Something you should know about PIR

October 09,2021

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PIR is the abbreviation of passive infrared, which is passive infrared technology. The full name of PIR is passive infrared detection (sometimes called passive infrared sensor).


The detection principle of PIR


PIR can detect infrared rays emitted by people or certain animals and convert them into electrical signals for output. When some crystals are heated, an equal number of charges with opposite signs will be generated at both ends of the crystal. This electric polarization caused by thermal changes is called the pyroelectric effect. Usually, the bound charge generated by the spontaneous polarization of the crystal is neutralized by the free electrons attached to the surface of the crystal from the air, and the spontaneous polarization moment cannot be expressed.


The pyroelectric sensor uses the pyroelectric effect and is a temperature sensitive sensor. It is composed of ceramic oxide or piezoelectric crystal elements. The two surfaces of the element are made of electrodes. When the temperature in the monitoring range of the sensor changes by ΔT, the pyroelectric effect will generate charges on the two electrodes ΔQ, that is, between the two electrodes a weak voltage ΔV is generated between the electrodes. Because of its extremely high output impedance, there is a field effect tube in the sensor for impedance conversion. The charge ΔQ generated by the pyroelectric effect will combine with the ions in the air and disappear. When the ambient temperature is stable, ΔT=0 and the sensor has no output. When the human body enters the detection area, due to the difference between the human body temperature and the ambient temperature, ΔT is generated, then there is a signal output; if the human body does not move after entering the detection area, the temperature does not change, and the sensor does not output, so this sensor can detect the human body or animal activities.


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The working principle and characteristics of PIR


In nature, any object higher than the absolute temperature (-273℃) will produce infrared spectrum. The wavelength of infrared energy released by objects at different temperatures is different, so the infrared wavelength is related to the temperature, and the radiant energy’s size is related to the surface temperature of the object.


The human body has a constant body temperature, which is generally around 37°C and emits infrared rays with a specific wavelength of about 10 um. The passive infrared probe works by detecting the infrared rays emitted by the human body. The infrared rays are enhanced and gathered into the pyroelectric element. This kind of element loses the charge balance when receiving the infrared radiation change of the human body, releases the charge outward, and then generates an alarm signal after being detected and processed. Passive infrared probe, the sensor consists of two pyroelectric elements connected in series or parallel, and the two electrical polarization directions are exactly opposite. The environmental background radiation has almost the same effect on the two pyroelectric elements, making it release The electrical effects cancel each other out, so the detector has no signal output.


The advantages and disadvantages of PIR


Different from the active infrared sensor, the PIR itself does not emit any type of radiation, has good concealment, low power consumption of the device, and low price. However, PIR also have disadvantages, such as:


  • ❌The signal amplitude is small, and it is easily interfered by various heat sources and light sources;


  • ❌Vulnerable to interference from radio frequency radiation;


  • ❌When the ambient temperature is close to that of the human body, the detection and sensitivity will decrease significantly, sometimes causing short-term failure.


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