> I'm sure your circuit works as advertised.
> 
> But for a 6" distance, the coupling between a hand and a sensor
> that is roughly 6"X6" is on the order of a picofarad. And the ability
> to reliably resolve a detection distance on the order of 6 inches
> requires a circuit that can resolve much smaller than a pF. I think
> your circuit can achieve this if the potentiometer is adjusted
> correctly and the noise is low enough

Hi Scott, yes, believe it not, the circuit does work quite well at
any scale of distance vs sensor size. You'll notice that the opamp
o/p is filtered. What you'd see as a hand approaches the sensor
(from memory, it's been a while since I had a scope on this) is a
decreasing amount of the 75kHz getting through, and the DC
component gradually increasing. Filtering adds that AC to the
DC for a cleaner signal. At a certain distance there is little AC
to smooth and a more rapid change of DC level

I have found that the capacitive qualities of people has quite a
range. I'm quite good at activating these but others aren't. Even
those with big hands. It mightn't sound right that physiology is
so different between people of the same sex and approximate
size, but that seems to be the case. It may even come down to
what they had for breakfast - eg electrolyte levels, blood
pressure, sweat gland activity, who knows what else. I've never
been troubled by static discharges, unlike those who pick up
very annoying charges in offices or cars. There's probably a
mechanism like that for capacitance

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