> The circuit is kind of similar to my first project as a
> real electronics engineer back in 9 years ago

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.
The output is split - one path goes to an op amp/comparator,
the other to an antenna (PCB or wire). The signal on the
antenna goes to the other terminal on the op amp/comparator.
A body approaching the circuit loads the antenna, or absorbs
the signal, I forget which, causing a change in the op amp and
thus triggering the alrm section. The one I made was quite simple
and stable and could detect movement in a room


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