| 摘要 |
PURPOSE:To enhance detection accuracy by effectively avoiding the erroneous emission of an alarm due to external light, by setting the frequency of the light emitted from a light emitting element so as to make the same higher than that of the light due to an illumination lamp. CONSTITUTION:The frequency of the light L emitted from a light emitting element 4 is set so as to be higher than that of the light LO emitted from an illumination lamp. When the light LO arrives at a light receiving element 5, the light receiving light SR outputted from the element 5 is outputted to a high pass filter 7 through an amplifying circuit 6. Because the frequency of the signal SR at this time is low, said signal SR can not pass through HPF7 and the output signal DCSH from a sample holding circuit 10 comes to a low level. Because of this, no effective output AR is selected from a comparing circuit 11 and no alarm 12 is emitted. As a result, the erroneous operation of a smoke sensor due to the external light of the illumination lamp can be prevented.
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