The acronym PIR in the term PIR motion detector stands for Passive InfraRed. PIR devices measure the infrared light emitted from objects. These systems use a photo detector to detect emissions by converting wavelength light into electrical currents. The currents are then run through the PIR computer. When the photo detector detects either large changes in the energy emitted, an alarm is triggered. Human movement can create these energy changes. The computer ignores smaller changes.
All objects emit infrared radiation. The human eye cannot see this radiation however it can be detected by electronic devices specifically built for this purpose. The term passive is used because the device does not produce an infrared beam but rather it detects the infrared radiation. Infra is used because the human eye is incapable of visually sensing the radiation that the device is able to detect The term red refers to the minimal energy that our eye can see represented by the color red.
When people enter an area that is monitored by a PIR, the warmer infrared current they produce is detected by a change from a cooler current. The movement of the people causes a movement of the warmer area of current. This moving area causes the computer in the device to set the alarm. An attempt to defeat the detection by using a thermal shield would be unsuccessful. The shield would still lead to the detection of movement of the spot of varying temperature and would then set off the alarm.
The approximate temperature of human skin is 34 degrees Celsius. This temperature is generally higher than any other temperature around it. When a person walks past or through a sensor, it is their skin temperature that causes the detection by the PIR. The computer in the PIR system contains a Fresnel lens which is located in front of the sensor. The lens concentrates the energy emitted over a wide area on the sensor and then separates it into hot and cold levels. A person passing through the separated zones causes the sensor to detect a varying radiation level.
A PIR system needs to be carefully placed to prevent false detections and false alarms. The system should be mounted so that the device is incapable of reading through a window. Headlights and sunlight may be able to be detected by a PIR, causing a false alarm. The device should also be positioned as far away as possible from a venting system. The movement of hot and cold air currents could cause the PIR to detect a change in temperature and once again, cause a false alarm.
PIR systems are available for a variety of environments. They can be used to protect both commercial and personal property. The most popular application is the use of the detection device in home security systems. The system can be installed both inside and outside to detect any movement that would be a threat to a home or a family.