Home > Learning Center > Passive Infrared (PIR) Detection Devices

Passive Infrared (PIR) Detection Devices

Homeowners concerned about home invasion should shop for passive infrared (PIR) detection devices. When it comes to detecting motion of an intruder these devices are more advanced than other alarms such as circuit alarms and radar-based detectors.

The systems are called passive because they sit idly and sense the infrared energy in the room. If a potential thief enters a room the PIR will detect a change in temperature and sound an alarm. In contrast, other systems are active because they generate their own energy to detect an intruder.
A passive infrared (PIR) detection system is like a memory bank that remembers how much infrared energy gets focused on its surface. The electronics in the device sit in a passive state and are connected to the input of a burglar alarm. If an intruder enters, he emits infrared energy which cannot be seen by the human eye, but can be picked up by electronic devices. When the sensor picks up the radiation change emitted by a burglar, it compares the old state of infrared energy to the new state. The device will then signal the alarm.

A savvy intruder might try to get around a passive infrared (PIR) detection device by using a thermal shield so that his body heat is not recognized. However, PIR can also detect a cold spot and still send a signal to the alarm.

Typically a PIR sensor is placed on a circuit board that has the equipment to interpret signals from the sensor. Homeowners should install the alarm so that the housing is located in a place where the entire security area can be monitored.

When infrared energy from a potential intruder reaches the passive infrared (PIR) detection device sensor array window, the infrared energy passes through a plastic housing. The plastic also keeps out dust and insects which could obstruct the sensor. The plastic housing is also useful because the infrared energy of insects could generate false alarms. False alarms are problematic because they cost local police money. In addition, if your neighbors get used to false alarms they become inured and are unlikely to act in the event of a real burglary alarm.

Placement of passive detection systems is very important. A PIR should not be placed near an HVAC vent that could blow hot or cold air onto the unit. While air does not emit much infrared energy, the air coming from a vent could change the plastic's temperature enough to confuse the electronic system.

Passive infrared (PIR) detection systems use a number of lenses or mirrors to give them an average range of about 25-30 feet. More expensive PIR's feature single segment mirrors and can cover more than 100 feet. One particularly expensive system on the market advertises coverage of 400 feet.

Consumers should note that they can protect a particular area like the floor space surrounding a valuable painting by purchasing a PIR with a very narrow coverage. Conversely, PIRs can also be designed for a broader angle of coverage.

PIRs sell at a wide range of price points. Entry level devices start at approximately $15 and a long-range detector sells for over $1,200. Before purchasing an expensive system homeowners should discuss with a security consultant or sales advisor the property that needs coverage.