Infrared sensors emit and/or detect infrared radiation to measure heat and detect movement. They use pyroelectric materials to generate an electrical voltage and trigger an alarm when detecting a sudden change in heat. Motion detectors with Fresnel lenses or parabolic mirrors are commonly used in residential and commercial security systems. Infrared sensors are desensitized to prevent false alarms, with a weight tolerance of 40-80 pounds for pets.
An infrared sensor is an electronic device that emits and/or detects infrared radiation to detect some aspect of its surroundings. Infrared sensors can measure the heat of an object and detect movement. Many of these types of sensors only measure infrared radiation, rather than emitting it, and are therefore known as passive infrared (PIR) sensors.
All objects emit some form of thermal radiation, usually in the infrared spectrum. This radiation is invisible to our eyes, but can be detected by an infrared sensor which accepts and interprets it. In a typical infrared sensor such as a motion detector, the radiation enters the front and reaches the sensor itself in the center of the device. This part may be composed of more than a single sensor, each of which being made of natural or man-made pyroelectric materials. These are materials that generate an electrical voltage when heated or cooled.
These pyroelectric materials are integrated into a small circuit. They are wired so that when the sensor detects an increase in heat of a small part of its field of view, it will trigger the motion detector alarm. It is very common for an infrared sensor to be built into motion detectors such as those used as part of a residential or commercial security system.
Most motion detectors have a special type of lens, called a Fresnel lens, on the surface of the sensor. A set of these lenses on a motion detector can focus light from many directions, giving the sensor a view of the entire area. Instead of Fresnel lenses, some motion detectors have small parabolic mirrors that serve the same purpose.
An infrared sensor can be thought of as a camera that briefly remembers what the infrared radiation of an area looks like. A sudden change in one area of the field of view, especially one that moves, will change how electricity passes from pyroelectric materials through the rest of the circuit. This will trigger the motion detector to trigger an alarm. If the entire field of view changes temperature, this will not activate the device. This prevents sudden flashes of light and natural temperature changes from activating the sensor and causing false alarms.
Infrared motion detectors used in residential security systems are also desensitized to some extent, with the goal of preventing false alarms. Typically, a motion detector like these will not register the movement of objects weighing less than 40 pounds (18 kg). With this change, pets will be able to move freely around the house without their owners having to worry about a false alarm. For households with large pets, sensors with an tolerance of 80 pounds (36 kg) are also provided.
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