> I mentioned RF and Theremins. They are basically the same
> as the capacitance method but use a much higher frequency.
> One alarm circuit I was thinking of has a simple RF oscillator.

Theremins actually use two different approaches.
The pitch section uses hand capacitance to detune a resonant "antenna"
circuit, and shift oscillation of that section against a reference
oscillator, producing the difference frequency as the pitch output.

The volume section runs at a different frequency, and uses the same
technique to shift the oscillator across a tuned circuit, resulting in
a varying output level, which is used as the volume control voltage.

The fun part with the theremin is the "antenna" circuit, which is
designed to maximize and hopefully linearize the pitch changes for a
very small hand capacitance change.  I stand about 2' from the
antenna, and on the low notes, my pitch hand ends up essentially on my
chest.  It's surprisingly sensitive, and fiendishly difficult to play,
let alone play well.


I'm sort of surprised nobody mentioned this other approach that meets
the criteria:

A hot solder iron.  Just listen for the scream.   :)
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