Abstract: The detector indicates the presence of a body (28) which is sufficiently conductive of Hertzian energy in a predetermined volume with the possibility of determining approximately its distance from the receiving aerial from a few millimeters to several meters. A stationary conductive body (28) has no effect on the detection mode. The electronic device consists essentially of an emitter (1) of pulses which are preferably modulated and, if necessary coded, a signal (10) from the emitter (1) after amplification (9) is radiated by the aerial (2). A receiver (13) receives directly the synchronization signal (4) and, if applicable, the coding signal (5) from the emitter (1). If a body (28) which is sufficiently conductive of Hertzian energy passes between or disappears from between the aerials (2, 2a, 26) and (11) the signal received on the aerial (11) varies sufficiently for the receiver (13) to act on a control (12) and signalling (15) mode.