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Fresnel lens is indispensable for human body induction module

Time:

2019-11-16

The pyroelectric human body infrared sensor of human body induction module is a device that converts the infrared light from human body or animal into electrical signal. It can be used to detect the existence of human body, so it is widely used in electronic products, such as security devices, anti-theft alarms, induction doors, automatic lights and intelligent toys, all of which are inseparable from Fresnel lens.

The pyroelectric human body infrared sensor of human body induction module can detect the infrared emitted by human body, but the detection distance is relatively short, generally within 2m. In order to improve the detection distance, a Fresnel lens is usually added to the front surface of the sensor, which can make the detection distance more than 10 m.

Fresnel lens is usually made of high-density polyethylene plastic. According to certain manufacturing methods, the lens is divided into several equal parts. The lens has two functions: one is to focus light; the other is to divide the detection area into several bright areas and dark areas. When a person enters a bright area of the detection area, the infrared light emitted by the human body is focused by the lens corresponding to the bright area, and then irradiates the sensitive element through the filter of the sensor to generate voltage for the sensitive element; when a person enters the dark area, the infrared light of the human body cannot reach the sensitive element, and the voltage at both ends of the sensitive element changes, that is, the sensitive element The voltage at both ends of the device changes with or without light, and the changed voltage is amplified by the FET and then output. The frequency of the sensor's output signal is related to the speed at which people move between bright and dark areas within the detection range. The faster you move, the higher the frequency of the output signal. If the person is fixed within the detection range, the sensor outputs a fixed voltage.