Capacitive proximity sensor

A capacitive proximity sensing system comprises sets of electrodes 1, 2 coupled to frequency selective circuits 3, 4 (e.g. LC resonant circuits), circuits 5, 6 (e.g. oscillators) responsive to the characteristics of the frequency selective circuits, means 7 for comparing the outputs of circuits 5 and 6 and decision means 9 for providing an output from the result of the comparison. Arrangements are disclosed for making the system sensitive to the rate of change of capacitance of the electrodes, the approach or withdrawal of a body then being detected rather than merely its presence or absence.