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A Passive Infrared Sensor (PIR sensor) is a type of electronic device commonly used to detect motion or changes in thermal energy in its surroundings. PIR sensors are often utilized for security and automation purposes in various applications, including home security systems, lighting control, and occupancy sensing. Here’s how a PIR sensor typically works:

Detection Principle:

PIR sensors work on the principle of detecting infrared radiation emitted by objects within their field of view. Specifically, they detect changes in the thermal energy or heat patterns in the environment.

Infrared Sensing:

  • PIR sensors consist of pyroelectric sensors, which are sensitive to infrared (IR) radiation. These sensors are typically made of pyroelectric materials, which generate an electrical charge when exposed to IR radiation.

Sensor Design:

  • PIR sensors are equipped with a specially designed optical system that typically includes one or more Fresnel lenses. These lenses are used to focus and direct the incoming IR radiation onto the pyroelectric sensor element.

Detection of Motion:

  • When a warm object, such as a human or an animal, moves within the sensor’s field of view, it causes a change in the distribution of IR radiation detected by the sensor. This change is interpreted as motion.

Signal Processing:

  • The electrical charge generated by the pyroelectric sensor is processed by onboard circuitry. The sensor typically includes a pair of pyroelectric sensors arranged in a differential configuration to detect changes in temperature more effectively.

Output Signal:

  • When motion is detected, the PIR sensor produces an output signal, often in the form of a digital pulse or voltage change, indicating the presence of a moving object.


  • PIR sensors are widely used for various applications, including security systems (to trigger alarms), outdoor lighting (to activate lights when motion is detected), energy-saving systems (to control lighting and HVAC systems based on occupancy), and automatic door opening systems, among others.

It’s important to note that PIR sensors are passive devices, which means they don’t emit any energy themselves but only detect changes in the incoming IR radiation. This makes them energy-efficient and suitable for battery-operated devices.

PIR sensors are valuable components in smart building systems, as they contribute to energy conservation, security enhancement, and automation. Their ability to detect the presence of warm objects or motion makes them a popular choice for a wide range of applications.

The infrared beam wavelength is longer than the visible light. The infrared beam is invisible but can be detected. The detector is made of a crystalline material that forms an electrically charged surface when exposed to heat or infrared radiation. The amount of electrical load that changes as the impact amount of infrared rays changes, and this load can be measured by a sensitive FET embedded in the sensor .

PIR Sensor Block Diagram

The sensor elements have a wide range of infrared rays, so a filtering window is added to the sensor. In this way, the detection range is adjusted to a range of 8- 14 μm which is more sensitive to living organisms.

Fresnel lens

fresnel lens can be attached to the front of the PIR sensor and the sensing distance can be increased up to 30m. Frensel lens is an invention for the lighthouse tower. He got his name from the inventor of the lens lens, Augustine Fresnel. The lens is polygonal, not cylindrical. The light that falls on the sensor when human or live moves, is cut again due to polygonal structure. From here it is understood that it is a living creature.

Figure 5 shows the diagram of the sensor circuit. The system consists of an infrared signal amplifier circuit, a comparator circuit, an output circuit and a power circuit.

Figure 5. Circuit diagram of infrared sensor


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