These are Electronic devices that detect the presence of Human beings and large Warm blooded animals (Birds and Mammals) from a distance of 5 meters or more. The most advanced form of Motion sensor is the Passive Infra Red sensor or PIR sensor. This electronic device measures the temperature in the infrared from the body of human beings and compares with the background temperature around the sensor and activates the alarm system. The heat from the human body is emitted in the form of Infrared ,and the PIR sensor can detect it from an angle of 45 degree.
The Motion detector is named as PIR sensor because it receives the Infrared from the human body passively and do not emit any infrared rays. The sensor has a Pyroelectric material in the form of a ¼ inch film. The Pyroelectric material used in the sensor may be Gallium nitride(GaN), Cesium nitrate(CsNO3), Cobalt phthalocyanine, or Lithium tantalite(LiTaO3) .The sensor forms the part of the integrated circuit having 1 – 4 pixels of the Pyroelectric material. The paired pixels are connected to the inputs of a differential amplifier. The differential amplifier cancel each other the PIR measurements and the average temperature in the field of view is removed from the electrical signal. So that the IR energy on the sensor will not trigger the alarm. This prevents false triggering when exposed to flashes of light. The differential amplifier also minimizes the common mode interference from electric fields.
The PIR sensor is available as single sensor or as PCB mounted module. A Fresnel lens is mounted in front of the sensor to focus the received IR rays into the sensor. Behind the Fresnel lens, there is filtering plastic window that allows only IR rays to enter into the sensor. The window limits the IR rays of 8-14 micrometers similar to the IR rays from the human body.
PIR sensor gives satisfactory results only if it is properly mounted on the wall. It should be mounted 2- 3 meters height from the ground level at an angle of 45 degree. Strong sunlight, electric light, light from vehicles should not fall on the sensor. Flashing lights (Lightning) can trigger the sensor that may give a false alarm.