Abstract: A detector unit for use in detecting unauthorized intrusion into homes, offices, factories and other buildings is described. The detector unit comprises a detector such as a passive infrared detector and a radio frequency transmitter that is connected to the detector and that transmits a detector signal when the detector is actuated, and battery means. Two batteries power the detector and the transmitter. A power control means is arranged so that when the power of the said first battery drops below a certain minimum the second battery is connected to the detector and the transmitter. A warning is given at the main control unit that the second battery is being used so that the user can now replace both batteries.