> There's a very clear writeup of the general-purpose I/O technique in
> this contest entry:
> 
> http://www.circuitcellar.com/microchip2007/winners/DE/MT2239.html

Thanks. I've downloaded the entry and will work through it

> There's a very simple technique for converting a differential variable
> capacitance (even a tiny one) directly into a DC voltage that varies
> by about a couple of volts or so

> If you're interested in pursuing this, let me know

If it's convenient for you I'd like to know more. A method I wasn't
expecting and would like to experiment

wbr
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