Applications and Circuits of Photoresistors
Photoresistor is one of the photoelectric sensor elements, which is a kind of element made by the photoconductivity effect of semiconductor. The resistance value changes with the intensity of the incident light. Generally speaking, the incident light increases, the conductivity increases, and the resistance value decreases. Generally, the photoresistor body is made into a thin film structure to absorb more light energy. The material used for the photosensitive layer of the photoresistor can be polycrystalline material or single crystal material, and single crystal material can be made into intrinsic type and Doped type.
What are the main application scenarios of photoresistors?
Visible light photoresistors are widely used in automatic control such as the automatic extinguishing of dirty beacon lights, railway signal lights, street lights and other lighting systems, which are mainly used in switching circuits. It is required that the resistance of the photoresistor changes with the light, the better, that is, the resistance change multiple should be higher. It is also used as photoelectric counter, mechanical "position detector" and automatic protection device, automatic water supply of faucet and automatic water stop device after people leave, photoelectric potentiometer, automatic exposure device of camera, etc.
The application field of infrared technology is very extensive. It has now become a technology that the world military attaches great importance to. Because infrared technology can work at night and can detect targets through smoke, it has received great attention and is widely used in night combat, reconnaissance and camouflage. For example, the current infrared forward-looking device used for reconnaissance can detect people, vehicles, cooking fires in the jungle, and other camouflaged targets on the ground from an altitude of about 1,500 meters. Most of this pretense is array elements. Reconnaissance satellites with infrared photography carry out reconnaissance from space. Even when taking pictures in the monitored area, no one has no vehicles, but the heat radiation produced by people or vehicles a few hours ago can leave traces on the film. The infrared camera can determine the position of tanks and military vehicles, which can be identified from an altitude of 230 kilometers
A target that can't be found by visible light. Infrared radar and infrared communication are also widely used.
Infrared technology can be used in industry to detect thermal joints of electrical equipment, heat setting and temperature control of cotton and polyester, furnace temperature measurement, infrared baking, detection of train hot shafts, and so on. It can also be used for infrared gas analysis, which uses the characteristic absorption of various gases in different infrared bands for component determination. Compared with general chemical analysis methods, it is sensitive, accurate, fast and can be continuously measured for automatic control. It is also widely used in metallurgy, chemical industry, fertilizer, environmental protection, etc.
Because temperature affects the physical, chemical and physiological processes of crops and soil, and affects the growth cycle and yield of crops, in agriculture, infrared technology is used mainly through the thermal infrared radiation of the crop itself and the ability of the crop to absorb or reflect infrared radiation. Measure the temperature of crop leaves to observe crop growth, detect pests and diseases, and accurately grasp drought conditions.
In forestry, infrared technology can be used to draw forest distribution maps and determine forest quality, fire sources, pests and diseases, etc. If the forest fire infrared detector is placed on the commanding height of the forest area, the surrounding forest area can be monitored instead of the construction observation, and forest fires that cannot be detected by the artificial observation can be found.
Infrared technology is mainly used to detect and monitor water and air pollution in terms of environmental protection.
In medicine, according to the infrared radiation emitted by the skin surface of the patient, an image of the infrared radiation distribution of the patient is formed by an infrared detector and an appropriate display system to reflect the changes in the skin temperature of the patient's surface. When comparing the radiation images, information about the pathological state can be obtained. It can diagnose diseases such as early cancer, arthritis, arterial disease and so on.
In addition, thermal infrared detectors are also used to measure the surface temperature of oceans, rivers, and lakes, survey water sources, and survey geothermal in dry areas.
Photoresistor application circuits
1. Photoresistor dimming circuit
The picture shows a typical light-controlled dimming circuit. Its working principle is: when the surrounding light becomes weak, the resistance value of the photoresistor RG increases, so that the partial voltage added to the capacitor C rises, and the thyristor conducts. The increase of the pass angle achieves the purpose of increasing the voltage at both ends of the illuminating lamp. On the contrary, if the surrounding light becomes brighter, the resistance value of RG decreases, which causes the conduction angle of the thyristor to decrease, and the voltage at both ends of the illuminating lamp also decreases at the same time, which makes the light dim, thereby realizing the control of the light illuminance.
Note: The rectifier bridge in the above circuit must give a DC pulsating voltage, and it cannot be filtered by a capacitor to become a smooth DC voltage, otherwise the circuit will not work normally. The reason is that the DC pulsating voltage can not only provide the basic conditions for the zero-crossing shutdown of the thyristor, but also enable the charging of the capacitor C to start from zero in each half cycle, and accurately complete the synchronous phase shift triggering of the thyristor.
2. Photoresistive light control switch
The light-controlled switch circuit with relay control output with photoresistor as the core component has many forms, such as self-locking bright excitation, dark excitation and precise bright excitation, dark excitation, etc. Several typical circuits are given below.
The figure is a sophisticated dark excitation delay relay switch circuit. Its working principle is: when the illuminance drops to the set value, the inverting terminal potential of the op amp IC rises due to the increase in the resistance of the photoresistor, and its output stimulates VT to be turned on, and the excitation current of VT makes the relay work, and the normally open contact is closed. The normally closed contact is disconnected to realize the control of the external circuit.