Infrared (PIR) sensors are used for detecting motion and reacting to infrared rays. As any living body is a source of such rays, the infrared sensor detects them and starts a given device, according to the scheme in which it participates. If the switched on device is an alarm system, the PIR sensor is called a security sensor, and if it switches a lighting system, the system is called automatic switching of lights. Infrared rays are naturally radiant heat, so each object in the field of the sensor has a temperature which is higher than the surrounding environment and activates the sensor for motion by transmitting infrared rays. The PIR sensor contains piro-crystals that turn the infrared rays into voltage that is proportional to their intensity, and the electrical signal closes a normally open contact (in a relay) in the main chain of the sensor, causing useful effect. Apart from the human body, a source of infrared rays can be different animals, operating heating appliances as well as sunlight.
Various constructive measures are taken in order to avoid false alarm. For example, to achieve detection only of wavelength emitted by the human body and to ignore the daylight the sensor is equipped with an optical filter, responsive only to wavelength of 8 to 14 micrometers, as this is the range of the rays of the human body. Infrared sensors are used increasingly in public spaces, thus saving a lot of bills. For example, many hotels place IR sensors, which switch the lighting in certain areas in long corridors between the rooms, thus illuminating only the used part of the interior of the hotel. In restaurants, bars, shops, theaters, discos, etc. customers often forget to turn off the lamps in the service areas, which they visit. To avoid this, the lighting there is activated by a PIR detector, set according to the usual time of visiting the premises.
Basic parameters of each infrared sensor, besides the supply voltage and the maximum commutated load, are also the sensitivity to light, which can usually be adjusted, the duration of the switched on condition after switching – also subject to regulation as well as the angular range of the lens. Important for the proper operation of the PIR sensor is the selection of the height of mounting on the wall and the choice of the location in general. To avoid errors, you need to read the instructions and to determine visually whether there are objects or angles, behind which the sensor will not be able to operate. The sensor must not be mounted opposite an area in the room where there is likelihood of a sudden change in the temperature, such as opposite air conditioning. The distance at which the sensor responds also needs to be taken into account, i.e. do not put a sensor with a range of 5 m into a 10 m room.
Motion sensors can be analog and electronic, passive (which only accept infrared rays) and active (microwave sensors).
The latter type emit waves of very high frequency and reabsorb the waves after their reflection in the surrounding objects and walls. When there is change in the frequency of the received waves, the sensor activates an electric circuit. These sensors can detect presence even behind closed doors, which determines their most common use. Another type of PIR sensor is the wireless sensor with a radio signal. It is powered by batteries and when actuated, it emits a radio signal to a remote device which performs a predefined algorithm. Another common combination is the combination of an infrared sensor with a microphone. This device allows the switching of lighting via a voice command. Except the light settings we also need to set the bandwidth of the sound to which the device reacts. Usually this is from 30 to 90 db. Of course, combinations of all the above-listed types are possible.
You can find on the market a combination of an infrared sensor and a lamp in a single housing,that can be plugged directly into the wall-plug. Thus even the biggest layman can provide automatic light switching in a particular area.
Another type of automatic lighting switching devices can be set for activation when the lighting is reduced under certain levels. A system of such PIR sensors can be used for illuminating gardens, parks, and the urban environment.