Pyroelectric infrared sensor and thermocouple are both thermoelectric infrared sensors based on the principle of thermoelectric effect. The difference is that the pyroelectric infrared sensor has a much higher thermoelectric coefficient than the thermocouple, and the internal thermoelectric element is composed of lead and mercury ferrotitanate ceramics with high thermoelectric coefficient, lithium tantalate, triglyferric sulfate and other filter window, and its polarization changes with the change of temperature.

In order to suppress the interference caused by its own temperature change  the sensor in the process of the two characteristics of the same thermoelectric element reverse series or connected into a differential balance circuit, so that non-contact detection of the infrared energy emitted by the object  and convert it into an electrical signal output. The purpose of introducing field effect tube in the structure of pyroelectric infrared sensor is to complete the impedance transformation. Since the thermoelectric element output is a charge signal, and can not be directly used , it needs to be converted into a voltage form  the resistance impedance is as high as 104MΩ, so the introduced N-channel junction field effect tube should be connected to a common leakage form , that is, a source follower to complete the impedance transformation.

The pyroelectric infrared sensor is composed of three parts: sensor detector, interference filter and field effect tube matcher. In the design, a high thermoelectric material should be made into a thin sheet of a certain thickness, and both sides of it are plated with metal electrodes, and then charged to polarize it, so that a pyroelectric probe is made. Since the voltage of electric polarization is polar, the probe element after polarization is also positive and negative polarity.

The structure diagram of a pyroelectric infrared sensor with two probe elements is presented. When used, the D end is connected to the positive terminal of the power supply, the G end is connected to the negative terminal of the power supply, and the S end is the signal output. The sensor connects two probe elements with opposite polarity and consistent characteristics together in order to eliminate interference caused by changes in the environment and itself. It compensates the sensor by using the principle that two interference signals of opposite polarity and equal size cancel each other inside. For the infrared radiation radiated to the sensor, the pyroelectric sensor focuses it through the Fresnel lens mounted on the front of the sensor and adds it to the two probe elements, so that the sensor outputs a voltage signal.

The high thermoelectric material used to make pyroelectric infrared detector is a kind of broad spectrum material, its detection wavelength range is 0.2 ~ 20μm. In order to have high sensitivity to infrared radiation in a certain wavelength range, the sensor is equipped with an interference filter on the window. In addition to allowing infrared radiation to pass through certain wavelength ranges, the filter can also shut out lights, sunlight and other infrared radiation.