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RE: A Simple Radio Circuit



Tom--
The simple circuit is used to detect amplitude-modulated signals. In
AM, a low frequency signal has been added to a high frequency "carrier",
so the amplitude of the carrier varies like the low frequency signal.
The composite signal is AC, and the diode is used to rectify the signal,
converting it to a pulsating DC (here "DC" implies a voltage with fixed
polarity, rather than a constant voltage). For many AM stations, the
amplitude of that pulsating voltage may be on the order of a volt or so,
especially for a local station. This is large enough to drive a high-
impedance earphone directly, which is why no extra amplification is
needed.
In the case of people picking up AM radio stations on their teeth,
what happens is that an oxide layer can form between a filling and the
tooth, and the oxide acts like a rectifier. The resulting current
pulses may be large enough to cause the filling to vibrate, and those
vibrations will be detected by the person's ears via bone conduction
of the vibrations. I also remember a case many years ago of a woman
repeatedly calling the police to report a man under her bed. It turned
out some of the springs in her mattress had rusted, and the oxide layer
was rectifying the signal from a local AM radio station. When the woman
laid her head on the mattress, she could hear the faint vibrations
caused by the rectified signal.

Steve Luzader
Frostburg State University
sluzader@fre.fsu.umd.edu