The working principle of the infrared emission tube is that the infrared beam emitted by the infrared emission tube is scattered by the smoke particles in the air. Of course, the intensity of the scattered light is proportional to the concentration of the smoke, so the intensity of the infrared beam received by the photosensitive tube will occur. Changes are transformed into point signals, and finally into alarm signals. The smoke sensor of the alarm is mainly completed by the optical labyrinth. In the labyrinth, there is a set of infrared transmitting and receiving photoelectric tubes, and the beam angle is 135 degrees.
When there is no smoke in the environment, the receiver tube cannot receive the infrared light emitted by the infrared transmitter tube, and the subsequent sampling circuit has no electrical signal change; when there is smoke in the environment, the smoke particles enter the maze to scatter the infrared light emitted by the transmitter tube. There is a certain linear relationship between the intensity of the scattered infrared light and the smoke concentration. The subsequent sampling circuit changes, and the main control chip built in the alarm judges these changes to confirm whether a fire alarm has occurred. Once the fire alarm is confirmed, the alarm sends a fire alarm signal and fire indication The light (red) lights up and the buzzer is activated.
Infrared alarms are divided into active infrared alarms and passive infrared alarms. Now let's introduce the differences and respective characteristics of these two alarms.
The active infrared intrusion detector is composed of a transmitter and a receiver. The transmitter is composed of a power supply, a light source and an optical system. The receiver is composed of an optical system, photoelectric sensor, amplifier, signal processor and other parts.
The active infrared detector is a kind of infrared beam blocking alarm. The infrared light-emitting diode in the transmitter emits a modulated infrared beam (the wavelength of this beam is about 0.8～0.95 microns) under the excitation of the power supply. After the action of the optical system, the parallel light is emitted. The light beam is received by the receiver, and the infrared photoelectric sensor in the receiver converts the light signal into a signal, which is processed by the circuit and sent to the alarm controller. The infrared rays emitted by the transmitter reach the receiver through the guard zone, forming a cordon. Under normal circumstances, the receiver receives a stable optical signal. When someone invades the cordon, the infrared beam is blocked, and the infrared signal received by the receiver changes. This change is extracted, amplified and properly processed, The alarm signal from the controller. At present, such detectors include two-beam, three-beam, and multi-beam infrared fences. Generally used in perimeter guards, the biggest advantage is that the guarding distance is long, which can reach more than ten times the detection distance of passive infrared.
Active infrared detector:
Detector that adopts active infrared method to achieve security alarm function. The active infrared detector consists of an infrared transmitter, an infrared receiver and an alarm controller. The optical systems respectively placed at the receiving and transmitting ends generally use optical lenses to focus the infrared light beam into a narrower parallel light beam, so that the energy of the infrared light can be concentratedly transmitted. Infrared light is in the invisible spectrum range of the human eye. If someone passes through this invisible barrier line, the infrared beam will be completely or partially blocked. The intensity of the electrical signal output by the receiving end will change accordingly, and the alarm controller will be activated to send out an alarm signal.
Passive infrared detector:
PIR (Passive infrared detectors) adopt the passive infrared method and have reached the security alarm function of detectors. Passive infrared detectors are mainly composed of optical system, pyroelectric sensor (or infrared sensor) and alarm controller. The detector itself does not emit any energy but only passively receives and detects infrared radiation from the environment. Once the infrared radiation of the human body comes in, the pyroelectric device will generate a sudden change of electric signal by focusing by the optical system, which will sound an alarm. The passive infrared detector mainly judges whether someone is moving based on the change of external infrared energy. The infrared energy of the human body is different from the environment. When a person passes through the detection area, the position of the different infrared energy collected by the detector changes, and then an alarm is issued through analysis.
Passive infrared alarms are aimed at detecting human radiation, so pyroelectric elements must be very sensitive to infrared radiation with a wavelength of about 10 μm.
In order to be only sensitive to the infrared radiation of the human body, its radiation surface is usually covered with a special Fresnel filter, so that the interference of the environment is obviously controlled.
The sensor consists of two pyroelectric elements connected in series or in parallel. Moreover, the two electrical polarization directions produced are exactly opposite, and the environmental background radiation has almost the same effect on the two pyroelectric elements, causing the discharge effects to cancel each other, so the alarm has no signal output.
Once a person invades the detection area, the infrared radiation of the human body is focused by a part of the mirror and received by the pyroelectric element, but the heat received by the two pyroelectric elements is different, and the pyroelectricity is also different and cannot be offset, and an alarm is issued after signal processing.
But the external environment is: not only the human body emits infrared energy, but many objects emit infrared energy under certain conditions, and this energy is particularly prominent in visible light, so the anti-white light interference of any passive infrared detector becomes an important index. This detection method performs very well when the indoor light is stable and the infrared energy is relatively constant. But the outdoor situation is different. For a long time, only a few manufacturers can do passive infrared detection outdoors. As the so-called one small step indoors and outdoors, three steps of science and technology content.
Active infrared detector equipment selection
1. According to the minimum and maximum temperature of the prevention site and its duration, select the active infrared intrusion detector with suitable working temperature; if the ambient temperature is too low, a special heater can be used to ensure the normal operation of the detector.
2. Active infrared intrusion detectors are severely affected by fog, and equipment with automatic gain function should be selected for outdoor use (this type of equipment will automatically adjust its sensitivity when the climate changes); in addition, the detection distance of the selected equipment should be set aside 20 A margin of more than% to reduce false alarms caused by climate change.
3. When using outdoors, you must use dual-beam or 3-beam active infrared intrusion detectors to reduce false alarms caused by birds and fallen leaves.
4. The wavelengths of infrared light-emitting diodes used in active infrared intrusion detectors are around 0.85μm and 0.95μm, respectively. The former has the phenomenon of red exposure, and its concealment is not as good as the latter.
5. It is not suitable to use active infrared intrusion detectors outdoors in foggy areas and areas where the environment is dirty and windy and sandy.
6. When using active infrared intrusion detectors in open areas or on fences or roofs, you should choose equipment with lightning protection functions.
7. When there is a folding wall and the distance is relatively close, reflective devices can be used to reduce the number of detectors used.
8. The maximum beam distance of the active infrared intrusion detector used outdoors should be more than 6 times the detection distance specified by the manufacturer.
