How your security system passive infrared sensor ( PIR ) works

AlarmTech - How your Infrared sensors work

Infrared Detectors (PIRs)

Most security systems use Infrared sensors, or Passive Infrared Detectors (PIR) to detect an intruder and trip an alarm. PIR detectors are electronic devices that detect an intruder by sensing his body heat when he enters or moves around the area of protection. The device consists of a mirror or lens that can focus the energy, a thermal sensor to detect the energy, and associated electronics to analyze the information. The area that a unit can protect is usually given in feet representing the length and width of it pattern, such as 35 X 35 feet. or 150 X 8 feet.

The mirror/lens divides the area of coverage into multiple zones of detection that we call "fingers". This can be visualized by holding your hand out and spreading your fingers. Each finger represents one optic zone of the detector. The PIR can only detect motion within one of its zones and is most sensitive to motion across the zones since this is what causes the greatest change in energy.

Many manufacturers make a selection of lens or mirrors for the detector so that the area of coverage can be changed by simply replacing the optics. This is particularly helpful if you decide to get a pet after your security system has been installed. Another feature of the mirror/lens is that you can mask, block out, any segment that may cause a problem by covering it with the tape supplied by the manufacturer. For example, if there is a radiator within one of the protected zones, you can place the tape on that segment and by doing so prevent it from causing a false alarm.

The thermal sensor used in today's PIRs is actually two (dual) or four sensors (quad) in a single unit. The two-element device, which is commonly called a Dual PIR (not to be confused with dual-tech which is a combination PIR and microwave sensor-both technologies have to trip before the unit goes into alarm)., divides each zone into two distinct, side-by-side detection areas. A person moving will first activate one of these sensors and then the other. This allows the PIR to reject alarms that originate from heated surfaces or even RF interference, since both of these are detected simultaneously by both elements.

A four-element device, which is commonly called a Quad PIR, further divides the zones into an upper and lower area. This allows the unit to compare the signals received, further reducing the possibility of false alarms.

PIRs incorporate additional features that help to minimize false alarms, such as white-light filters and pulse-count circuits. The white-light filters, for example, prevent the device from falsing (false alarming) when exposed to a source of direct light where the pulse-count circuit tells the unit not to activate unless it detects motion multiple times in rapid succession.

Some PIRs have a special Pet Alley lens that prevents them from detecting motion near the floor. This option can allow your pet to roam the area when the system is on, as long as it does not jump or climb on something inside the detection area.

Proper PIR placement is essential for good protection and false alarm reduction. The following guidelines should be considered when using PIR detectors:

Match the PIR to the area. Using a unit that has a 100-foot range in a twenty foot room can cause false alarms.
Do not mount the unit facing windows or any source of rapidly changing heat or cold air.
Seal all openings into the unit to prevent insects and drafts from entering the unit and causing false alarms.
Mount the unit so that an intruder will have to walk across the detection pattern, rather than towards the detector.
If using a Pet Alley lens, carefully examine the room to ensure that the pet cannot climb high enough to activate the unit.
Be careful with Mylar balloons and other hanging objects. A PIR is capable of detecting them when they are within the detection area and if they can move when the air is turbulent.
Mask any zones that may cause false alarms.
Last but certainly not least carefully walk test each unit to accurately determine the area of coverage.