Abstract: A radio frequency transmitter is adapted to be worn by a worker in a confined space. The transmitter includes a manual switch and a mercury switch which, when tilted by reason of the worker getting out of his normal work position, closes to cause the transmitter to emit a signal which is received by a receiver positioned outside of the confined space. The receiver closes a switch to put the coil of a relay across a power supply. The relay closes two switches, one being an alarm switch which causes energization of a strobe light and a siren; the other establishes a holding circuit by-passing the receiver actuated switch. The holding circuit includes an interrupt switch to turn off the alarm. There is a time delay in the relay circuit to avoid false alarms. Once the alarm is sounded, termination of the radio frequency signal alone will not stop the alarm; the switch in the holding circuit must also be opened.