It is a detection device that can trigger an alarm when an intruder approaches it. In the proximity detector, there is usually a high-frequency LC oscillating circuit, and the LC loop of the oscillating circuit is connected with external metal parts through wires. When the human body approaches, the resonant frequency of LC circuit will change due to electromagnetic coupling in space, which will lead to the change of oscillation frequency. The detection circuit of the detector can recognize this frequency change and send out a warning signal. When approaching the circuit design of the detector, we need to pay attention to several key technical points: ① the selection of frequency, the detection sensitivity is low when the frequency is too low, and the false alarm is easy to appear when the frequency is too high, so we should try to avoid the radio frequency point; (2) The power consumption is low, and the proximity detector is sometimes made into a small portable alarm, which needs battery power supply. Using battery power supply is also conducive to improving the anti-interference ability of the circuit and reducing false alarms; (3) The resonance frequency of 3)LC oscillator circuit is also influenced by external environmental factors (such as temperature and humidity), so it is meaningless to detect the slow change of oscillation frequency, but the sudden change of oscillation frequency should be detected, and only the "sudden change" of oscillation frequency is related to possible cheating.

The main advantages of proximity detection system are versatility and versatility. It can be used to protect almost any object from interference from a few meters away. Once someone approaches or touches jewelry boxes, filing cabinets, doors and windows for theft, it will trigger an alarm, but the normal business work nearby can be carried out as usual.

Proximity detectors are more suitable for indoor use, such as protecting some special objects such as desks, filing cabinets, safes, etc., and can also be used to protect doors and windows. Usually, the protected object is metal, which can actually form a part of the protection circuit, so as long as someone tries to destroy the system, an alarm will be triggered immediately. A sensor that can detect the position of a moving fixed object is called a motion detector. In fact, movement is everywhere, the earth is turning, and everything on the earth is "moving". What we want to detect here is actually relative motion, such as objects placed on the desktop being removed from the desktop, parked vehicles being started or moved, and so on.

In order to detect the movement of the guarded object, we must find out the physical quantity changes that can be produced by the movement, at least: mechanical method, optical method, electromagnetic method and vibration detection method. Mobile detection equipment is most suitable for the protection of valuable and confidential special items such as filing cabinets and safes, and is also suitable for combining with other systems to prevent thieves from entering through walls. The validity of mobile detector is closely related to the correctness of its application. It is often used to protect some special objects and people in protected areas. Light travels in a straight line, so it is called "light". If the propagation path of light is blocked, the light will be interrupted and cannot continue to spread. Active optical intrusion detector uses the linear propagation characteristics of light to detect intrusion, which consists of optical transmitter and optical receiver. The transmitter and receiver are installed separately, and an optical warning line is formed between the transceivers. When the intruder crosses the warning line, the light is blocked, the receiver loses its illumination and sends out an alarm signal.

Generally, infrared radiation outside the visible spectrum is selected as the light source of the transmitter, so that intruders cannot detect the existence of warning lights. In order to avoid the interference of natural sunlight, two technical measures are usually taken:

① Add a color filter on the light receiving window of the receiver to filter other light;

(2) Modulating the amplitude (intensity) of transmitter light, specifically: using infrared light-emitting diode as the light-emitting device of transmitter light source, and modulating the voltage or current of transmitter light source power supply with modulation signal with frequency of several KHz, so that the light intensity emitted by transmitter also changes according to the law of modulation signal. In the receiver, the infrared receiving diode is used to receive the optical signal, and the signal is amplified by an amplifier with a tuning loop, so that the interference of other signals with different frequencies in the modulation signal can be filtered out. Sunlight is a stable light without any modulation, and the signal generated on the receiving diode is naturally filtered out and there is no response. Using the physical principle of "blackbody radiation", as long as the temperature of an object is higher than absolute zero, it will continuously radiate light around, and the wavelength of the radiated light is related to the temperature of the object. At normal body temperature, the human body can emit far infrared rays, which are invisible to the naked eye, but infrared sensors can detect them, so intruders can be found. The core component of this detector is pyroelectric infrared detection element, which is equipped with a "Fresnel" lens made of transparent plastic, so it can monitor a certain spatial range, with convenient installation, high sensitivity, no need for auxiliary light source, low power consumption and low cost. It is a relatively popular electronic security product component.

According to the working principle, infrared detectors can be mainly divided into infrared detectors, microwave infrared detectors, passive infrared/microwave infrared detectors, glass broken infrared detectors, vibration infrared detectors, ultrasonic infrared detectors, laser infrared detectors, magnetic switch infrared detectors, switch infrared detectors, video motion detection alarms, sound detectors and so on.

Infrared detectors can be divided into active infrared detectors and passive infrared detectors according to their working modes.

Infrared detectors can be divided into point-controlled infrared detectors, line-controlled infrared detectors, surface-controlled infrared detectors and space-protected infrared detectors according to different detection ranges.

