PIR sensors are more complicated than many of the other sensors explained in these tutorials (like photocells, FSRs and tilt switches) because there are multiple variables that affect the sensors input and output. To begin explaining how a basic sensor works, we’ll use this rather nice diagram.
The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here is not really doing much and so we see that the two slots can ‘see’ out past some distance (basically the sensitivity of the sensor). When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected.
The IR sensor itself is housed in a hermetically sealed metal can to improve noise/temperature/humidity immunity. There is a window made of IR-transmissive material (typically coated silicon since that is very easy to come by) that protects the sensing element. Behind the window are the two balanced sensors.
Working Principle of PIR motion sensor
All the objects, living things having a temperature more than absolute zero emit infrared radiations in their surroundings. Warmer the object, more the infrared radiations are emitted.
The PIR sensor is a thermal infrared sensor (IR Sensor) which detects the motion of objects on reading these infrared radiations’ variations in its nearby environment.
The sensor has two equal slots made up of Pyroelectric material that is very sensitive to Infrared. When the sensor is not active, both its slots sense the same amount of Infrared radiations coming from any object, door or wall, etc.
When a warm body comes in the range of the PIR motion sensor, it passes through both the slots one after the other. The moment it intercepts the first half of the sensor, a positive differential change is caused between the two halves. Similarly, when it leaves the detection area a negative differential change occurs between both the slots. These changes in the pulses are the indication to the sensor that there is some motion in its detecting zone.
- Burglar Alarm Systems or Intrusion Detection System,
- Automated Home Appliances,
- Automated Doors,
- Parking Areas,
- Automated Lightings,
- IR Thermometers,
- Night Vision Cameras,
- Gas Analyzers,
- Moisture Analyzers,
- Smoke and Fire Detectors.
The applications of the PIR sensor are based on the requirement of the system, such that the energy is not wasted and the system is automated without human intervention. The system is thus designed with low power, cost reliability and exhibits more immunity to noise.
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