The intrusion alarm system is an electronic system that can send out an alarm signal in time when an intruder is detected on the site. It generally consists of sensors (alarms), transmission systems and alarm control panels. When the sensor detects an unexpected situation, it generates an alarm signal, which is sent to the alarm control panel through the transmission system, and sends out sound, light or other alarm signals.

There are many types of sensors (alarms), which can be divided into microwave, PIR, laser, ultrasonic and vibration according to the different physical quantities detected. According to different transmission methods of electrical signals, it can be divided into wireless transmission and wired transmission. Here mainly introduces the PIR motion sensor.

PIR motion sensor is an alarm device composed of infrared emission and reception technology. According to the working principle, it can be divided into two types: active and passive.

1 Active PIR motion sensor

The active PIR motion sensor is composed of two parts: receiving and sending devices. The transmitting device emits a beam of infrared rays to the receiving device installed several meters, or even hundreds of meters away. When it is blocked, the receiving device sends out an alarm signal. Therefore, it is also a blocking alarm, or a beam sensor. Usually, the transmitting device is composed of a multivibrator, a waveform conversion circuit, an infrared light-emitting tube, and an optical lens. The oscillator generates a pulse signal. After waveform conversion and amplification, the infrared light-emitting tube is controlled to generate infrared pulse light. The infrared light is converted into a thinner infrared beam through the focusing lens and directed to the receiving end.

The receiving device is composed of optical lens, infrared photocell, amplification and shaping circuit, power driver and executive mechanism. The photoelectric tube converts the received infrared light signal into an electrical signal, and after shaping and amplifying, the alarm device is activated. Active PIR motion sensors have a longer transmission distance, because infrared rays are invisible light sources, it is difficult for intruders to detect and avoid them, and the defense boundary is very clear.

The active PIR motion sensor is a point-type and line-type detection device. In addition to being used as a point warning and line warning for a single machine, in order to effectively prevent it in a wider range, it can also use multiple machines to adopt optical wall or optical network installation. Form a warning blockade or a warning blockade network, and even form a three-dimensional warning area. A single optical router consists of a transmitter and a receiver.

Dual optical routers consist of two pairs of transmitters and receivers. The two pairs of receiving and sending devices are opposite to each other, in order to eliminate cross misshooting. Multiple light paths constitute a warning surface. Reflecting a single light path constitutes a warning zone.

2 Passive PIR motion sensor

The passive PIR motion sensor does not radiate energy into space, but relies on receiving infrared radiation from the human body to issue an alarm. Any object with temperature is constantly radiating infrared rays to the outside world. The surface temperature of the human body is 36-37°C, and most of its radiation energy is concentrated in the wavelength range of 8-12um. The structure of passive PIR motion sensor can be divided into infrared detector (infrared probe) and alarm control part.

The passive PIR motion sensors are currently the most widely used infrared detectors, which are sensors that convert human infrared radiation into electricity. If the human infrared radiation is directly irradiated on the detector, it will of course cause a temperature change and output a signal, but in doing so, the detection distance will not be long. In order to extend the detection distance of the detector, an additional optical system is required to collect infrared radiation. Usually, a metal-coated plastic optical reflection system or a Fresnel lens made of plastic is used as a focusing system for infrared radiation. In the detection area, the infrared radiation energy transmitted by the human body through clothing is accepted by the detector lens and focused on the pyroelectric sensor. When the human body (intruder) moves in this monitoring range, it enters a certain field of view sequentially, and then walks out of this field of view. The passive PIR motion sensor will see the moving human body for a while, but not for a moment. So the infrared radiation of the human body constantly changes the temperature of the pyroelectric body, causing it to output one corresponding signal one by one, which is an alarm signal.

(1) Main features of the passive PIR motion sensor

• Since it is passive and does not actively emit infrared rays, it consumes very little power and is easy to install.
• Compared with microwave alarms, infrared wavelengths cannot pass through ordinary buildings such as bricks and cement. When used indoors, there is no need to worry about false alarms caused by outdoor moving targets. When multiple passive PIR motion sensors are installed indoors with a large area, because they are passive, there will be no problem of mutual interference of the system.
• Works independently of sound, ie sound does not cause it to generate false alarms.

(2) Installation of the passive PIR motion sensor

• Depending on the spot sensor model, it can be mounted directly on the wall, on the ceiling or in a corner.
• Pay attention to the detection range and horizontal viewing angle of the sensor.
• Do not aim the sensor at the heater and air outlet duct of the air conditioner.
• Do not aim the sensor at strong light sources and doors and windows exposed to direct sunlight.
• Be careful not to have tall shelters or interference from electric fan blades in the warning area, and do not install it in places with strong electricity.