The surface charge produced by thermoelectric effect is temporary. As soon as it appears, it is quickly neutralized by various ions in the air. Therefore, when making an infrared sensor with pyroelectric effect, a mechanical periodic shading device is often added in front of its elements, so that spontaneous polarization charges appear periodically, only when detecting moving objects.
Because the output impedance of pyroelectric sensor is very large and the output voltage signal is very weak, field effect transistor and thin film resistor Rg are added inside the sensor to transform its impedance. Rs is the load resistance, and some pyroelectric sensors do not have Rs, so they need to be connected externally. In order to achieve different detection purposes, a filter is set on the window where infrared rays enter to filter out unnecessary spectra.
As a pyroelectric infrared sensor for human detection, it is mostly composed of two parts.
The characteristics of the two elements are as follows: first, when the energy emitted by human beings hits the two elements in turn, the output of the two elements is twice as high as that of one element because of their series connection; In addition, due to the reverse connection of the two components, the same and simultaneously input energy will cancel each other out, such as infrared interference in sunlight and errors caused by environmental temperature changes. The application circuit of pyroelectric infrared sensor to prevent outsiders from invading, such as when someone enters the protected field of vision, the sensor generates AC induced voltage, the frequency of which is related to the moving speed of human body, usually around 5 ~ 6 Hz. It can control the moving human body from 10 to 12m, and the visual field is about 70 degrees.
Component selection:
A 1, A2, A3: corresponding operational amplifier integrated circuits, such as LM324, can be selected.
IC 1: A three-terminal stable integrated circuit, such as 78L00.
IC2 and IC3: Any 555 time-base integrated circuit.
PIR: A binary pyroelectric infrared sensor with a wavelength of 8~ 10um and a Fresnel lens with a focal distance of 15~20mm, such as LS-064 or P2288. If there is no Ps in it, it should be external.
K: 12V DC relay, whose contact can control the flashing of alarm lamp, select JRX- 13F.
R 1、r 12: 1kω； R2:47kω； R3、R5、R8、R9: 100kω； R4、R7、r 13: 10kω； R6:270kω； r 10: 1.5mω；
r 1 1: 15kω； r 12:4.7kω； r 14:8.2kω； r 15:2kω； r 16:3.3kω； r 17:2.2mω； R 18、R20:20kω； r 19: 1mω； r 2 1:200kω； RP:4.7kω.
C 1、C6、C 13、c 14: 100 uf； C2、C3、C5、C9、c 12: 10uF； C4、C8:0.47 uf； C7:680 p； C 10、C 1l:0. 1/W .
V2: NPN transistor, model 90 13.
D2 d 1, D4 D3: 1n 4004 diode.