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PRI Passive Infrared (PIR) Motion Sensors protect space by 'looking' for changes in infrared (heat) energy levels caused by movement of an intruder. The passive infrared detectors are passive devices. i.e., they do not transmit any form of energy. When a person enters and moves within the detecting zone, his body which is at its normal temperature of 98.6 degree F., radiates more (or less) thermal energy than the surroundings and such minute difference in thermal energy is detected by the PIR. In simple words, PIR keeps a picture of the heat levels in it's view and keeps on comparing the new view with the stored one. In case there is any change then they activate the alarm.

PIR's come with a wide variety of detection patterns to meet various needs of indoor movement sensing. Depending upon the type of protection needed and the size of the protection area / volume, the specific parameters of angular and area coverage can be chosen from the vast array of PIR types available. The standard PIR has a wide angle coverage with a typical range of 20 to 30 feet. The Narrow angle PRI is for long range covering a long and narrow corridor. The Extra wide angle PIR is for sensing movements through an angle of 150 degrees. Then there are certain specialized PIR:

  Roof counted PIR to detect movement in a circular space in the room.

  Curtain type vertical pattern to detect access through windows, doors and other entry points and also to give protection to objects hung on a wall

  Curtain type horizontal pattern to detect access through ceilings or skylights

When installed outside the entrance, this may be used to warn of any one approaching the entrance. Lighting also can be coupled to come ON automatically along with the chiming / alarming on sensing movements. PIR's can be used as a stand-alone system (when connected with an output device) or it can also form part of an integrated system along with other types of security devices. If PIR is used as a sensor in a larger security system which is controlled by a Control Unit (26) , then the Control Unit (26) decides about the action to be taken.

Some PIR's units have a built in key pad to enter the password for activation and deactivation. Some come with a remote control to activate and deactivate. Such PIRs usually have a built in alarm also, which keeps on till it is deactivated.

Avoiding False Alarms With PIR

PIR must not point towards a heat producing source like Refrigerator, AC, heater etc. Also, while installing it should be so positioned that the intruder cannot approach the PIR without coming in view of the PIR else he can point the PIR elsewhere or deactivate the same without sounding the alarm. Precaution is required if there are pets in the house. PIR's are sensitive enough to detect dogs and cats. There are special lens available (or a tape can be put on lower part of the existing lens) so as to avoid detection close to the ground. At the same time, it should be kept in mind that the intruder can also crawl and avoid detection. So placement and subsequent testing of PIR's is a must to avoid false alarms.

If PIR is being installed outdoors then one must keep in mind that the large birds and even pigeons can activate the alarm. So, it must be placed at such a location that PIR does not aims at the flight path of the birds. Due to such restrictions it is not always possible to use a PIR outdoors but it is one of the most used security device.