Understanding of passive infrared detection

About infrared:

Whenever people mention the word "infrared", perhaps an infrared-related application reflected in the minds of many people is the infrared remote control of home TV sets. Yes, this is correct. There is an infrared light emitting element on the remote control and an infrared light receiving device in the controlled appliance. When we press a button of the remote control, the electronic circuit in the remote control will transmit the corresponding code of this button through the infrared emission tube on the top of the remote control. When the infrared detection element in the controlled electrical appliance "sees" these optical code signals, it converts them into electrical signals, and then processes and recognizes the signals through subsequent circuits to perform some operations. It is this series of actions that fulfills our needs-such as the channel switching of TV programs. This seemingly simple operation actually involves two conversions of electricity-light and light-electricity. Infrared detector is a product invented by using this feature.

The human body has a constant body temperature, generally at 37 degrees, so it emits infrared rays with a specific wavelength of about 10UM. The passive infrared probe works by detecting the infrared rays of about 10UM emitted by the human body. About 10 UM of infrared rays emitted by the human body are enhanced by the Fidel filter and then concentrated on the infrared sensor source. Infrared induction sources usually use pyroelectric elements, which will lose the charge balance when the temperature of the infrared radiation of the human body is received and release the charge outward. The subsequent circuit can generate an alarm signal after detection and processing.

1) This probe is aimed at detecting human radiation. Therefore, the pyroelectric element must be very sensitive to infrared radiation with a wavelength of about 10 UM.
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2) In order to be sensitive only to the infrared radiation of the human body, its radiation surface is covered with a special Fidel filter, so that the interference of the environment is obviously controlled.
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3) A passive infrared probe whose sensor contains two pyroelectric cells connected in series or parallel to each other. Moreover, the two polarized electrodes are made in the opposite direction. The environmental background radiation has almost the same effect on the two pyrolysis elements, causing the discharge effect to cancel each other, so the detector has no signal output.

4) Once a person intrudes into the detection area, the infrared radiation of the human body is focused through part of the mirror surface and received by the pyroelectric element, but the heat received by the two pyroelectric elements is different, the pyroelectricity is also different, and cannot be cancelled, after signal processing And the alarm.

5) The Fidel filter has different focal lengths (sensing distances) according to different performance requirements, resulting in different monitoring fields of view. The more fields of view, the tighter the control.

Classification of passive infrared detectors:

According to the wiring method (signal transmission method), it can be divided into wired and wireless;
Classified by installation method: wall-mounted, ceiling-mounted,
Classified by place of use: indoor, outdoor,
According to product technology classification: ordinary detector, intelligent detector (with automatic pulse number adjustment), double detector (dual detector), composite (infrared + microwave),
According to the detection range: wide-angle (spatial), curtain, directional;

In actual use, many manufacturers combine several technologies to improve the quality of the detector, such as: intelligent dual-screen wall-mounted curtain detector, multiple wide-angle infrared detector, etc ...

Installation of passive infrared detectors:


The biggest advantage of the wireless passive infrared detector is that it is very convenient to install and can be quickly installed without damaging the decoration of the house. However, there are shortcomings that need to replace the battery and the wireless channel may be interfered. If you consider installing before the decoration, it is more reasonable and economical to install a wired passive infrared detector, and it is not prone to failure after long-term use.




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