Abstract: To monitor that infra-red radiation element 22 within housing 10 has not been disarmed, for instance by rendering opaque an infra-red transparent element 16 masking a window 12, in the housing 10, through which radiation may enter and be focussed by optical system 18 onto the element 22, an infra-red radiation generator 32 comprising a low-temperature lamp 34 is mounted externally of the housing 10 so that element 22 receives radiation therefrom and provides a signal to a processor 30. The latter will provide an alarm in the event of absence of radiation from the generator 32 and/or change in the radiation reaching the element 22 commensurate with approach of a person.

Abstract: In the receiver of an outdoor intruder detection apparatus, which receiver is spaced from and receives signals from a microwave transmitter and provides an alarm output upon the occurrence of change in the received signal, eliminator means is provided for eliminating pulses which may be superimposed on the signals from nearby radar. Such eliminator means comprises a wide band amplifier which serves, by way of a monostable to actuate a switch which gates out, in synchronism with the superimposed pulses, the detected signals from an aerial and diode as supplied by way of amplifiers and a delay line to further signal-processing circuitry.