Detection principle of pyroelectric infrared sensor

　　Pyroelectric infrared sensors can detect infrared rays emitted by people or some animals and convert them into electrical signals for output. When some crystals are heated, the same amount of charges with opposite signs will be generated at both ends of the crystal. This kind of electrode phenomenon due to thermal change is called pyroelectric effect. Generally, the bound charge generated by the spontaneous polarization of a crystal is neutralized by free electrons attached to the crystal surface from the air, and its spontaneous polarization electric moment cannot be displayed. When the temperature changes, the gravity center of positive and negative charges in the crystal structure shifts relatively, and the spontaneous polarization changes, the crystal surface will have charge depletion, which is proportional to the degree of polarization. Figure 1 shows the principle of the formation of the pyroelectric effect.

　　Pyroelectric sensor is a temperature sensitive sensor, which uses the pyroelectric effect. It is composed of ceramic oxide or piezoelectric crystal element. The two surfaces of the element are made into electrodes. When the temperature within the monitoring range of the sensor is Δ When T changes, the pyroelectric effect will generate charges on the two electrodes Δ Q. That is, a weak voltage is generated between the two electrodes Δ V。 Because its output impedance is very high, there is a FET in the sensor for impedance transformation. Charge generated by pyroelectric effect Δ Q will disappear with the ions in the air. When the ambient temperature is stable, Δ T=0, sensor has no output. When the human body enters the detection area, due to the difference between the human body temperature and the ambient temperature Δ T. Then there is signal output; If the human body does not move after entering the detection area, the temperature will not change and the sensor will not output, so this sensor can detect the activities of human body or animals.

　　The sensor is mainly composed of shell, filter, pyroelectric element (sensing element), FET, etc. Wherein, the filter is set at the window to form a window through which infrared rays pass. The filter can well filter the short wavelength (below 5um) of sunlight and fluorescent light. Pyroelectric element (inductive sensitive element) The inductive element converts the weak change of the infrared signal with the wavelength between 8-12um into an electrical signal. In order to be only sensitive to the infrared radiation of the human body, its radiation surface is usually covered with special Fresnel lenses, so that the interference of the environment is obviously suppressed.

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