A crystal set is a very basic AM receiver that can be used without batteries.
Theory of operation
A commonly used schematic for a crystal set is this:
An AM signal will appear on the antenna.
The LC circuit will have a high resistance on the resonant frequency and a low resistance on the other frequencies. Therefore, all frequencies except the resonant one are shorted to ground. Only the wanted frequency, the resonant frequency of the LC circuit, will be detected by the "Crystal diode".
This crystal diode is a diode with a very low forward voltage, about 200-300mV. This is needed because the signals on the antenna are very weak. With a too high forward voltage, no signal would appear on the cathode of the detector diode.
High frequencies are filtered away by the fixed-value capacitor. It has a low impedance for high frequencies, so that they're shorted to ground. Only the audible frequencies continue to the crystal earphones.
These crystal earphones must have a high impedance (hi-Z). Otherwise, the output doesn't match the earphones and you won't be able to hear anything.
