Photoelectric type fire detector

Claim 1

A photoelectric type fire detector comprising: a light emitting element; a light receiving element which receives scattered light emitted from said light emitting element and scattered by smoke particles; an amplifier which amplifies an output signal of said light receiving element; and a control circuit, coupled to said light emitting and light receiving elements and to said amplifier, for alternately and repeatedly operating in fire monitoring mode and self-testing mode time intervals, said control circuit comprising: (a) means for detecting a smoke density according to an output signal of said amplifier during each fire monitoring mode time interval and for generating an alarm signal when the smoke density exceeds a predetermined level; (b) means for setting an output range defined by an upper threshold and a lower threshold; (c) means for increasing an amplification factor set in said amplifier during each self-testing mode time interval relative to an amplification factor set in said amplifier during each fire monitoring mode time interval; (d) means for comparing a level of said output signal of said amplifier with said output range during each self-testing mode time interval; (e) means for counting a number of times in which the level of said output signal of said amplifier deviates from said output range; (f) means for setting a threshold value for said number of times; and (g) means for detecting an abnormality in said photoelectric-type fire detector when said number of times exceeds said threshold value and for generating an error signal when detecting said abnormality.