Photocell detectors detect smoke within the system. They actuate the blower motor controls and other devices to perform the following:
1. Shut off the entire system.
2. Shut down the supply blower, close outside-air and return-air dampers and runs.
3. Run supply-air blower, open outside-air dampers, close return-air dampers, and stop return-air blower or exhaust fans.
4. Run supply-air blower, open outside-air dampers, close return-air dampers, and run return-air blower or exhaust fan. Actuation occurs when smoke within the unit exceeds a density that is sufficient to obscure light by a factor of 2 or 4 percent per foot. A key switch is used for testing. Two detectors are used. One is located in the return air section. The other is located in the blower section downstream from the air filters.