a sensor electrode is capacitively coupled to the environment . electric charges carried by surrounding objects induce corresponding electric charges on the sensor electrode . a high input impedance circuit senses change in charge on the electrode and provides a first varying signal indicative of that change . a second circuit compares the first signal against a threshold level and provides a second signal indicative of the movement . a pair of sensors may be included to cancel out extraneous environmental chasrges . difference between charges on each sensor electrode of the pair is compared in a circuit which provides signal indicative of the difference . the varying signal is compared against a threshold to provide a signal indicative of the movement .