Pyroelectric sensor is a kind of sensor which can detect infrared ray emitted by human or animal and output electric signal. As early as 1938, pyroelectric effect was proposed to detect infrared radiation, but it was not paid much attention to. It was not until the 1960s that the research on pyroelectric effect and the application of pyroelectric crystals rose again.

Objects existing in nature, such as human body, fire, ice and so on, emit infrared rays, but the wavelengths are different. The human body temperature is 36-37 ℃, and the infrared wavelength emitted is 9-10um, which belongs to far infrared region; the infrared wavelength emitted by the heating body at 400-700 ℃ is 3-5um, which belongs to mid infrared region. The pyroelectric infrared sensor is not affected by day and night. It can be used for monitoring day and night. It is widely used in anti-theft alarm, induction lamp and other fields.

Principle of pyroelectric sensor

Pyroelectric infrared sensor generally consists of cap, socket, infrared filter, sensitive element, field effect transistor or digital chip.

Based on the characteristic that the spontaneous polarization of pyroelectric material produces charge movement with the change of external temperature, the change of infrared energy radiated by human body is detected in a non-contact way, which is converted into electrical signal and output in the form of voltage or current, and the signal is amplified by power amplifier to achieve the purpose of control circuit.

The acceptable pass through wavelength of infrared filter window material is as follows:

Spectrogram of 5.5um long wave pass filter

The figure above shows a typical reference figure of 5.5um long wave pass infrared filter, and the curve is the average value of infrared transmittance. The window material is a semiconductor wafer which has been treated by special vacuum coating.

Characteristics of pyroelectric sensors

The pyroelectric infrared sensor can play the most important role only when it is used with Fresnel lens. Without Fresnel lens, the detection radius of the sensor may be less than 2 meters, and with Fresnel lens, it can reach 10 meters or even further. The Fresnel lens is made of common polyethylene and is mounted in front of the sensor.

The principle of Fresnel lens is shown in the figure below

When the light passes through the lens unit, the light and dark visible area and blind area are formed on its reverse side. Each lens unit has only a very small field of view angle. The visible area is inside the field of view angle, and the blind area is outside. By changing the infrared radiation of human body in the light and dark area, the temperature of pyroelectric sensitive element is changed, so that it outputs one weak electrical signal after another. The frequency range of output signal is about 0.1 ~ 10Hz, which is determined by Fresnel lens, human motion speed and the characteristics of pyroelectric infrared sensor.

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