Abstract: An intruder detector system such as a microwave fence having a transmitter 10 and receiver 12 produces a signal proportional to the disturbance created by an intruder. The signal is processed by a microwave detector 14 and processor 16 and supplied, as DC voltage output to two channels 18, 20. First channel 18 compares the DC voltage strength to a pre-set value stored in a first threshold detector/comparator 22. On the DC voltage exceeding the pre-set value an alarm relay 24 is actuated causing actuation of an alarm (not shown). Second channel 20 passes the voltage through a first filter 26 which removes high frequency signal variations characteristic of a fast moving intruder. Thus the signal which is compared with a second pre-set value stored in a second threshold detector/comparator 28 relates only to slow moving intruders and the preset value can therefore be set for a lower level of disturbance as would be caused by a crawling human intruder.