On Thu, Apr 13, 2006 at 02:48:39PM -0700, William Chops Westfield wrote:
> A long time ago I built a "touch switch" that essentially worked
> because a human body is a reasonably good antenna for the 60Hz Hum
> that's all around us these days.  Touch an otherwise unterminated
> pin of a generic op-amp configured for "infinite" gain, and I got
> a nice 60Hz waveform at the output suitable for driving a flipflop
> and relay...
> 
> Since then, op amps have only gotten better.   Is it possible to
> build a circuit NOW that reliably detects 60Hz hum with only a
> small internal antenna?  How close to power wiring would it have
> to be?  In particular, can I make reliable lithium power-fail
> safety lights that I can just stick to a wall anywhere in the
> average USian home?

You should look up Quantum Research Group's QTouch series of ICs, for
instance the QT110 chip. They work by measuring the capacitance change
when someone touches an electrode. They are very simple and rugged, lots
of people use them at my school. Fairly low power and they just
magically work.

-- 
pete@petertodd.ca http://www.petertodd.ca
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