Abstract: The design of the human body thermometer is mainly to meet the needs of fast and non-contact measurement of human body temperature. It mainly introduces the working principle of the pyroelectric infrared sensor and the structure and equivalent circuit of the pyroelectric sensor p7187 that is most suitable for infrared detection of the human body. The working principle of the infrared thermometer based on the pyroelectric sensor p7187 is discussed, the design and implementation method of the system are discussed, and the applicable conditions of the temperature measuring system are briefly introduced. The application shows that the design has wide clinical applicability.
Pyroelectric infrared detector is a photoelectric detection device composed of pyroelectric infrared sensor, Fresnel lens and electronic circuit. He can detect infrared rays radiated by human body movements without contact and convert them into electrical signals for output. This article introduces the principle and basic design circuit of a p7187 pyroelectric sensor for rapid measurement of human body temperature.
1. Pyroelectric effect
The surface of some ferroelectric substances (PZT, LiTaO3, etc.) receives infrared radiation energy, and the surface changes in temperature. As the temperature rises or falls, the charge changes on the surface of these substances. This phenomenon is called Pyroelectric effect. Figure 1 shows the movement of the crystal surface charge with temperature changes.
It can be seen that when the pyroelectric element is irradiated by infrared rays, its internal polarization changes greatly, and its changed part is discharged as a charge, and the charge is taken out from the outside to become the output voltage of the sensor. It can be seen that the pyroelectric sensor only has an output voltage when the temperature changes.
2 Equivalent circuit of p7187 pyroelectric sensor
Common pyroelectric sensors are p2613, p3782, p7187 and so on. According to Faraday's law, the body temperature of the human body is about 37 ° C, and the wavelength of the most radiated infrared light is about 10μm, while p7187 is more sensitive to the wavelength in the range of 7-20μm. , And install a pair of electrodes on the inside and outside of the light-receiving surface to take out the electric charge, which can sensitively capture the measured object or light source, and has a very high sensitivity. The two light-receiving electrodes are connected in reverse series, which can effectively prevent the background fluctuation and the malfunction caused by interference light (one is the malfunction caused by environmental changes, and the other is the malfunction when using the light modulator). When the two light-receiving electrodes are irradiated with infrared rays at the same time, the output voltage cancels each other without output. Only when the human body moves does it have a voltage output. The output voltage accurately reflects the movement of the human body.
Figure p7187 pyroelectric sensor equivalent circuit diagram
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