The front-end detection part of the intrusion alarm system mainly consists of various types of detectors, among which the most important is the intrusion detector. Intrusion detectors typically consist of sensors, signal processors, and output interfaces. Intrusion detectors mainly include active infrared motion sensors, PIR motion sensors, microwave detectors, microwave and PIR motion sensors, ultrasonic detectors, vibration detectors, door contacts, ultrasonic and PIR motion sensors, etc. The most commonly used intrusion detector is the PIR motion sensor.

1 About Infrared

Whenever people mention the word “infrared“, perhaps one application related to infrared that comes to mind for many people is the infrared remote control for household televisions. Yes, that’s very correct. The remote control has an infrared light emitting component, while the controlled TV has an infrared light receiving device. When we press a button on the remote control, the electronic circuit in the remote control will emit the corresponding code of the button through the infrared emission tube at the top of the remote control. When the infrared detection components in the controlled TV receive these optical code signals, they convert them into electrical signals, and then process and recognize the signals through subsequent circuits to perform some operations. These actions fulfill our needs, such as channel switching for TV programs. This seemingly simple operation actually involves two conversions of electricity to light and light to electricity. PIR motion sensors are products invented using this feature.

2 Composition of PIR motion sensors

PIR motion sensors mainly consist of optical systems, pyroelectric infrared sensors, operational amplifiers, signal processors, alarm outputs, and other components. Its core component is a pyroelectric infrared sensor, which can detect changes in thermal radiation within a three-dimensional defense space through the coordination of optical systems.

3 Working principle of PIR motion sensors

The human body has a constant body temperature, usually around 37 degrees, and the wavelength of infrared radiation from the human body is 3-50 μm. Among them, 8-14 μm accounts for 46%, with a peak wavelength of 9.5 μm. PIR motion sensors mainly detect and receive infrared radiation from the outside. The sensor itself does not emit any energy and is only sensitive to the infrared band emitted by the human body. PIR motion sensor mainly receives infrared radiation at a wavelength of 8～14μm and performs its work. The infrared emitted by the human body is enhanced by a Fresnel filter and then concentrated on the infrared sensing source.

Infrared sensing sources usually use pyroelectric components. When there is human movement in the prevention area, the infrared emitted by the human body is focused on the pyroelectric infrared sensor through an optical lens. The pyroelectric infrared sensor senses the infrared signal and outputs a thermoelectric signal, which is very weak and mixed with many interference signals. To achieve this, a special thermoelectric signal processing circuit needs to be designed to amplify the thermoelectric signal while filtering out the interfering clutter signals.

The sensor adopts an operational amplifier, combined with peripheral circuits, integrated into an amplification circuit with bandpass filter function. It separates the thermoelectric signal and amplifies it thousands of times. And through parameter optimization design, combined with the comparator circuit, further eliminate the interference caused by flowing hot air, and analyze and judge the separated thermoelectric signal after two-stage amplification. Determine whether an alarm is triggered based on pre-set judgment criteria. If an alarm is detected, it output an alarm signal.

4 Types of PIR motion sensor optical systems

The PIR motion sensor optical system includes three types: Fresnel lens, parabolic reflector, and occlusion film.

The Fresnel lens has two functions: The first function is focusing, which refracts (reflects) the thermally released infrared signal onto the PIR. The second function is to divide the alert area into several bright and dark areas. A Fresnel lens is a convex lens that projects the infrared image of an object onto the surface of a pyroelectric element.

The optical system composed of parabolic reflectors has the advantages of low thermal energy absorption, low scattering loss, high efficiency, and high imaging accuracy, making it suitable for use in long-distance and large-scale infrared detectors.

The occlusion film does not have the effect of focusing imaging on the radiation source, and the effect distance of the viewing area is very close. It is usually used to form the downward viewing function of infrared sensors.

The pyroelectric infrared sensor’s pyroelectric components and preamplifier circuit are packaged inside a circular metal cap. The top square opening of the metal cap is embedded with a silicon infrared filter that is resistant to cold white light, and there are metal pins at the bottom, which are power pins, ground pins, and thermoelectric signal output pins.

Pyroelectric devices are the core components of pyroelectric sensors, which are dynamic energy conversion components that convert thermal radiation into current. The electrical characteristic property of a pyroelectric device is a flat capacitor with a thermoelectric crystal thin film as the dielectric. As the temperature changes, the spontaneous polarization charge on the surface of the thermoelectric crystal has a property of following the change in scale, that is, thermal radiation can cause a change in the capacitance of the capacitor, and this characteristic can be used to detect the changing thermal radiation. Pyroelectric infrared sensors include three types: signal, dual, and quaternary. Currently, dual and quaternary sensors are mainly used.

5 The characteristics of PIR motion sensors

(1) PIR motion sensors are aimed at detecting human radiation. Therefore, pyroelectric components must be very sensitive to infrared radiation with a wavelength of about 10µm.

(2) In order to be only sensitive to human infrared radiation, special Fresnel filters are usually covered on its radiation surface, which significantly controls environmental interference.

(3) PIR motion sensor, whose sensor consists of two pyroelectric elements connected in series or parallel with each other. Moreover, the two polarization directions produced are exactly opposite, and the environmental background radiation has almost the same effect on the two pyroelectric components, causing them to produce a pyroelectric effect that cancels out each other, resulting in no signal output from the detector.

(4) Once a person invades the detection area, the infrared radiation from the human body is partially focused through a mirror and received by a pyroelectric element. However, the two pyroelectric elements receive different amounts of heat, and the pyroelectric effect cannot be offset. After signal processing, an alarm is triggered.

(5) According to different performance requirements, Fresnel filters have different focal lengths (sensing distances), resulting in different monitoring fields of view. The more fields of view, the more intensive of the monitoring.

6 Advantages and disadvantages of PIR motion sensors

• Advantages: It does not emit any type of radiation and it is safe and reliable. The power consumption of the device is very low. Good concealment. Low price.
• Disadvantages: Easy to be interfered by various heat sources and sunlight sources. Passive infrared penetration is poor, and the infrared radiation of the human body is easily blocked and not easily received by the sensor. Susceptible to interference from radio frequency radiation. Due to environmental temperature limitations, when the ambient temperature is close to the human body temperature, the detection and sensitivity significantly decrease, sometimes causing short-term malfunctions.

Different measures can be taken to avoid false alarms and missed alarms in response to the shortcomings of PIR motion sensors. On the one hand, it is to adopt new technologies to enhance anti-interference capabilities; On the other hand, attention should be paid to installation.

7 Classification of PIR motion sensors

• Classified by wiring method (signal transmission method): wired, wireless
• Classified by installation method: wall mounted, ceiling mounted
• Classified by place of use: indoor, outdoor
• Classified by product technology: ordinary sensor, intelligent sensor (with automatic pulse number adjustment), dual sensor, composite sensor (infrared + microwave)
• Classified by detection range: wide-angle (spatial), curtain type, directional type

In practical use, many burglar alarm manufacturers combine several technologies to improve the quality of detectors, such as intelligent dual detection wall mounted curtain sensors, multiple wide-angle infrared sensors, and so on.