Emergency Radio
Crystal Emergency Radio Circuit
Crystal Emergency 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.
Emergency Radio
Emergency Radio Circuit
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.
Crystal Emergency Radio Circuit
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.
Possible uses:
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 crystal emergency radio 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.
Amplified Crystal Emergency Radio
An improved crystal emergency radio with a two-stage audio amplifier. The 2N3904 (or any other general-purpose NPN) acts as a preamplifier while the LM386 boosts the signal high enough to drive low impedance Walkman headphones or perhaps even a small speaker.
Crystal Emergency Radio Amplifier Circuit
Crystal Emergency Radio Pros and Cons
Problems:
The same rules that we’ve already been through at the previous circuit above.
Possible uses:
Make a real AM receiver! With a decent antenna and good ground you should be able to catch a station or two. A 12-turn air core coil 5mm in diameter should get you somewhere between 5 and 15 Mhz, for example. You might also experiment with different coils for other band with crystal emergency radio.
See also: RC Radio Circuit – RF Wireless Circuit
