PIR Sensor
A Passive Infra-red Sensor is an electronic device that detects the motion or movement of some person or animal by sensing the heat emitted from the body. It sets off an alarm automatically at the approach of these warm beings to within a certain specific radius within its view. The 'passive' in the Passive Infra-red sensor means that the device does not emit the infra-red rays. It merely detects the emission from another source. All objects emit a black body radiation which is not seen by our eye but can be detected by electronic devices like the PIR sensor. The PIR sensor principle is used in motion detectors and lights.
In the PIR sensor motion detector, motion is gauged by the temperature of the object moving into the view of the detector. Should a wall come into view, the PIR sensor detects its presence by its temperature which differs from that of a living being and the interpretation thereby becomes obviously different. The sensor has the electronics to do this. The pet immune PIR sensor does not react to the motions of the pet but a person will be detected.
A PIR remote thermometer is another form of PIR sensors. Here the temperature of a remote object is measured. This is calibrated towards the Infra-red emission of the object and then the interpretation as to what type of object it could have been is made.
The PIR sensor lights automatically gets switched on when a person moves into its area of coverage. It will not react if the person stands still. This effect occurs because a moving person emits more infra-red radiations than a person standing still. This effect is really dramatic in nature and has been utilized in the production of magic and horror movies. One becomes thrilled if one does not know the principle behind the PIR sensor lights which go on and off presumably at the whim of a magician.
The complete circuit in a PIR sensor for motion detection is placed in a plastic housing and the PIR sensor positioned where maximum monitoring is required whether indoor or outdoor. The plastic does not hinder detection of motion by the PIR sensor. It cuts off dust and insects which could obstruct the view of the PIR sensor. The light or emission can be focused onto the PIR sensor by either Fresnel lenses or parabolic mirrors. A filtering lens selects emission between 8 and 14 micrometers wavelength which would detect human beings. The PIR sensor is similar to a camera which remembers the amount of infra-red energy focusing on it.
The PIR sensor can be activated by electric power or it can work on its own electronic energy.
When you switch on the power, the electronic energy becomes quiescent. When an object moves into the view of the PIR sensor, the temperature or infra-red radiations emitted will be sensed by remembering the cooler temperature earlier when the object was not there. A chip is the part on the PIR sensor which has this function. When the object moves, the point on the chip moves simultaneously thereby setting off the alarm or warning system. However a still object will not be recorded as an obvious difference in emissions of infra-red rays is not detected by the PIR sensor. If somebody tries to trick the PIR sensor by holding a thermal shield in between, his movement will be recorded as a cold spot moving across.
It is to be remembered that the positioning of the PIR sensor is of utmost importance to prevent false alarms. A strong light from a headlight or the sun's reflection from a window can set off false alarms unnecessarily disturbing the neighborhood and your peace of mind. An air vent should not be blowing air onto the PIR sensor. Though there is no emission involved, the temperature of the plastic can change, disturbing the PIR sensor's function. The better equipped PIR sensors are able to conduct a surveillance of even 5000 square feet area.
One drawback seen in PIR sensors is that it happens to be more sensitive on a cold day rather than a hot one. The optimum temperature is between fifteen and twenty degrees Celsius. At higher temperatures the field of view becomes lesser and thereby some motions may be missed by the PIR sensor and it takes more time to take pictures. If the temperature drops below ten degrees Celsius, the view covered by the PIR sensor becomes wider and the distance under surveillance becomes further. The working of the PIR sensor is thereby considered better on a colder day.
Some sensors used in home security systems are combinations of a PIR sensor and a microwave sensor. This makes the sensor more sophisticated and useful. These sensors afford a better protection than the PIR sensor alone or microwave sensor alone. The PIR sensor is a worthwhile addition to any home security system
| 
