Abstract: In a fire, smoke or intruder detection system including a master unit and satellite units with remote sensors, the satellite units send alarm transmissions to the master unit over a radio frequency channel. In order to register satellite units with the master unit each satellite unit is provided with a unique identification code and, after suitable triggering, sends a signal including this code to the master unit over a line-of-sight communications channel. The master unit registers satellite unit identification codes received on the line-of-sight channel and disregards radio frequency transmissions if they include unregistered satellite unit identification codes.

Abstract: The microwave section of a dual technology sensor comprising microwave and infrared sections may be periodically tested. This is done by arranging for the output of the microwave section to be monitored at times corresponding to microwave pulses and at times coresponding to no microwave pulses. If the microwave section is operating properly then a difference should be seen in the output between the "pulse" and "no pulse" times. Because this test can involve a change in the average energy output by the microwave section it can also affect the infrared section of the sensor which often inadvertently receives radiation from the microwave source. Thus the infrared section may be tested using this effect.

Abstract: In an intrusion detector, one or more surveillance zones are imaged using an optical system including a mirror surface and a lens spaced within a small distance of one another and, preferably, in contact. In use, radiation received from said surveillance zone(s) passes through the lens, is reflected from the mirror, re-traverses the lens and falls on a sensing element. In preferred embodiments of the invention the lens is a Fresnel lens and is arranged to create look-down zones for a detector which also monitors forward, side and/or upward surveillance zones.