Duct Smoke Detector Head for D300A. The D282A-DH Replacement Duct Smoke Detector Head detects the large smoke particles which typically result from wood, paper, and fabric combustion. It is a conventional 24 VDC photoelectric smoke detector head for use with the D300A and D300A-HV conventional duct detector housing units. It uses an infrared (IR) LED light source and a silicon photo diode to measure light reflected from particulates in a detection chamber. After two consecutive measurements exceeding the threshold level, the unit signals an alarm condition. The detection chamber, designed for reliable smoke entry characteristics, is protected by a micro-fine insect screen to minimize nuisance alarms. The sensitivity stability and high signal-to-noise ratio of this detector permits its use in a wide range of environmental conditions. The detector head has a moisture-proof reed switch that reacts to an external magnet for testing. The magnet test simulates a 4% to 6% smoke obscuration, which places the detector in an alarm condition. This test is especially useful in environmentally unstable or clean areas. There are built-in LEDs that flash to indicate the unit is powered. The LEDs latch on steady in an alarm condition.