Passive Infrared Sensor 08
Passive Infrared Sensor 08 : PIR-Based Motion Detector: (5) Burglar Alarm Applications: When used as part of a burglar alarm, the electronics in the PIR typically control a small relay. This relay completes the circuit across a pair of electrical contacts connected to a detection input zone of the burglar alarm control panel. The system is usually designed such that if no motion is being detected, the relay contact is closeda 'normally closed' (NC) relay. If motion is detected, the relay opens, triggering the alarm. (5. 1) Placement: Manufacturers recommend careful placement of their products to prevent false alarms (detections not caused by an intruder). They suggest mounting the PIRs in such a way that the PIR cannot "see" out of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (such as from a vehicle headlight or sunlight) can overload the sensor and cause a false alarm. A person moving on the other side of the glass would not be "seen" by the PID. That may be good for a window facing a public sidewalk, or bad for a window in an interior partition. They also recommended that the PIR not be placed in such a position that an HVAC vent would blow hot or cold air onto the surface of the plastic which covers the housing's window. Although air has very low emissivity (emits very small amounts of infrared energy), the air blowing on the plastic window cover could change the plastic's temperature enough to trigger a false alarm
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