Optical smoke detectors incorporate a pulsingLED located in a chamber within the housing ofthe detector. The chamber is designed toexclude light from any external source.At an angle to the LED is a photo-diode whichnormally does not register the column of lightemitted by the LED. In the event of smokefrom a fire entering the chamber, the lightpulse from the LED will be scattered andhence registered by the photo-diode. If thephoto-diode “sees” smoke on the two followingpulses, the detector changes into the alarmstate and the indicator LED lights up. Thedetector housing is identical to that of theionisation detector but has an indicator LEDwhich is clear in quiescent state but producesred light in alarm.
The optical smoke detector has a moulded self-extinguishing white polycarbonate case with wind resistant smoke inlets. Nickel plated stainless steel wiper contacts connect the detector to the base. Inside the smoke detector case a printed circuit board has the ionisation chamber mounted on one side and the signal processing electronics on the other.