A wireless security system in which a sensor for detecting a security violation is provided with a radio frequency transmitter adapted to excite the receiver of a control unit at a different location, the transmitter of the sensor radiating a signal responsive to the occurrence of a security violation. The sensor also having means to periodically excite the transmitter after the lapse of a period of time to indicate that the sensor is in proper working order, that period varying randomly. The control unit is provided with a memory and stores responses to the random signals, and at intervals long with respect to the random periods, samples the memory. The control unit responds to the absense of a stored response from the transmissions of the sensor.
In a preferred construction, a plurality of sensors are employed with a single control unit, each of the sensors radiating a unique encoded signal. The control unit may thus determine by sampling which of the sensors has failed to radiate a signal received by the control unit during the sampling interval of the control unit.