Infrared alarm can be divided into active type and passive type. Active infrared alarm is an alarm that actively emits infrared rays. When the infrared ray hits an obstacle, it will bounce back and be received by the probe of the alarm. If the probe detects that the infrared ray is still, that is, the red line is constantly emitting and bouncing, then the alarm will not give an alarm. When a moving object violates this invisible red line, the probe will detect the abnormality and give an alarm.
The main materials of pyroelectric infrared sensor are lithium tantalate (LiTao3), triethyl sulfate (LATGS) and lead zirconate titanate (PZT). It is a piezoelectric crystal such as barium titanate, with electrodes on the upper and lower surfaces and a black film on the surface. When infrared ray is intermittently irradiated, its surface temperature rises, which changes the atomic arrangement inside the crystal, that is, it causes spontaneous polarization charge. This phenomenon is called pyroelectric effect, which is a kind of pyroelectric effect.
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.