Figure 17-17.  Engine fire switch operation. with a smoke detector. The smoke detectors monitor air  in the cargo compartments for smoke. The fans bring air  from the cargo compartment into the smoke detector.  Before the air goes in the smoke detector, in-line water  separators remove condensation and heaters increase the air  temperature.  [Figure 17-19]  The optical smoke detector consists of source light emitting  diodes (LEDs), intensity monitor photodiodes, and scatter  detector photodiodes. Inside the smoke detection chamber,  air flows between a source (LED) and a scatter detector  photodiode. Usually, only a small amount of light from the  LED gets to the scatter detector. If the air has smoke in it,  the smoke particles reflect more light on the scatter detector.  This causes an alarm signal. The intensity monitor photodiode  makes sure that the source LED is on and keeps the output  of the source LED constant. This configuration also finds  contamination of the LED and photodiodes. A defective  diode, or contamination, causes the detector to change to  Smoke Detector System Figure 17-18.  Cargo fire detection warning. 17-15 Engine fire switch (P8) Switch positions L E F T Disch 1 2 L E F T 1 2 DISCH Switch locked Switch released electrically Switch pulled Switch released manually Electrical connector Push-pull contacts (internal) Engine fire override switch Solenoid energized Engine fire warning light Spring return (switch open) Rotary contacts (internal) Discharge 2 (switch closed) Spring return (switch open) Discharge 1 (switch closed) Center Engine fire override switch pushed Cargo fire/engine control panel • Fire/overheat test switch FWD cargo fire warning light • AFT cargo fire warning light Speaker (2) Glareshield panel • 2 Master warning lights