Abstract: The present invention is particularly useful with an optical smoke detector system of the Tyndall type. The system includes a chamber, an emitter for periodic energization to provide radiation within the chamber, and a sensor for providing an electrical information signal related to the amplitude of radiation passing from the emitter. In the absence of particles of combustion, radiation impinges on the sensor at a reference level. When particles enter the chamber they are struck and diffuse the radiation from the emitter, raising the level of the signal produced by the sensor. The invention includes a supervisory arrangement for checking the normal pulse received by the sensor when no particles are in the chamber, thus checking the efficacy of the emitter and sensor as well as the chamber operation.