections
Aerial/Antenna
You will have noticed there are no batteries or any other form of power supply associated with your crystal receiver.
This is because all the power required to "drive" your receiver is "collected" by the aerial system. The "collected" power is measured in micro-volts (millionths of a volt!). Therefore, the longer and higher the aerial is, the more power it will "collect" and thus the better will be the performance of your receiver.
Earth
As important as the aerial, a good connection to earth will give improved performance to your receiver.
You could try clipping the earth wire to a cold water copper pipe or to an unpainted section of your central heating system
NB. It is NOT advisable to use the earth connection of the Domestic Electrical System
Components List
1 6 Position Connector Block - marked CB
2 Tuning Coil with sliding core - marked L1
3 Crystal Diode* - marked D1
4 100PF Capacitor - marked C1
5 Resistor- marked R1
6 High Impedance Ear Piece - marked EP
7 2 Link Connecting Wire - marked Red
8 3 Link Connecting Wire - marked Green
*NB When installing Diode (D1) ensure the identifying band is as shown on Layout plan i.e. The Band should be on the right hand side
If you are making up your own kit, the following information may be helpful:
C1 100pF ceramic capacitor
D1 BAT42 Schottky diode - a germanium diode such as OA90 or OA91 can be used
L1 60 - 70 turns 10mm diameter of 30 - 34 SWG enamelled copper wire on paper former to fit
loosely round ferrite rod
R1 33k resistor although anything between 25k and 100k will work satisfactorily
1 piece ferrite rod 10 mm diameter approx 50mm (2 inches) long
crystal earpiece - not magnetic
6-terminal screw connector block - use the 3A or 5A version as others are too big
3 wire links made from insulated connecting wire each approx 30mm (1 1/2 inches) long. Peter made up a 2-way link with red insultation (to link connectors 1 and 3) and a 3-way link with freen insulation (to link connectors 2, 5 and 6)
Possible problems
The wires from the coil and earpiece are too thin so tend stick into the connector block at the side of - instead of under - the screw and hence don't get fastened down. Solder some thicker bits of wire to the ends - with the ear piece leads insulate the solder joint by taping them or adding heat-shrink.
We stuck double-sided sticky tape to the screw connector block so that it could be stuck to the layout on to the cardboard pack. However, fixing the coil was a bit more difficult - we initially glued some balsa wood to the coil and then stuck tape to the bottom of the balsa but found that the tape lost its "stick" over time - when the constructors peeled the backing strip off the tape on the balsa, the entire tape came away from the balsa wood. Next time round, we'll try using Blu-tak or similar stuff to fasten down the coil and do away with the balsa wood.
Using a variable inductance avoided having to solder together a coil and capacitor. The capacitor was mounted on the screw connector block which reduced the number of components floating aorund.
With a crystal set you need a good aerial. At the Halifax Charity Gala we used about 15m of wire suspended 5m high which allowed us to receive 5 MF broadcasting stations. Although sdding an earth increased the output, we didn't always use it as the volume was sufficient without the earth.
Layout plan
Assembly instructions
NoteThe six terminals of the screw connector block are numbered 1 to 6 from the left hand end to the right - the upper end is marked A and the lower end is marked B. On the HADARS kit, all the components were mounted on the layout diagram marked inside the wallet and shown above.
Remove the backing paper from the screw connector block and align and fix on to the layout plan
Fit C1 between terminals 2A and 2A - screw down terminal 2A but do not screw down terminal 3A
Fit D1 between terminals 3A and 4A - screw down terminal 3A but do not screw down terminal 4A
Note which way round to mount D1 - the mark on the diagram matches the mark on the component
Fit the earpiece between terminals 4A and 5A and screw down both terminals
Fit R1 between terminals 4B and 5B - screw down terminal 4B but do not screw down terminal 5B
Fit a wire link between terminals 1B and 3B - screw down terminal 1B but do not screw down terminal 3B
Fit a wire link between terminals 5B and 6B - screw down terminal 6B but do not screw down terminal 5B
Fit a wire link between terminals 2B and 5B - screw down terminal 5B but do not screw down terminal 2B
Fit L1 between terminals 2B and 3B - screw down both terminals
Connect aerial wire to terminal 1A and screw down terminal
Connect earth wire to terminal 6A and screw down terminal
Operation
Having checked that all the above connections are correctly in place, then using the earpiece to listen, gently slide the ferrite rod core in or out of coil L1 until you hear a radio station. Having identified the station, you can mark the sliding core "handle" as a form of calibration.
Well done and happy listening
Troubleshooting
It is very unlikely that a component will be faulty. We have found that most problems occur because the connecting wires or the ends of the components have not been inserted under the screws in the connector block.
Credits
The original idea came from Zac, now M6ZAQ, Peter G4NTA did all the work in creating the wallets and kits; David G3PTU, John G7ELX, Barry M6PEC, John M0JPA , Peter G0JKW, Anthony G0WFG and Martin M0GQB all hepled on the day and guided people through the construction and testing.
