How does a photoelectric smoke detector work?

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How does a photoelectric smoke detector work?
FRONT VIEW OF PHOTOELECTRIC SMOKE DETECTOR
LED STATUS INDICATOR LIGHT. Blinks RED when
Normal Stay solid RED when in alarm.
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BACK VIEW OF PHOTOELECTRIC SMOKE DETECTOR
COMMON TERMINAL
TRANSMITTER TERMINAL
COLLECTOR TERMINAL
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INSIDE VIEW OF PHOTOELECTRIC SMOKE DETECTOR
COLLECTOR
TRANSMITTER Emits a Infrared (IR) beam
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INSIDE VIEW OF PHOTOELECTRIC SMOKE DETECTOR
COLLECTOR
NOTICE When no smoke is present the IR beam does
not hit the collector
Infrared (IR) beam
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Once the IR beam hits the smoke the beam
disperses. When the IR beam is displaced enough
it will be picked up by the collector. This will
cause the smoke detector to go into alarm.
COLLECTOR