In the intelligent security system, the anti-intrusion alarm system is an early warning system to prevent theft and vandalism. The biggest difference between it and the video surveillance system is early prevention. How does the anti-intrusion alarm system provide early warning? Today we will learn about the front-end sensing equipment (detector) of the anti-intrusion alarm system. The most widely used technology for front-end sensing equipment is passive infrared technology.
. What is passive infrared?
First, let’s understand what infrared radiation is. Why can passive infrared detector detect moving human bodies?
Any object in nature can be regarded as a source of infrared radiation. This is because when the temperature of the surface of an object is higher than absolute temperature zero (i.e. -273°C), thermal radiation will be generated, and the spectrum generated by the thermal radiation is mainly in the infrared band. For example, when the incandescent lamps we usually see emit light, about 10% of the radiated light is visible light, and the remaining 90% is almost infrared rays. So our human body is also a source of infrared radiation. For the detector, this infrared energy is not emitted by the device itself, but passively received, so it is called passive infrared.
The human body's infrared radiation
radiation ability varies from person to person, and the radiation ability of different parts of the human body is also different. Our human body and other indoor background objects not only differ in infrared radiation energy, but also have different peak wavelengths of infrared radiation. The peak wavelength of infrared radiation radiated by the human body is about 10 μm. In other words, as long as we use a detector to capture the infrared radiation source with an infrared peak at 10μm, we can detect the moving human body. and passive infrared detector use this feature to detect moving human bodies.
Passive infrared detector internal
detector external Fresnel lens
2. What sensor can detect infrared radiation from the human body?
There are two key components in the passive infrared detector, one is the Fresnel lens , and the other is the pyroelectric sensor . The most critical thing about is this pyroelectric sensor, which can convert the infrared radiation energy of the human body into electrical energy, and filter out the 10μm infrared wavelength that is only sensitive to the human body through the filter on the front of the pyroelectric sensor.
Pyroelectric sensor
But the pyroelectric sensor has a characteristic, that is, once the infrared radiation energy does not change, there is no signal output, so The Fresnel lens has two functions. One is to focus the infrared energy of the human body into the pyroelectric On the electrical sensor, another function is to divide the detection area into several sensitive areas and non-sensitive areas. That is to say, when the human body moves in front of the Fresnel lens, it passes through the sensitive area for a while and the non-sensitive area for a while. The infrared energy focused on the pyroelectric sensor continuously changes, and at the same time, it outputs changing electrical signals, which are amplified. Process, and finally control the relay to output the alarm signal. As shown in the following figure
summary: When the human body passes through the passive infrared detector, the Fresnel lens in front of the detector divides the detection area into a sensitive area and a non-sensitive area. When the human body passes through the sensitive area and the non-sensitive area, the pyroelectric sensor It detects the infrared radiation energy of 10μm infrared wavelength emitted by the human body, and continuously outputs the detection electrical signal, which is processed and amplified to output an alarm signal. In other words, as long as we enter this detection area, the detector has already discovered us.
