There are many different types of sensor lights on the market, and the following types of sensor lights are the most common: sound and light control sensor lights, human infrared sensor lights, radar sensor lights. Today we will learn about the key components of the human infrared sensor light – pyroelectric sensor.
Infrared human body sensor light can be in a silent situation, to achieve the light on action. It is mainly through a pyroelectric sensor plus Fennel lens and peripheral control circuits.
The pyroelectric sensor can sense the thermal radiation released by the human body temperature, and the Fennel lens can collect and focus the infrared radiation released by the human body.
When someone appears in the sensing range, the Fennel lens will be released by the human body heat infrared radiation focused on the pyroelectric sensor, and then by the pyroelectric sensor output a control model to the rear section of the circuit, to achieve the light on and off control.
But all objects with temperature will produce heat radiation, different temperature objects radiate different wavelengths, and the human body has a constant body temperature, so it will radiate a specific length of infrared radiation, and PIR human infrared sensor can accept sensing this wavelength, resulting in a change in the current, triggering the alarm.
Infrared pyroelectric body sensors work by detecting the infrared rays emitted by the human body.
The main principle is: the human body emits about 10μm of infrared light through the Fresnel lens enhancement gathered to the pyroelectric element PIR (passive infrared) detector.
When the human activity, the infrared radiation emission position will change, the element will lose charge balance, the pyroelectric effect occurs to the outward release of charge, infrared sensors will be through the Fresnel lens of the infrared radiation energy changes converted into electrical signals, that is, thermoelectric conversion.
In the passive infrared detector detection area without human movement, the infrared sensor senses only the background temperature, when the human body into the detection area, through the Fresnel lens, pyroelectric infrared sensor senses the human body temperature and the difference between the background temperature, the signal is captured and the system has been the existence of the detection of the data is compared to determine whether or not there are people and other infrared sources to enter the detection area.
Passive infrared sensors have a key component, the Fresnel lens.
Fresnel lens has two roles: one is the role of focusing, that is, the pyroelectric infrared signal refracted in the PIR; the second is the detection area is divided into a number of bright and dark zones, so that the detection area of the moving objects/people can enter the PIR in the form of temperature changes in the form of changes in the pyroelectric infrared signal.
Generally will also be matched with a low noise amplifier, when the ambient temperature on the detector rises, especially close to the normal body temperature of the human body (37 ℃), the sensor’s sensitivity decreases, through which the gain is compensated to increase its sensitivity.
Infrared body sensor two key use of environmental factors: temperature difference and movement, for stationary, or the target and the ambient temperature difference is not large, does not apply.
Human infrared sensors can only be installed indoors, and their sensitivity is highly dependent on their installation location. Correct installation of the human infrared sensor should meet the following conditions:
1、The installation location should be away from the heating, air conditioning, refrigerators, furnaces and other air temperature changes in sensitive places
2、Do not directly to the window, to prevent the window of the hot air disturbance and personnel walking will cause false alarms
3、Do not install the doorway, air ducts and other strong air flow activity of the place
4、The detection range shall not have a partition screen, furniture, large bonsai or other isolation.
Fresnel lens is a French physicist Augustin Fresnel (Augustin Fresnel). Fresnel (Augustin. Fresnel) invented, he initially used this lens design in 1822 for the establishment of a glass Fresnel lens system – lighthouse lens.
A Fresnel lens is a microstructured optical element that resembles a dartboard when viewed from the front, consisting of a ring of concentric gardens.
A Fresnel lens is simply a lens with equidistant teeth on one side of the lens, through which light bandpasses (reflections or refractions) of a specified spectral range can be achieved.
Traditional bandpass optical filters for polishing optical equipment are expensive. Fresnel lenses can greatly reduce the cost. A typical example is the PIR (Passive Infrared Detector).
PIRs are widely used in alarms. If you take a look at one, you will see that on each PIR there is a little plastic cap. This is the Fresnel lens. The inside of the little cap is engraved with teeth. This Fresnel lens limits the frequency peak of incident light to about 10 microns (the peak of human infrared radiation).
Pyroelectric infrared sensors
Pyroelectric infrared sensors are mainly made of a material with a high thermoelectric coefficient, such as lead zirconate titanate-based ceramics, lithium tantalate, and titanium triglycinate sulphate, etc., into a detector 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 due to their own temperature rise. By the detection element will detect and receive the infrared radiation into a weak voltage signal, installed in the probe by the field effect tube amplification to the external output.
In order to improve the detection sensitivity of the detector to increase the detection distance, generally installed in front of the detector a Fresnel lens, the use of Fresnel lens special optical principles, in front of the detector to produce an alternating change in the “blind zone” and “high sensitivity zone”, in order to improve its detection and reception sensitivity. To improve the sensitivity of its detection and reception.
When someone walks past in front of the lens, the infrared rays emitted by the human body will continuously alternate from the “blind zone” into the “high sensitivity zone”, so that the received infrared signals are input in the form of strong and weak pulses, thus strengthening its energy amplitude.
Fresnel lens and amplifier circuitry, can amplify the signal more than 70 dB, so that pyroelectric infrared sensors can be detected within the range of 10 ~ 40 metres of human action.
The central wavelength of infrared radiation from the human body is 9~10–um, and the wavelength sensitivity of the detection element is almost stable in the range of 0.2~20–um.
At the top of the sensor opened a window equipped with a filter lens, the filter can be passed through the light wavelength range of 7 ~ 10 – um, just suitable for the detection of human infrared radiation, and other wavelengths of infrared radiation by the filter to be absorbed, so that the formation of an infrared sensor specifically used to detect the radiation of the human body.