> What would be lowest cost method of detecting
> a hand moving through a volume of space.

Hi Cedric,

It's possible to detect a hand or any other conductive object at a
distance of 6" by using capacitive sensing. However, you'll need a system
capable of resolving roughly 10fF changes. Most capacitance sensing chips
on the market are incapable of achieving this kind of resolution. However,
I believe Analog Devices claims 100 aF resolution. The PSoc and Quantum
capacitance sensing chips are too noisy.

Another approach is to use an all-analog sensing scheme:

http://www.4qdtec.com/pdet.html

It's been specifically optimized to detect females over males!

Scott

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