The detector consists of a sensor and a signal processor. In the intrusion detector, the sensor is the core of the detector, and it is a physical quantity conversion device, which usually converts physical quantities such as pressure, vibration, sound and light intensity into electrical quantities (voltage, current, resistance, etc.). ) easy to handle. The function of the signal processor is to amplify, filter and shape the electric quantity converted by the sensor, so as to make it a signal that can be transmitted smoothly in the transmission channel of the system.

infrared alarm detector

Any object with a temperature higher than 0K can generate thermal radiation, while the thermal radiation spectrum generated by an object with a temperature lower than 1725℃ is concentrated in the infrared region, so all objects in nature can radiate infrared heat. However, the wavelength and distance of infrared radiation produced by any object due to its physical and chemical properties and its temperature are different, and it is usually divided into three bands.

Near infrared: wavelength range 0.75 ~ 3 μ m.

Mid-infrared: wavelength range 3 ~ 25 μ m.

Far infrared: wavelength range 25 ~1000μ m.

The wavelength of infrared light radiated by human body is 3 ~ 50μ m, of which 8 ~ 14μ m accounts for 46%, and the peak wavelength is 9.5μm m. ..

passive infrared detector

At room temperature, everything has radiation. The higher the temperature, the stronger the infrared radiation. People are warm-blooded animals, and infrared radiation is also the most stable. We call it passive infrared, that is, the detector itself does not emit any energy but passively receives and detects infrared radiation from the environment. A few seconds after the detector was installed, it has adapted to the environment. When no human or animal enters the detection area, the infrared radiation at the scene is stable. Once the infrared radiation of the human body comes in, it will be focused by the optical system, which will cause the pyroelectric device to generate a sudden electrical signal and give an alarm. The warning line formed by passive infrared intrusion detectors can generally reach tens of meters.

Passive infrared detector is mainly composed of optical system, thermal sensor (or infrared sensor) and alarm controller. Its core component is infrared detector, which can detect the change of thermal radiation in a three-dimensional prevention space through the cooperation of scientific system. The detection wavelength range of infrared sensor is 8 ~ 14 micron, and the infrared peak wavelength of human radiation is about 10μm, which is just within this range.

Passive infrared detectors can be roughly divided into single-beam type and multi-beam type according to their different structures, alarm ranges and detection distances. Single beam PIR adopts reflective focusing optical system, and uses curved mirror to focus the infrared radiation emitted by the target on the infrared sensor. The field of view of this detector is narrow, generally below 5, but the action distance is far, which can reach 100 meters. Therefore, it is also called linear remote control passive red detector, which is suitable for protecting long and narrow corridors, passages and shielding doors and windows, fences and so on. Multi-beam type adopts lens focusing optical system, and at present, multi-beam infrared plastic lens-Fresnel lens is mostly used. This kind of lens is made of special plastic at one time, and several small lenses are arranged on an arc surface. The early warning range is in the state of multiple single beams in different directions, forming a three-dimensional fan-shaped thermal sensitive area, forming a three-dimensional early warning. Fresnel lenses are divided into several rows from top to bottom, with more lenses on the top and fewer lenses on the bottom. Because people's faces, knees and arms have strong infrared radiation, but they are just facing the lens above. There are few lower lenses, one is because the infrared radiation of the lower part of the human body is weak, and the other is to prevent the infrared radiation interference of small animals on the ground. The warning visual angle of multi-beam PIR is much larger than that of single-beam PIR, which can be greater than 90 horizontally and 90 vertically, but the action distance is close. All the lenses are focused on the built-in pyroelectric device, so the sensitivity is high. As long as someone walks in the field of view of the lens, it will give an alarm.

Infrared light penetration is poor, so there should be no tall objects in the prevention area, otherwise someone walking in the shadow will not be able to give an alarm, and do not face heat sources and strong light sources, especially air conditioning and heating. Otherwise, the changing hot air flow will lead to false alarms. In order to solve the problem of object occlusion, a passive infrared intrusion detector for ceiling was invented. It is installed on the ceiling for warning within 360 degrees downward. As long as it is within the protection range, no matter which direction it invades, it will trigger an alarm. Widely used in banking halls, shopping malls, public activity areas and other places with large space.

Passive alarm detector is widely used because of its good detection performance, convenient deployment and low price. Its disadvantage is that the false alarm rate is higher than active detection.

Active infrared detector

The active infrared detector consists of an infrared transmitter, an infrared receiver and an alarm controller. Generally, optical lenses are used in optical systems placed at the receiving end and the transmitting end respectively, which can focus infrared beams into thin parallel beams and make infrared energy transmit centrally. Infrared light is in the invisible spectral range of human eyes. If someone crosses this invisible blockade line, it will inevitably block the infrared beam in whole or in part. The intensity of the electrical signal output by the receiver will change accordingly, thus starting the alarm controller to send out an alarm signal. The active infrared detector should not give an alarm when it is blocked by small animals, leaves, dust, rain, snow and fog, but will give an alarm when it is blocked by people or objects with a considerable volume. Because the beam is narrow, the transceiver should be installed firmly and reliably, and should not be affected by ground vibration and produce false alarm due to displacement. The optical system should be kept clean and maintained. Therefore, the active detector detects a point-to-point range, not a surface. Its characteristic is high detection reliability. However, if a space is protected, it needs multiple active detectors, which is expensive. Active detectors are often used in museums to protect a single piece of valuable cultural relics, as well as to block the doors and windows of factory warehouses, the corridors of shopping centers, the exits of parking lots, the balconies of homes and so on.

Active infrared detectors can be divided into single beam, double beam and four beam. According to the different positions of transmitter and receiver, it can be divided into opposite installation mode and reflection installation mode. The receiver in reflective installation mode does not directly receive the infrared beam emitted by the transmitter, but receives the infrared beam reflected by a mirror or a suitable reflector (such as the smooth paint layer on the surface of lime wall and door panel). When the position and direction of the reflecting surface changes or one of the infrared emitting beam and the reflected beam is blocked, the receiver cannot receive the infrared reflected beam and sends out an alarm signal. When more detectors are used for preventive arrangement, attention should be paid to eliminating the cross-fire of beams.