Electrical test circuit for optical particle detector

Abstract

A photoelectronic smoke detector has walls around a dark chamber to which smoke, but not light, is admitted, and through which light is directed from an exciter light source on a path to a limited area of a chamber wall. A first photocell views light scattered from the through path by particles in the path primarily to produce an alarm signal, and also receives background light scattered from the chamber wall outside the limited area. A second photocell is disposed to receive background light substantially only from the limited area and other areas outside the source light path. The first cell produces a first signal in response to background and particle-scattered light. The second cell produces a second signal corresponding to background scatter. The first and second cells, preferably photovoltaic photodiodes, are coupled in opposition in a circuit whose output consequently is substantially independent of the background light. A remotely operated test relay or similar switching means has contacts which switch the second photovoltaic diode from a connection in which it opposes the signal of the first photocell as just described to a connection in which it reinforces the first photocell signal thereby to raise the joint photocell output to alarm level in the absence of particles and test operation of the light source, first photocell and subsequent alarm circuits.