Abstract: A scatter type of smoke detector wholly supplied by a dry cell battery includes a clock circuit applying energy pulses to an LED light source which directs light pulses on a smoke sensing path. Light pulses scattered by smoke generate detection pulses in a photo diode whose amplitude is dependent on the smoke density. The clock pulses and detection pulses are applied to a control circuit including a dual, data-type flip-flop logic circuit and a threshold circuit driving an alarm horn. If the smoke density, and hence the detection pulse amplitude, exceeds a predetermined level the control circuit energizes the alarm in a continuous mode. If the battery drops below a preselected level the control circuit is actuated in an intermittent mode.

Abstract: An optical smoke detector has a housing which forms a dark chamber into which light is directed on a path viewed by a photocell. Light scattered from the path by smoke causes the cell to trigger an alarm. The housing includes an interior wall around the dark chamber and an exterior wall exposed to ambient light. A wire mesh extending between offset ports in the exterior and interior walls screens insects from the chamber and obstructs light from passing directly to the interior part.

Abstract: A simplified smoke or other particle detector circuit includes a light emitting diode in series with a compensating photocell and a photocell sensing LED light scattered from particles. In parallel with the LED is a high resistance, and in parallel with the photocells is a low resistance, the resistances having a common junction coupled to a threshold circuit input as is the output of the photocells. Sensing smoke produces a photocell output which triggers the threshold device to generate an alarm signal. Failure of the LED produces a voltage at the resistance junction which similarly triggers the threshold device to alarm.