How Does an Infrared Motion Sensor Work?

A motion sensor light.

If security is of great importance to you and you do not want a cheap magnetic alarm then an infrared motion detector may be what you are looking for. An infrared motion detector, unlike typical alarm systems, use a "broken plane" trigger to set off the alarm. An infrared motion detector uses a combination of radiation and motion in order for it to be triggered. There is some basic and not-so-basic science involved with an infrared motion detector and the following article will help to explain how they work.

Radiation as Infrared

The majority of alarms you find will be triggered by lasers or magnets. While these are effective they are not at all picky and will sound whenever the conditions are meant. Magnetic alarms are stuck to door and window and once the magnet pulls away from its sister unit, the alarm will sound. Laser alarms will have a beam that is projected and once the beam is broken the alarm will sound. Infrared works on the same principles but is more complex and accurate.

Not many people know it but infrared is a form of radiation that is harmless and exists in an electromagnetic spectrum. This is a spectrum of light that can't be seen. This is because the electromagnetic spectrum is longer than visible light, but can still be detected. Any object that produces heat also produces an infrared signal. Humans and animals will give off the strongest instances of infrared radiation. This is the spectrum that an infrared motion detector detects.

Infrared Motion Detector and its Parts

One thing that is a constant between the different infrared motion detectors on the market is the parts inside of the units. Every infrared motion detector has the same parts which all work together in order for it to work properly and efficiently. These parts are: fresnel lens, which are used to focus the thermal energy; an IR filter, which filters the thermal energy to the infrared spectrum; a pyroelectric sensor, which senses changes in temperatures within the field of vision of the motion detector; the amplifier, which increases data from the pyroelectric sensor; and the comparator, which filters collected data in order to compare the output shapes to the rest of the area

Quality and sensitivity of these components will vary for each motion detector that you find on the market.

How it Works Together

When you turn on the infrared motion detector is begins to scan the area surrounding the detector within its range. It will take an initial reading in order to learn a baseline of the area. When an object within the eye of the infrared motion detector moves within the detection area, the motion detector will sound. Unlike other alarm systems, you can set the sensitivity on most of these motion detectors. This is a very important feature if you live within a wooded area. More expensive infrared motion detectors can distinguish the difference between a human and an animal based on the overall temperature that is given off by the subject.