What is a PIR Motion Sensor Alarm?

The PIR Motion Sensor operates using the pyroelectric effect and a Fresnel lens.  The PIR Motion Sensor Switch is mainly composed of a pyroelectric infrared sensor, which uses the principle of infrared induction to identify infrared rays of specific wavelengths emitted by humans.  Pyroelectric infrared sensors are mainly made of a material with a high pyroelectric coefficient.  One or two detection elements are installed in each detector, and the two detection elements are connected in series with reverse polarity to suppress the temperature rise due to the interference generated.  The detected and received infrared radiation is converted into a weak voltage signal by the detection element, which is amplified by the field effect tube installed in the probe and then output.

In addition to the "pyroelectric sensor", the key components also include the "Fresnel lens".  Fresnel lenses are installed in front of the detectors, using the special optical principle of the lens to create an alternating "blind zone" and "high sensitivity area" in front of the detector to improve the sensitivity of detection and reception.  When someone walks in front of the lens/the PIR motion sensor, the infrared rays emitted by the human body continuously alternate from the "blind area" to the "high sensitivity area", so that the received infrared signal is input in the form of intermittent pulses, thereby enhancing its energy amplitude.  In summary, the Fresnel lens helps focus the infrared radiation emitted by the human body onto the pyroelectric element of the infrared sensing source of the PIR sensor, which improves the efficiency of the PIR Motion Sensor System in detecting the presence of a human body.

How do an alarm PIR motion sensor work?  PIR motion detector sensor alarm utilizes the pyroelectric effect to detect infrared radiation emitted by the human body and convert it into electrical signals. This sensor contains a pyroelectric element.  When a person enters its detection area, the temperature difference between the human body and the surrounding environment causes the infrared radiation temperature of the pyroelectric element to change, thereby breaking the charge balance inside the element and releasing the charge.  These charge changes are detected by the circuit, which produces a signal output.  When a person moves and breaks the charge balance on the pyroelectric element, the circuit detects these changes and controls the switch to the "on" state accordingly.  If the person leaves the detection area and does not return within the set delay time, the sensor will no longer sense the presence of a human and the switch state will then switch to "off".

The detection wavelength range of PIR sensors is 5-14μm, which covers the central wavelength of human body radiation of 8-12μm, with 9.36μm as the peak wavelength.  To improve the absorption efficiency of these wavelengths, the surface of the detection element is usually coated with a gold-black absorption layer to ensure that the absorption rate in the range of 2-15μm exceeds 90%.  The filter design of the sensor allows infrared light with wavelengths of 5.5μm to 14μm to pass, ensuring that the sensor only responds to infrared radiation from humans or animals.  Importantly, PIR sensors detect temperature changes, not infrared radiation of fixed wavelengths.  This means that if objects remain stationary or move too fast within the detection range, the PIR sensor will not be able to detect them.

Alarm PIR motion sensor has significant advantages, including no radiation, low power consumption, good concealment and affordable price.  However, they also have some disadvantages, such as susceptibility to interference and weak infrared penetration ability.

Advent Model I2 Pir Motion Sensor Alarm