Abstract: An apparatus for detecting and signalling the presence and absence of movement of an object moving randomly in space and time has a portable transmitter carried by the moving object, such as a human operator. The transmitter transmits a carrier frequency signal so long as the operator continues his random movements. A detector associated with the transmitter detects the random movements and has sensing means which is a sensor that responds to the operator's movements and is movable randomly in three dimensions in response to the movements. The detector has a photo-responsive switch activated by the movement sensor which interrupts the emission of the carrier frequency signal when the human operator is substantially immobile and permits the transmission once the operator renews his movement. A receiver is positioned in an area for receiving the carrier frequency signal.