Abstract: A self-diagnostic circuit for use in alarm systems and the like, of the kind having a plurality of alarm detection switches connected in a loop, indicators associated with each of the switches for indicating locally the detection of an alarm condition and a central controller for reversing voltage polarity in the loop whenever an alarm condition is detected by one of the switches. The self-diagnostic circuit comprises: a first switch for detecing an alarm condition connected in an alarm loop; a second switch automatically activatable in response to operation of the first switch; an alarm indicator automatically operable in response to activation of the second switch; and, an automatically operable circuit for preventing deactivation of the second switch until after diagnosis of alarm system operation, even when the first switch no longer detects the alarm condition. Any such first switch exhibiting erratic switching behavior can be easily identified by continued operation of the associated alarm indicator.

Abstract: A diagnostic status indicating apparatus is used with an alarm system having a plurality of normally-closed switch contacts which are opened upon occurrence of an alarm condition. Light-emitting diodes are connected across the normally-closed switch contacts at an opposite polarity to that of the current normally passing through the contacts in the operating mode. In order to diagnose the alarm-generating condition of the electrical contacts, a switch removes the operating voltage and connects a test voltage at a reverse polarity, whereupon the LED for any open circuited electrical contact is activated. The device is preferably based upon a double-pole-double-throw switch and includes indicators for a test condition and for a ground fault.