[Lefty]

Oh yes, I remember crystal radios. I had a philco crystal detector, but I too could only get one station as I lived only a mile from a heavy weight 24/7 AM (KCBS) station that swamped out all other stations.

There are still active crystal groups that publish plans and other stuff. I'm sure a goggle search would turn some up.

Theory? Well lets see. There is usually a tuned L/C input circuit to try and select a given frequency, then the RF signal is rectified to pulsating DC by the crystal (usually germanium). The crystal is nothing more then a primitive diode. Then a small bypass capacitor shunts the RF component to ground but lets the amplitude modulation (audio frequency) pass onto the headphone. As there is no power, there is no amplification avalible so sensitive high impedence headphone were required to help with the otherwise very weak signals. A long wire outdoor antenna is needed so as to try and get as strong an input signal as possible. 100 ft. signal wire routed as high as possible is the usual recipe.

FM? no, only audio amplitude (AM) modulation can be demodulated with this simple rectification method.

I think a simple crystal kit would be a great way to get a child or grandchild interested in electronics if given at the right age, say around 10 or so.

Lefty