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In our country, PIR ( Passive Infrared Sensor ) sensors, which are called motion sensors, can detect the temperature of a human or a flame. Humans or warm-blooded animals produce infrared light that can be detected by such sensors. On the front of the sensor there are numerous fresnel lenses that focus the heat rays on the IR sensor at various points. The beams falling on the sensor are interpreted as live / not with the sensor and circuit connected to it.

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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