[Viacheslav] built a keypad that uses human capacitance to detect key presses. Unlike normal keys which close a physical connection, his project detects touch through the PCB substrate. He uses the analog comparator of an AVR ATmega8 to detect the moment of zero crossing and then measures the time it takes to discharge in order to detect key presses.
I use analog comparator to detect the moment of zero crossing and the charge is evaluated by measuring time. Builtin AVR ADC probably wouldn’t be very useful here because the charge that can be built up is very small.
Update: [Viacheslav] left a comment to let us know we had it wrong. Instead of using the ADC, the analog comparitor is used. This change has been made above.