This Passive Infrared Alarm System ( PIR alarm ) can turn on a lamp or alarm when it detects heat from an object such as a moving person. The name PIR (Passive Infrared) is given to the sensor because it receives infrared rays passively and does not emit any infrared rays. The lamp or alarm remains on for a few minutes depending on the mono time setting of the PIR sensor used. Ready-to-repair PIR sensors are available with Fresnel lenses to increase sensitivity and filter ambient light. Its range is about 5 meters at a 45 degree cone angle.
Passive Infrared
Humans cannot detect infrared rays, but they emit infrared from the body as heat, like all other warm-blooded animals (animals whose body temperature remains constant – mammals and birds). This is exploited in motion detectors to detect the presence of humans. PIR sensors (Passive Infrared sensor) are used to activate an alarm system using infrared in the form of body heat from the human body. The PIR sensor is an electronic device that measures infrared in its field of view. Motion is detected by the sensor when an infrared source like human passes in front of the sensor with a temperature and comparing another temperature like that of the wall on which the sensor is mounted.
About PIR sensor
The PIR sensor is mounted on a PCB with other semiconductors. The complete set is mounted in a housing with a Fresnel lens molded into the front. Behind the lens there is a small window through which infrared enters the sensor. The window is covered with transparent plastic that only allows infrared rays to enter the sensor and prevents visible light. The filtering window limits IR rays to 8-14 micrometers, similar to IR rays from the human body.
The PIR sensor is a solid-state device made of ¼ inch pyroelectric material as a thin film. The pyroelectric material can be gallium nitride (GaN), cesium nitrate (CsNO3), cobalt phthalocyanine, lithium tantalite (LiTaO3) etc. The sensor is made as part of the integrated circuit with 1 to 4 pixels of pyroelectric material. The paired pixels are connected to the inputs of a differential amplifier. Differential amplifiers cancel PIR measurements and the average temperature in the field of view is removed from the electrical signal. So that the IR energy from the sensor does not trigger the alarm. This prevents false triggering when exposed to flashes of light. The differential amplifier also minimizes common-mode interference from electric fields. The circuit in the PIR sensor can be connected to a relay switch and the sensor resets when turned on and remains in standby.
Circuit operation
The PIR Sensor module is a pre-assembled board with circuitry to compare the infrared rays from the object and the nearby area such as the mounting wall. When a person enters range of the sensor, the Fresnel lens focuses infrared energy on the sensor that previously detected the coldest area. This change in energy level heats the sensor and creates a hot spot on it. The Module then supplies about 3 volts (Check the output. On some brands, the output is active low). The high output saturates T1 and it conducts. When T1 conducts, T2 also conducts to trigger the relay. The AC or Alarm lamp can be connected through the Common and NO contacts of the relay switch. When the person moves, the hot spot also moves, which causes the electronic circuit to deactivate the relay. If the person tries to mask the sensor with some protection, the sensor develops a cold spot which also activates the alarm. Thus, the PIR sensor is foolproof.
The circuit can be powered by 12 volt regulated power supply or battery.
PIR Sensor Assembly
Fixing and focusing the PIR sensor is important to provide maximum sensitivity and avoid false alarms. Mounting the device on the wall at a 45 degree angle and 2 to 3 meters high will give good results. The sensor may saturate if strong sunlight and headlights from vehicles fall on the sensor. Such positions should be avoided.
Circuit diagrams
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