In addition to the above distinction, there are other ways of division. In practical application, different types of infrared detectors can be reasonably selected according to different use conditions to meet different safety requirements.

As a sensing device, infrared detector is used to detect the intrusion behavior and various abnormal situations of intruders. All kinds of intelligent buildings and ordinary buildings have many places that need safety precautions. These places also have various security purposes and requirements according to the actual situation. Therefore, various infrared detectors are needed to meet different safety requirements.

According to the actual site environment and the safety requirements of users, reasonable selection and installation of various infrared detectors can better achieve the purpose of safety prevention. When the infrared detector is not properly selected and installed, there may be security loopholes, which can not achieve the security rigor, giving intruders an opportunity, thus bringing undue losses to security.

The infrared detector is required to have the functions of anti-disassembly and anti-destruction. When the infrared detector is damaged, the transmission line is short-circuited or artificially disconnected, and the protective cover is illegally tried to open, it should be able to generate alarm signal output; In addition, infrared detectors should have certain anti-interference measures to prevent all kinds of false alarms, such as preventing the harassment of pets and small animals and resisting false alarms caused by changes in environmental conditions.

The sensitivity and reliability of infrared detectors interact with each other. Reasonable selection of detection sensitivity and different anti-interference measures of infrared detector can improve the performance of infrared detector. The performance of infrared detectors in different environments is determined by different anti-interference measures. Understanding the performance and characteristics of various infrared detectors and rationally configuring different infrared detectors according to different use environments are the key links of the anti-theft alarm system. After the thermal detector absorbs infrared radiation, the temperature rises, which can make the detection material produce thermoelectric electromotive force, resistivity change, spontaneous polarization intensity change, or gas volume and pressure change. By measuring these physical properties, the absorbed infrared radiation energy or power can be determined. Various thermal detectors can be made by using the above different properties:

(1) Liquid mercury thermometer and pneumatic Golay battery: using the thermal expansion and cold contraction effect of materials.

⑵ Thermocouple and thermopile: The thermoelectric effect of thermoelectric electromotive force can be generated between different materials by using temperature gradient.

⑶ uncooled infrared imaging array with time oscillator: infrared detection is realized by using the principle that vibration frequency is sensitive to temperature.

⑷ bolometer: Thermal effect of resistance or dielectric constant of materials-temperature rise caused by radiation changes the resistance of materials-is used to detect thermal radiation. Semiconductor resistors are widely used because of their high temperature coefficient. Thermometers are often called "thermistors". In addition, due to the appearance of high temperature superconducting materials, superconducting detectors with sudden resistance change near the transition temperature have attracted attention. If room temperature superconductivity becomes a reality, it will be the most noticeable detector in 2 1 century.

5. Pyroelectric detector: some crystals, such as triethylene sulfate and barium strontium niobate. When the temperature rises under infrared radiation, the intensity of spontaneous polarization changes. As a result, a small voltage perpendicular to the direction of spontaneous polarization is generated between the two outer surfaces of the crystal, so that the power of infrared radiation can be measured. After the photon detector absorbs photons, its electronic state changes, thus causing photon effects such as internal photoelectric effect and external photoelectric effect. The number of absorbed photons can be determined by photon effect.

⑴ photoconductive detector: also known as photoresistor. After the semiconductor absorbs enough photons, some carriers in the body change from bound state to free state, which increases the conductivity of the semiconductor. This phenomenon is called photoconductive effect. Photoconductive detectors made of photoconductive effect are divided into

Polycrystalline thin film type and single crystal type.

⑵ Photovoltaic detector: mainly uses the photovoltaic effect of pn junction. Infrared photons with energy greater than the band gap excite electron-hole pairs in the junction region and its vicinity. The existing junction electric field makes holes enter the P region and electrons enter the N region, and there is a potential difference between the two parts, so the external circuit has voltage or current signals. Compared with photoconductive detectors, the detection rate of background limit of photovoltaic detectors is 40% higher, and there is no need to apply bias electric field and load resistance, so it does not consume power and has high impedance.

(3) Light-emitting-Schottky barrier detector: metal and semiconductor contact to form Schottky barrier, and infrared photons are absorbed by PtSi through Si layer, so that electrons gain energy to transition to Fermi level, leaving holes to enter Si substrate through the barrier, and electrons in PtSi layer are collected to complete infrared detection.

⑷ Quantum well detector (QWIP): Two kinds of semiconductor materials grow alternately in a thin layer by artificial methods to form a superlattice, and an energy band mutation occurs at the interface, so that electrons and holes are confined in a low potential energy well, and the energy is quantized to form a quantum well. Infrared detectors can be made by using the principle of energy level electron transition in quantum wells. Because only the polarization vector perpendicular to the growth plane of superlattice works in the incident radiation, the photon utilization rate is low; The electron concentration in the ground state of quantum well is limited by doping, so the quantum efficiency is not high. Narrow response spectral region; Low temperature requirements are high.