A Simple Crystal AM Radio
that Needs No Batteries
Feb 2004 by Rick Andersen
The simplest radio of all is the Crystal Receiver. This is the radio that soldiers in foxholes during World War 1 built out of scraps of wire, rusty razor blades, and telephone headsets. The crystal radio needs no battery power at all! It runs off of the microwatts of power present in the signal that reaches its antenna. Amazing as this is, the tradeoff is that you need a long wire antenna and a halfway decent earth ground connection to make it work. This is no pocket portable! But it works forever!
The entire radio consists of just a coil, a tuning capacitor, a germanium diode as the "detector", a crystal earphone, and a resistor (resistor is actually optional).
The crystal earphone is a high-impedance device that is very sensitive to voltages rather than currents; the older magnetic headphones and 8 ohm earphones of today are current-operated; i.e., their impedance is medium (2000 ohms) down to as low as 8 ohms. A crystal earphone will give an audible "click" when you touch its two leads together, even when out of the circuit. You can also hear a soft 60-cycle AC hum when you squeeze the leads in your fingers. So much of the magic of a crystal radio lies in the electric sensitivity of the earphone.
The sensitivity of a crystal earphone, in conjunction with the voltage amplification of a resonant L-C tuned circuit, enables you to receive the incredibly weak radio signals of AM broadcast stations within about 10-20 miles during daytime, but from hundreds of miles away at night. A long wire antenna thrown out the window to a tree - 100 feet or so is a really good performer - will pull in a lot of stations at night when the AM band goes "DX" (long-distance reception, due to ionospheric shifts between daylight and nighttime). A good ground can make all the difference in the world between a marginally-performing crystal radio and a stellar performer.
Drive a metal ground rod, or, alternately, run a ground wire from your receiver to a cold water pipe in your basement, using a clamp. Less efficient, but still workable, is an old trick I learned as a kid: run your ground wire to that little screw in the middle of the plastic cover on the wall electrical outlet - it's grounded to the power company's ground and will also make your radio pull in more stations. (DON'T connect anything to the prongs of the plug socket itself!!! ELECTRIC SHOCK HAZARD!!!)
The antenna wire goes to a tap somewhere on the coil; I've found about halfway down the coil to be a good spot to connect the antenna; experiment and do what works best at your location. Some people connect the detector diode to a tap, about 1/3 down from the top of the coil; I have found that this does NOT work as well for me... a lot depends on the antenna length. A crystal radio is a piece of "art"; expect to experiment a little until you find just what works best.
If you've never built a crystal radio before, be advised: you must use a Germanium diode such as a 1N34 or 1N60 for the detector; don't try to use a silicon diode such as the 1N914/1N4148 small signal switching diodes commonly used elsewhere in Electronics. The silicons turn on at around 0.6 volts; the germanium types, while older and "leakier" in the reverse direction, turn on at about 0.2v and are thus more sensitive-- definitely needed in this application where there are no batteries or amplifiers.
As shown in the schematic, I have a fondness for connecting my radio projects to a Radio Shack amplified speaker. Sometimes those crystal earphones get intermittent and don't work well unless you rap them on the table (lightly); sometimes I just get fatigued from having an earphone plugged into one ear and it's nicer to have the sound coming out of a speaker a few feet away.
You might think that crystal sets are only good for AM broadcast, not for shortwave (world-band) reception from 1.8 to 30 MHz, because the power used by most communications services in that range is much less than that used by AM broadcasters. But the Big Guns European broadcasters pack a wallop too, and a friend of mine has built more than one crystal receiver that works in the lower shortwave range (about 3-11 MHz) surprisingly well. So a long antenna, a good ground, and a sensitive earphone will enable you to listen to world-wide broadcasts with an extremely simple radio that uses no power except that of the incoming stations! That's the magic of crystal radios.