Pyroelectric infrared sensor Preface Pyroelectric infrared sensor is a very potential applications of the sensor.It can detect people or animals, the infrared transmitter and converted into anelectrical signal output. As early as 1938, it was proposed detection using pyroelectric infrared radiation effect, but not taken seriously. Until the sixties, with the laser, infrared technology is developingrapidly, it has contributed tothepyroelectric effect and research on pyroelectric crystals application development. In recent years, along with the rapid development of integrated circuit technology, as well as the characteristics of the sensor depth study of the relevant application specific integrated circuit processing technology is also growing rapidly. This article first describes the principle of the pyroelectric sensor, and then describe the relevant ASIC processing technology.Pyroelectric effectIn nature, any more than the absolute temperature (-273K) objects will have infrared spectra, objects at different temperatures the wavelength of infrared energyreleased is not the same, so the level of infrared wavelengths is related to temperature, and radiation energy size and surface temperature.1µm wavelength of visible light is usually less, but more than 1µm light the human eye can not see, but can be an appropriate instrument to detect the energy oradiation.When some of the crystal is heated, the crystal will have an equal number of both ends of the opposite sign of charge, such as heat of changes in the polarization phenomenon, known as the pyroelectric effect. Typically, the crystals produced by the spontaneous polarization bound charge is attached to the air from the surface of free electrons in the crystal and in itsspontaneous polarization electric moment can not be demonstrated. When the temperature changes, positive and negative charges in the crystal structure of the relative center of gravity shifts, the spontaneous polarization changes, the crystal surface will have run out of charge, charge depletion is proportional to the polarization degree of the situation, Figure 1 shows thepyroelectric The principle effect of the formation.Pyroelectric effect can produce a crystal or call the pyroelectricpyroelectric body components, the material commonly used in single crystal (LiTaO3, etc.), piezoelectric ceramic (PZT, etc.) and polymer film (PVFZ, etc.) [ 2] When represented LiTaO3 pyroelectric material is self-polarized state, the absorption of infrared incident, the crystallization of the surface temperature change, since the polarization has changed, crystal surface charge becomes imbalanced, the imbalance taken out of charge voltage can be measured by infrared. Pyroelectric materials produced only when the temperature changes the voltage, if the infrared radiation has been, there is no imbalance in voltage, if no infrared radiation, the crystal surface charge on the in an unbalanced state, so the output voltage Pyroelectric infrared sensors, infrared light irradiation due to gain or lose heat and shelter, resulting in voltage output. In principle should be independent of the wavelength, but the material made by the pyroelectric sensor has a light transmission window, the translucent windows and wavelength selection of a relationship. To SiO2 as the window material, such as sensors, it can almost all visible light, while others near the window material can only 4µm wavelength of light, and some wavelengths of light through 6.1µm, and some through 8µm ~ 14µm wavelength of light, so use a different window materials which can be confirmed that the heat generated by the wavelength of light.Quantum-type infrared detectorQuantumponse, response sensitivity and infrared wavelength. The energy generated by each incident photon E = hc / λ = 1124λ Where, h - Planck constant, h = 4.14 × 10-15 (evs) = 6.625 × 10-34 (JS) c - the speed of light, c = 3 × 1010cm / s 1µm infrared light energy 1.24eV, 10µm infrared light energy of 0.12eV, compared with the visible light, infrared light energy smaller. Quantum-type infrared sensor light conductive type and is divided into two kinds of light force. Components of light conductive material PbS, PbSe, Hg, Cd, Te, etc., it is the use of infrared radiation to reduce the characteristics of impedance detection signal to obtain; and light force type is in Ge, IrSb formed on a substrate such as semiconductor PN Results, when the infrared radiation generated when the light force, Ge band gap for the 0.6ev, Ge diode of 0.6µm and 1.9µm more sensitive to infrared light, infrared li ght when the wavelength of the incident in the 0.6µm ~ 1.9µm, in the PN junction force formed with the increase of the amount of incident light, thereby detecting amplified output signal.Pyroelectric sensorPyroelectric sensor using the pyroelectric effect is, is atemperature-sensitive sensor. It consists of oxides or ceramic piezoelectric crystal components, the component made of two surface electrodes, when the temperature within the sensor monitoring the change ∆T, the pyroelectric effect in the two electrodes will produce the charge ∆Q, ie between two electrodes in a weak voltage ∆V. Pyroelectric infrared sensors and thermocouples are based on the principle of thermoelectric thermoelectric infrared sensors. Different pyroelectric infrared sensors is the thermoelectric coefficient is much higher than the thermocouple. Chart Pyroelectric infrared sensor structure and the internal circuit shown in Figure 2. Sensors are mainly case, interference filters, pyroelectriccomponent PZT, FET, FET and other components.FilterHuman central wavelength of infrared radiation 9 ~ 10 - um, the wavelength sensitivity of the detection component in 0.2 ~ 