Abstract: Each of the inputs of an analog multiplexer are connected to a plurality of capacitive keyswitches and a reference capacitor. At the first multiplexer scan position, the reference capacitor is charged through a charging resistor and when a predetermined voltage is reached a threshold amplifier is caused to respond. The time period required to charge the reference capacitor is stored and compared with the time period required to charge each of the keyswitches. A control signal is generated in response to this comparison providing a signal representative of the capacitive value of the keyswitch in relation to the reference capacitor and thus the operative condition of the keyswitch.