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Optical smoke detector

  • US 4,249,169 A
  • Filed: 05/18/1979
  • Issued: 02/03/1981
  • Est. Priority Date: 05/18/1979
  • Status: Expired due to Term
First Claim
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1. An optical smoke detector, having an examination zone through which smoke is adapted to pass, comprising(a) a light source in position to illuminate said zone and any smoke therein,(b) a photo-resistive light detecting cell visible to said zone and electrically connected in parallel circuit to said light source, said cell characterized by a temperature coefficient that increases the resistance of said cell with an increase in ambient temperature and decreases the resistance of said cell with an increase in ambient temperature,(c) a resistance element electrically connected in series with and on the input side of said source and characterized by a temperature coefficient that decreases the resistance of said element with an increase in ambient temperature and increases the resistance of said element with a decrease in ambient temperature, and,(d) alarm means including a voltage responsive switching device operatively connected to said light detecting cell on the output side thereof and adapted to generate an alarm when the resistance of said light detecting cell exceeds a predetermined level,(e) said light source being a monochromatic light emitting diode characterized by a light output that increases and decreases linearly with increases and decreases in applied current.

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