Abstract: An intrusion detection loop employing an end-of-line resistor that operates with a voltage window. As the voltage window is increased, the sensitivity of the system decreases. As the window is decreased, the sensitivity of the system increases. By utilizing two comparators each having one input coupled to the loop, one can specify one voltage window when the system is disarmed and hence place the system at maximum sensitivity thereby detecting any borderline problem. In any event, the same two comparators operate to provide a different voltage window when the system is armed and hence desensitizing the loop and therefore reducing alarm probability. In this manner one can determine during the disarmed condition that the loop is operating with a borderline resistance and still employ the system in the armed condition to assure that there will be a reduced alarm probability in view of an undesirable change of the loop resistance.

Abstract: A fault indicator apparatus employs a single indicating device such as a light emitting diode (LED) to provide an indication of all zones within a plurality of zones which exhibit a predetermined condition being monitored. In particular, the invention is employed in a multi-zone intrusion system where a single indicator provides an output indicative of all zones which are not secure. The apparatus causes the indicator to blink the zone numbers in a continuous sequence and for example, if zones 2 and 5 were not secure the indicator would provide two flashes indicative of zone 2 and then provide a predetermined pause while then providing 5 flashes indicative of zone 5. The user by counting the flashes can then immediately determine which zones in the plurality of zones have been violated.