Photoelectric smoke sensing and alarming device

Claim 1

A photoelectric smoke sensing and alarming device of a type having a pulse generator, a chamber, a light emitting element disposed in said chamber and adapted to emit a light beam in the form of pulses in cooperation with said pulse generator, and a light-receiving element disposed in said chamber and adapted to receive the light from said pulses of light beam, so as to sense the presence of smoke in said chamber through a detection of scattering of said light beem caused in said chamber by said smoke, comprising: an amplifier having gain control and connected to said light-receiving element for producing an output upon reception of light by said light-receiving element; a threshold detecting comparator for producing a pulse output when the output of said amplifier exceeds a predetermined threshold level; a delay circuit coupled to the pulse generator for producing a delayed pulse output in synchronism with the output of the comparator; a gate circuit having a pair of inputs respectively connected to the output of the comparator and the output of the delay circuit for delivering a first output upon simultaneous receipt of output pulses from said comparator and said delay circuit and for producing a second output upon the occurrence of the delayed output pulse of the delay circuit in the absence of the output pulse of said comparator; and a flip-flop circuit coupled to said gate circuit and adapted to be set by the first output from said gate circuit, wherein an alarm is generated over a time length which is at least as long as the time between successive of said pulses generated by said pulse generator, only when at least n (n>2) successive of said first outputs are delivered by said gate circuit to said flip-flop circuit, said second output of said gate circuit being used to reset the flip-flop circuit in the absence of successive first output signals from said gate circuit.