Capacitive Proximity Sensing
     Here’s a neat magic trick for a user interface. It’s called capacitive proximity sensing. It can detect the presence of a finger or a hand or a body on the opposite side of a wall. This can be really handy when you want the observer to interact with something but have the interface be completely invisible.
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    The technique involves three parts: One, an oscillator circuit that uses a metal plate as the capacitor that effects the frequency. Two, a frequency divider that averages out all the garbage that might be picked up by the metal plate. and Three, a circuit that detects changes in the average frequency.
                                       
 Here’s a rough schematic using a very simple circuit. In the schematic I have it lighting an LED but this could go into the Logochip or another circuit.
I have data sheets for the two Integrated Circuits used in the circuit here:
          the CD40106     and the CD4040