Mechanical Virtual Theremin

I built Arthur Harrison's minimum theremin last night, and it was easy to
put together and worked "right out of the box".

Was trying to think of a way to make a virtual theremin, found that it
already exists.

http://www.simple-media.co.uk/music/vsti/fvspook%20keys.htm

So I now want to use sensors to drive oscillators similar to a Theremin or a
Tannerin. What type of magnetometer would be sensitive to a permanent
magnet being brought into proximity to it? I want a cheap sensor that can
be driven by OpAmp.


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