How does the human sensor light detect the presence of people?
The infrared human body induction lamp can turn on the light silently. It is mainly composed of a pyroelectric sensor, a Fresnel lens and a peripheral control circuit.
The pyroelectric sensor can sense the thermal radiation released by human body temperature, and the Fresnel lens can collect and focus the infrared rays released by the human body.
When someone appears in the sensing range, the Fresnel lens focuses the infrared radiation released by the human body heat onto the pyroelectric sensor, and then the pyroelectric sensor outputs a control model to the back-end circuit to realize the light on and off control.
Working principle
Any object with temperature will generate thermal radiation to the outside, and the wavelengths radiated by objects with different temperatures are also different. The human body has a constant body temperature, so it will radiate a specific length of infrared rays, and the PIR human infrared sensor can receive and sense this wavelength, resulting in current changes and triggering an alarm.
The infrared pyroelectric human body induction sensor works by detecting infrared rays emitted by the human body.
The main principle is: the infrared rays of about 10μm emitted by the human body are enhanced by the Fresnel lens and gathered on the pyroelectric element PIR (passive infrared) detector. When a person moves, the emission position of the infrared radiation will change, the element will lose the charge balance, and the pyroelectric effect will occur to release the charge outward. The infrared sensor converts the change of infrared radiation energy through the Fresnel lens into an electrical signal, that is, thermoelectric conversion.
When there is no human movement in the detection area of the passive infrared detector, the infrared sensor only senses the background temperature. When the human body enters the detection area, the pyroelectric infrared sensor senses the difference between the human body temperature and the background temperature through the Fresnel lens. After the signal is collected, it is compared with the existing detection data in the system to determine whether there is really someone or other infrared source entering the detection area.
Passive infrared sensors have a key component-Fresnel lens.
Fresnel lens has two functions: one is focusing, that is, refracting the pyroelectric infrared signal on the PIR; the other is dividing the detection area into several bright and dark areas, so that the moving objects/people entering the detection area can generate changing pyroelectric infrared signals on the PIR in the form of temperature changes.
Generally, a low-noise amplifier is also matched. When the ambient temperature on the detector rises, especially when it is close to the normal body temperature of the human body (37°C), the sensitivity of the sensor decreases, and the gain is compensated through it to increase its sensitivity.
Two key environmental factors for the use of infrared human body sensors: temperature difference and movement. For static or small temperature difference between the target and the environment, it is not applicable.
Installation requirements
Human body infrared sensors can only be installed indoors, and their sensitivity is closely related to the installation location. The following conditions should be met for the correct installation of human infrared sensors:
The installation location should be away from places where air temperature changes are sensitive, such as heaters, air conditioners, refrigerators, and stoves
Do not face the window directly to prevent false alarms caused by hot air disturbances and people walking outside the window
Do not install in places with strong airflow, such as doorways and air ducts
There should be no screens, furniture, large bonsai or other partitions within the detection range.
Fresnel lens
The Fresnel lens was invented by French physicist Augustin Fresnel, who originally used this lens design in 1822 to build a glass Fresnel lens system – a lighthouse lens. The Fresnel lens is a microstructured optical element that looks like a dartboard from the front and consists of concentric circles.
A Fresnel lens, in simple terms, has equidistant teeth on one side of the lens, through which the light bandpass (reflection or refraction) of a specified spectral range can be achieved.
Traditional bandpass optical filters for polished optical equipment are expensive. Fresnel lenses can greatly reduce costs. A typical example is PIR (Passive Infrared Detector).
PIR is widely used in alarms. If you take a look, you will find that there is a small plastic cap on each PIR. This is a Fresnel lens. The inside of the small cap is engraved with teeth. This Fresnel lens can limit the frequency peak of the incident light to about 10 microns (the peak value of infrared radiation from the human body).
Pyroelectric infrared sensor
Pyroelectric infrared sensor is mainly made of a material with a high thermoelectric coefficient, such as lead zirconate titanate ceramics, lithium tantalate, triglycan titanium sulfate, etc., to make a detection element with a size of 2*1mm.
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 interference caused by the increase in their own temperature. The detection element converts the detected and received infrared radiation into a weak voltage signal, which is amplified by the field effect tube installed in the probe and output to the outside.
In order to improve the detection sensitivity of the detector and increase the detection distance, a Fresnel lens is generally installed in front of the detector. Using the special optical principle of the Fresnel lens, an alternating “blind area” and “high sensitivity area” are generated in front of the detector to improve its detection reception sensitivity.
When someone walks in front of the lens, 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 pulses that are sometimes strong and sometimes weak, thereby increasing its energy amplitude.
The Fresnel lens and the amplifier circuit can amplify the signal by more than 70 decibels, so that the pyroelectric infrared sensor can detect the actions of people within a range of 10 to 40 meters.
The central wavelength of infrared rays radiated by the human body is 9 to 10μm, and the wavelength sensitivity of the detection element is almost stable in the range of 0.2 to 20μm.
A window with a filter lens is opened at the top of the sensor. This filter can pass light with a wavelength range of 7~10μm, which is just suitable for detecting infrared radiation from the human body. Infrared rays of other wavelengths are absorbed by the filter, thus forming an infrared sensor specifically used for detecting human body radiation.
