Abstract: Apparatus for detecting the presence of intruders includes a plurality of sensors. The apparatus not only operates an alarm to indicate the presence of an intruder, but, also, identifies the location in which the intrusion has occurred. There are two groups of sensors at each discrete location or zone. One group of sensors detects the presence of an intruder by the activation of one or more sensors. Another group of sensors discriminates against externally generated sounds to reduce false alarms. In the preferred embodiment, both groups of sensors utilize the same cable. An intrusion detection multiplexer and a discriminator multiplexer are employed to enable the sensors of each zone respectively to be sequentially detected and identified through the same apparatus.