Research on Passive Infrared Detector Technology

　　Passive infrared detectors are one of the most commonly used devices in intrusion alarm systems. Since its introduction, the technology used has been continuously developing, and each technology targets different problems. Some are mainly for filtering white light, some focus on reducing the impact of high-frequency interference, and some are specifically for solving false alarms caused by small animal activities; there are two technologies that just emphasize two opposing aspects, such as pulse counting technology and one-step triggering technology; some two technologies have opposite application principles, but the results are the same, such as spread spectrum technology and ultra-narrow band technology in wireless anti-theft systems. As construction maintenance and marketing personnel in the alarm service industry, you should have a certain understanding of these technologies, so that you can formulate design and construction plans in a targeted manner according to the use environment, so as to achieve the purpose of using materials in a quantitative manner and reducing false alarms. Based on the infrared detector models that have been installed more in China in recent years, the author has preliminarily summarized the technologies they use into the following categories.

　　Use high-quality materials

　　Select mirror material: Improve the material used in Fresnel lens to make it have good wavelength selection characteristics. While allowing infrared light emitted by the human body to pass through, it blocks other wavelengths of light in white light as much as possible to eliminate white light interference. A good quality lens can eliminate white light interference up to 20,000 candlepower, which is equivalent to the runway lights of an airport shining at 2 meters, and the detector will not cause false alarms.

　　Double shielding film: A special conductive filter film is attached in front of the sensor, which is called a double shielding film. The function of this film is to reduce the influence of light sources such as sunlight, car headlights, and fluorescent lights. When the sun shines into the room in the morning or evening, the illumination can reach 50,000 Lux. After using this shielding, the light intensity can reach 100,000 Lux without false alarms; on the other hand, this film also has the function of resisting RF radio frequency interference, so that the detector has an anti-interference ability of 20V/M or 30V/M. (20V/M is equivalent to a 10W wireless transmitter within 1m without false alarms, and 30V/M is equivalent to a 10W transmitter at a distance of 0.3m-0.5m without false alarms.)

　　Magnetic protection relay: This relay can prevent wireless electromagnetic interference and external force damage. When there is a strong magnetic field near the detector, the general relay lacks magnetic field shielding and may not work properly, causing the detector to fail. With the magnetic protection relay, the strong external magnetic field will no longer affect the operation of the relay.

　　Improved optics

　　Spherical lens: When the detector is working, its detection range should be opened at a certain angle up and down and left and right. Within the effective detection range, its sensitivity should be kept consistent as much as possible. In order to improve the uniformity of the detection sector, a spherical lens is used to keep the focal length of each detection area on the lens consistent and the sensitivity unchanged.

　　Sealed optical system: A heat-insulating plastic box is used between the lens and the circuit board. The sensor is placed in the sealed box under relatively constant temperature conditions to reduce interference caused by heating of circuit components. Carbon black sponge is used around the sensor to absorb white light, which can reduce external white light interference and also prevent dust, moisture and insect invasion.

　　Control the detection area: Use curtains and pet-proof mirrors to avoid the range where small animals usually move, and do not detect the area where they move (such as the detection area is a certain distance above the ground) without affecting the detection of intruders.

　　Mirror adjustment and zoom function: Due to the different heights of the roof, the requirements for ceiling infrared detectors are also different. By adjusting the rotating mirror, the focal length and detection height can be changed to adapt to different environmental requirements.

　　Infrared reflection focusing: There are three types of optical systems that match infrared sensors, namely reflection type, transmission type and refraction type. Among them, the reflection type has the highest sensitivity and the longest detection distance. Fresnel lens is a transmission type with the lowest sensitivity, and the refraction type is in the middle. Fresnel lens is small in size, easy to seal, good in stability, and low in price, so the transmission type system is currently used more at home and abroad. However, the use of reflectors to gather infrared energy is more effective than Fresnel lenses, so the gain of the detector signal amplification circuit can be made lower, the signal-to-noise ratio is higher, and false alarms caused by interference signals can be better prevented. A few passive infrared detectors imported from abroad use reflection focusing, which has achieved better anti-false alarm effects.

　　Temperature compensation and light sensitivity adjustment

　　Temperature compensation circuit: The strength of the signal detected by the passive infrared detector is closely related to the difference between the human body temperature and the ambient temperature. Generally, the human body temperature is quite different from the ambient temperature. When the intruder moves, the sensor receives a large amplitude change in the infrared signal, triggering an alarm. When the ambient temperature is close to the human body temperature, the sensor receives a small amplitude change in the infrared signal when the intruder moves, so it is possible that the signal is less than the trigger threshold and no alarm will be triggered. Therefore, the detector needs to compensate the circuit gain according to the change in ambient temperature.

　　Conventional temperature compensation is linearly increasing, that is, the higher the temperature, the greater the compensation. When the ambient temperature is higher than the human body temperature, excessive compensation is obviously unreasonable and prone to false alarms. A better compensation method is to use a parabola, so that the detector can maintain basically consistent sensitivity within a relatively large temperature change range.

　　Photosensitive adjustment: Due to the large change in light intensity during the day and night, if the detector works during the day and night, the working characteristics during the day and night will change greatly. The detector circuit uses the characteristic that the resistance of the photoresistor changes with the change of light intensity. According to the brightness of the surrounding area and the working time, it automatically adjusts the receiving sensitivity and determines the different working modes during the day and night, which can effectively prevent false alarms caused by strong light interference during the day.

　　Optimizing circuit design

　　Detector working voltage: The working voltage range is the basic parameter of the detector. It seems simple, but it is very important. The principle of the detector working voltage value is that it should be lower than the working voltage of the control host. When the AC power is interrupted and the system works on the backup battery, the power supply voltage will gradually decrease. If the detector working voltage design range is small, the host can still work normally, but the detector can no longer maintain a normal state, and the system will continue to generate false alarms.

　　Power supply filtering: Using electronic filters can effectively filter out the interference caused by noise in the power supply to the circuit. The noise that the electronic filter needs to filter out is mainly 50HZ/60HZ AC frequency and its harmonics, thereby reducing false alarms caused by power supply interference.

　　One-step trigger technology: mainly used in places with no interference (such as well-sealed basements, etc.) but with high sensitivity requirements. On the basis of not increasing false alarms, this technology can generate a pulse when a moving object passes through the detection area to trigger an alarm, and the response is very sensitive.

　　Pulse counting technology: The Fresnel lens divides the detection coverage into a certain number of detection zones. When a temperature change occurs between two adjacent zones, a pulse signal is triggered. Setting a multi-pulse start mode can reduce false alarms caused by accidental reasons to a certain extent. In this mode, the detector uses digital counting to measure the number of sector trigger edges triggered by the intruder from the beginning, and triggers an alarm signal according to the pre-set number of digital pulse counts. However, this method will reduce the sensitivity.

　　Energy accumulation technology: The energy is determined based on the detected signal strength, and an alarm is generated when the threshold is reached. Small objects have a small coverage area, such as only occupying a part of a mirror detection area, or occupying only one or even less than one area in the far, medium, and near detection areas. Therefore, the signal strength generated does not meet the threshold requirement, and no alarm will be generated.

　　Anti-high-frequency interference technology: When there is high-frequency interference, the interference signal strength increases. If the threshold of the detector trigger remains unchanged, the signal-to-noise ratio decreases, which can easily lead to false alarms. After adopting the anti-high-frequency interference technology, the threshold of the detector will increase accordingly as the interference signal increases, so that the signal-to-noise ratio remains basically unchanged, thereby reducing false alarms caused by high-frequency interference.