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I'm 3D printing a series of motorised automata. I want to display these like an arcade in my window that fronts the street.

To make them interactive I want anyone interested to be able to switch the motor on themselves, but to do this the switch needs to be accessible outside of the window which is double glazed.

Is there a capacitive touch sensor switch which can handle both the two thicknesses of glass and the air gap between?

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You sometimes see interactive displays in shops such as estate agents, but sometimes they don't work well, and double-glazing and weather can definitely be problems.

But I'd recommend thinking about some non-capacitive method

Consider instead a simple optical proximity sensor (LED and phototransistor), perhaps in infrared. Very cheap and easy to try. Be aware a lot of windows are opaque to IR, but you may find yours isn't. (Try with television remote and camera in your phone should show it.)

Same idea but use a laser pointer and a narrow-angle detector.

Or a photodetector adjusted to switch on when a very bright light is on it (mobile phone torch?)

If you've got enough computing power (Raspberry Pi + camera, perhaps) you could have a face or hand detector.

A hand detector which says how many fingers you're holding up could animate a particular robot -- this seems an especially captivating idea. Highly recommended article on this: OpenCV for counting fingers

Or face detecotr? "I see you looking!" Blog post on faces)

Other less-good ideas: bluetooth (would work but interaction is ugly), wifi (same), rfid (is it taking my money!).

See also this question.

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One way to improve the sensitivity of the touch sensors would be to put foil on both sides of the window.

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It should be possible. A while back I made an water sensor that was based on an RC oscillator with a small capacitance and large resistance (a few pF and 100 kΩ or so). A tiny change in capacitance would give an easily measurable change in frequency. You’d want a fairly large (palm-sized) pad.

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