20 - um range is almost constant. The top of the sensor chip has opened a window with filter, this filter through the wavelength range of 7 ~ 10 - um, just right for the detection of infrared radiation in the human body, while the other infrared wavelengths by the filter film to be absorbed, thus forming a special body for detection of infrared radiation sensor. Thermoelectric components Thermoelectric materials will be a certain thickness of the sheet, and in its metal electrodes deposited on both sides, and then power on its polarization, this will make the pyroelectric detectors. As the voltage increases polarization is extremelynature and therefore, the detection element is polarized positive 、 negative in nature. Thermoelectric element within the Seebeck coefficient by the high iron titanate ceramic lead and mercury, and lithium tantalate、 Triglyceride iron sulfate, and its polarization changes with temperature. In order to suppress the temperature change due to their interference can be generated process will be the same two features reverse thermoelectric element connected in series or poor way of balancing the circuit, to suppress its own temperature as a result of the interference. Thus able to detect objects in non-contact infrared energy emitted changes and converts it to electrical signal output.FET impedance transformationPyroelectric infrared sensors in the structure aims to introduce complete FET impedance transformation. Thermoelectric element as is the charge output signal, resistance up to 104M , so the introduction of the N-channel JFET common drain pipe should be connected in the form of that the source followerto complete the impedance transformation. Because of its high output impedance, so the sensor has a field effect transistor for impedance transformation. Pyroelectric effect will be produced by the charge ∆Q with combination of ions in the air disappeared, when the ambient temperature stability constant, ∆T = 0, the sensor no output. When the body into the detection zone, because the body temperature and ambient temperature differential, resulting in ∆T, is a signal output; if the body does not move into the detection zone, the temperature does not change, the sensor has no output, so this sensor can detect body or animal activities.Reference SelectionThe commonly used models of pyroelectric infrared sensors are P228, LHl958, LHI954, RE200B, KDS209, PIS209, LHI878, PD632 and so on. Pyroelectric infrared sensor is usually a 3 pin metal package, the pins are supply-side (internalswitch D pole, DRAIN), signal output (internal switch S pole, SOURCE), ground (GROUND) . Pyroelectric infrared sensors are the main working parameters Voltage (commonly used pyroelectric infrared sensor operating voltage range 3 ~ 15V) Wavelength (typically 7.5 ~ 14 µ m) Source voltage (usually 0.4 ~ 1.1V, R = 47k ) Output signal voltage (typically greater than 2.0V)Note the use of infrared modules:1, the human body are highly sensitive sensor device module, its high power requirements, the regulator must be a good filter, such as cascading 9V battery may not work because of the larger resistance, advises clients to use the LM7808 regulator chip 0.1UF regulator and then through the 220UF capacitor filtered supply. 2, the module does not work properly when connected load, after load connected to the work of disorder, because the power capacity of a small load is power, work load due to voltage fluctuations lead to the module malfunction, the other reason is that the load may causeinterference with electrical work, such as relays or solenoids and other inductive load will produce back EMF, 315M will be launching board work will affect the electromagnetic radiation module. Solution is as follows: A, power supply filter inductor increases. B, by using different load and voltage of the module, for example: load with 24V operating voltage, 12V voltage module, during which three-terminal regulator with the LM7812 isolation. C: with a larger capacity power supply. 3, the body work environment sensor module should avoid sunlight, direct exposure to bright light, if the work environment has a strong radio frequency interference shielding measures can be used. In case of strong flow interference, close doors and windows or to prevent convection. Sensitive area to avoid facing the heating appliances and objects, and debris easily by the wind and clothing 4, the human sensor module must be assembled in a sealed box, otherwise the outputsignal has been there. Sensor (PIR) and the lens has a focusing distance, usually 20-30mm range of adjustment. 5, infrared detectors to detect if the required angle of less than 90 degrees, you can block the lens with opaque tape or cut narrow the lens to achieve. 6, the human body sensor module using a dual probe, the body's limbs and head movement direction and the sensor sensitivity are closely linked, if you set the improper installation, will affect the induced effects. 7, module probe (PIR) for assembly and welding the other side of the circuit board. Probe can also be extended with the dual-core shielded cable, 2 meters length should be as well.ConclusionPyroelectric infrared sensor has low price, technical performance and stability, development using simple One such feature, in addition to the above monitoring alarm and automatic switching of typical applications, in many of its It has wide application areas, such as automatic starting or stopping the air conditioner, water dispenser, TV, automatic Camera filmingthe activities of humans and animals, or digital camera, with the development of electronic technology, heat Pyroelectric infrared sensors will be more widely used in the field of automatic control.。
