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.

Abstract: Two side-by-side elongate terminals support, from below, one or a pair of electrically-conducting bodies which contact the terminals and thus provide electrical continuity between the terminals so that vibration will cause the body or bodies to lift from one or both of the terminals and break the continuity.

Abstract: To prevent unintentional alarm initiation in an intruder detection system of the kind in which a transmitter emits radio waves across an area to be monitored and a receiver senses changes in the resultant signal, which initiation might otherwise occur as a result of environmental changes or natural causes such as the entry of a small animal into the monitored area, a receiver for such a system is adapted to sense the received signal at predetermined intervals of time and only to cause alarm initiation in the event that the change occurring in any one of said predetermined intervals exceeds a predetermined amount. Such an arrangement serves, therefore, to compensate automatically for changes, in the received signal, arising from environmental changes.