Emergency Crystal radio
requires no power supply to operate because it uses energy from the signals it receives. Performance is – like one should expect – poor. Nevertheless, with a good antenna and proper ground, it shouldn’t be too difficult to tune to a strong AM station.
While this emergency crystal radio circuit is very simple, many points should be observed. The variable capacitor is relatively hard to get in a store these days, so your best bet to find one could be a junked AM receiver. Connect the case to the antenna and the central rod to ground. Otherwise, you’ll de-tune the circuit every time you touch the capacitor.
Next comes the coil. Originally, 60-80 turn air core coils of large diameter (10cm) were used. A less bulky alternative could be 30 turns of enamel-coated magnet wire wound on a ferrite rod. It works for me, at least.
Detection and demodulation of the signal is done by a germanium diode. In the US, 1N34 seems to be very popular. Here in Europe, AA119 is more common and offers similar performance.
Finally, you’ll need high-impedance headphones. These are also quite rare. Ordinary Walkman phones won’t work directly. Add an audio output transformer to drive them or build a small amplifier like shown on the next page.
Problems:
To hear anything, you’ll need a very long antenna. Use any hookup wire at least a few meters in length. Make sure that no power lines are nearby! A good ground is also vital, like a cold water pipe. Otherwise, plant a metal rod as deeply in the ground as possible and it should work well.
In these times of incredible advance in technology, such a simplistic receiver doesn’t seem to have any real use at all. It’s a nice project for the more nostalgic types, though. Since no power is required for operation, having such a receiver could prove handy in an emergency situation or just to hear the news when you’re out in the woods backpacking
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