A PIR detector is a motion detector that senses the heat emitted by a living body. These are often fitted to security lights so that they will switch on automatically if approached. They are very effective in enhancing home security systems.
PIR stands for Passive Infra-Red. The sensor is passive because, instead of emitting a beam of light or microwave energy that must be interrupted by a passing person in order to sense that person, the PIR is simply sensitive to the infrared energy emitted by every living thing. When an intruder walks into the detectors field of vision,.
A PIR sensor light is designed to turn on when a person approaches, but will not react to a person standing still. The lights are designed this way. A moving person exhibits a sudden change in infrared energy, but a slower change is emitted by a motionless body. Slower changes are also caused by gradual fluctuations in the temperature of the environment. If the light were sensitive to these slower changes, it would react to the sidewalk cooling off at night, instead of the motion of a burglar.
If you have a PIR light, you may notice that it is more sensitive on cold days than on warm days. This is because the difference in temperature between the ambient air and the human body is greater on cold days, making the rise in temperature easier for the sensor to detect. This has drawbacks, though; if the sensor is too sensitive, it will pick up things you don't want it to - such as the movement of small animals. The optimum temperature for PIR sensor lights is between 15 and 20grdC. At temperatures of 30grdC or higher, the sensor's field of view will decrease, and it will take more time to pick up infrared emissions. If the temperature drops to below 10grdC, the opposite occurs: the field of view widens, and the sensor will pick up infrared sources that are smaller and farther away.
No motion detection system is perfect, but PIR sensors are by far the most sensitive and advanced option. PIR sensor lights are ideal additions to any home security system.