Active infrared sensor
The transmitter of the active infrared sensor emits a modulated infrared beam, which is received by the infrared receiver, thus forming a cordon composed of infrared beams. When encountering leaves, rain, small animals, snow, dust, fog, no alarm, people or considerable volume of objects will be alarmed.
Active infrared detector technology mainly adopts one send and one receive, which belongs to linear prevention, and has now developed from the initial single beam to multiple beams, and can also be double-sent and double-received, which minimizes the false alarm rate, thereby enhancing the stability and reliability of the product.
Because infrared is a detection medium with good incoherence of environmental factors (good incoherence for sound, lightning, vibration, various artificial light sources and electromagnetic interference sources in the environment); At the same time, it is also a product with good coherence of target factors (only the target that blocks the infrared beam will trigger an alarm), so the active infrared sensor will be further promoted and applied.
Passive infrared sensor
Passive infrared sensors work by detecting infrared rays emitted by the human body. The sensor collects infrared radiation from the outside world and then concentrates it on the infrared sensor. Infrared sensors usually use pyroelectric components, which will release an electric charge when the temperature changes after receiving infrared radiation, and generate an alarm after detection and processing.
The sensor is designed to detect radiation from the human body. Therefore, the radiation sensitive element must be very sensitive to infrared radiation with a wavelength of about 10 μm. In order to be sensitive to the infrared radiation of the human body, its radiation surface is usually covered with a special filter, so that the interference of the environment is obviously controlled.
A PIR sensor consists of two pyroelectric elements that are connected in series or parallel to each other. Moreover, the two polarization directions are opposite, and the ambient background radiation has almost the same effect on the two pyroelectric elements, so that the release effect of the two pyroelectric elements cancels each other, so the detector has no signal output.
Once the intruder enters the detection area, the infrared radiation of the human body is focused through part of the mirror, so as to be received by pyroelectric electrons, but the heat received by the two pyroelectric electrons is different, and the pyroelectric is also different, which cannot be canceled out, and the alarm is generated by signal processing. Passive infrared sensors are widely used in human infrared detectors.
According to the different energy conversion methods, infrared sensors can be divided into photonic and pyroelectric types.
Photonic infrared sensors
Photonic infrared sensors are sensors that work using the photon effect of infrared radiation. The so-called photon effect refers to the fact that when infrared rays are incident on some semiconductor materials, the flow of photons in infrared radiation interacts with the electrons in the semiconductor materials, changing the energy state of the electrons, thereby causing various electrical phenomena.
By measuring the changes in the properties of electrons in semiconductor materials, it is possible to know the intensity of the corresponding infrared radiation. The main types of photon detectors include internal photodetectors, external photodetectors, free carrier detectors, QWIP quantum well detectors, etc.
The main characteristics of the photon detector are high sensitivity, fast response speed, and high response frequency, but the disadvantage is that the detection band is narrow and generally works at low temperature (in order to maintain high sensitivity, liquid nitrogen or thermoelectric refrigeration is often used to cool the photon detector to a lower working temperature).
Pyroelectric infrared sensor
The pyroelectric infrared sensor uses the thermal effect of infrared radiation to cause the temperature change of the element itself to detect certain parameters, and its detection rate and response speed are not as good as that of photonic sensor.
However, since it can be used at room temperature and its sensitivity is wavelength-independent, it has a wide range of applications. Pyroelectric infrared sensors using the ferroelectric pyroelectric effect are highly sensitive and have been widely used.
Pyroelectric effectWhen some insulating materials are heated, as the temperature rises, an equal number of charges will be generated at both ends of the crystal with opposite signs. This phenomenon of electropolarization due to thermal change is known as the pyroelectric effect. The pyroelectric effect has been used in pyroelectric infrared sensors for nearly a decade. Crystals that produce pyroelectric effects are called pyroelectrics, also known as thermoelectric elements. Commonly used materials for thermoelectric components include single crystal, piezoelectric ceramics and polymer films.
Structure of Pyroelectric Infrared SensorThe pyroelectric infrared sensor consists of the following four main parts:
(1) Aluminum substrates and field-effect transistors (FETs) that make up the circuit;
(2) ceramic materials with pyroelectric effect;
(3) window material that limits the wavelength of incident infrared;
(4) Shell TO-5 type cap and header.
Because when the detector element is used alone, there is a shortness of the detection distance, and the follow-up circuit of the obtained signal is not easy to process, so the infrared assembly is mostly used for detection at present. The infrared assembly consists of a pyroelectric infrared sensor, a lens, a measurement conversion circuit, and a sealed tube shell. Lenses can expand the detection range and improve the sensitivity of the measurement; The measurement conversion circuit can complete the signal processing process such as filtering and amplification; The sealed tube shell prevents malfunction caused by external noise. This kind of assembly is small in size, low in cost, and versatile, so it is widely used.
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