Semiconductors change their properties by adding a trace of trace elements. Phototransistors are an important derivative. Vision is the most important feeling of the human body. Therefore, I think it is too subtle to control the circuit through light, and the photosensitive diode transistor just happens to accomplish this task. Because the phototransistor also has amplification, it is more widely used than diodes.
Phototransistors are used to measure lightness and are often used in conjunction with light-emitting diodes as signal receiving devices. The following examples illustrate the various functions.
Measuring brightness
In the classroom library, many times the fluorescent lights are on during the day. In the dormitory, the fluorescent lamps are often staying up all night, and the students are already insensitive to this waste. Some students go to the classroom in the morning. Although the classroom is very bright, they still need to turn on the lights. Although a fluorescent lamp will not waste a lot of resources, but the accumulation is small, the waste is great. Therefore, we can install a control circuit in the classroom. When the brightness reaches a certain level, the fluorescent lamps in the classroom and in the dormitory will not start. We can use a phototransistor to attach an electromagnetic relay to complete this circuit. The choice of lighting points is a key, because not every classroom is the same brightness, we can use multi-point sampling to achieve this. For example, in 20 classrooms, phototransistors are placed. We can set them up. If all or most of them are very bright, the fluorescent lamps will not start normally, saving energy.
Another situation is that if one day the sky is covered with dark clouds and the brightness is not enough, the fluorescent light can be turned on. But soon the clouds will drift and the weather will be fine, and the fluorescent lamps will not automatically turn off. It also causes a lot of waste. A lighting spot can be installed outside the classroom where the lighting point is located. When the difference between indoor and outdoor strength is reduced to a certain range, we can think that the function of the fluorescent lamp can be neglected, and the fluorescent lamp will automatically turn off.
In another case, if the outside of the classroom is raining, the fluorescent light in the classroom is lit. At this time, a lightning bolt outside the window makes the outside bright and the fluorescent lamp is turned off, which causes trouble. So avoid this problem. The method is to install a counter in the circuit, so that the brightness difference can be maintained for a certain time before the fluorescent lamp is forcibly turned off.
In summary, we can use a phototransistor to design a circuit, so that the fluorescent lamp can not be started normally or forced to close to achieve energy saving. Of course, the feasibility of this method is not very high from now on, and the high cost of the circuit modification may affect the implementation. However, I think it is indeed possible to save money through the characteristics of the phototransistor.
Two photoelectric isolation
Another function of the phototransistor is to transmit signals, optocouplers (optICal)
Coupler, the abbreviation for OC) is also known as opto-isolator, referred to as optocoupler. The optical coupler transmits electrical signals in light as a medium. It has good isolation for input and output electrical signals, so it is widely used in various circuits. It has become one of the most versatile and versatile optoelectronic devices. An optical coupler generally consists of three parts: light emission, light reception, and signal amplification. The input electrical signal drives a light-emitting diode (LED) to emit a certain wavelength of light, which is received by the photodetector to generate a photocurrent, which is further amplified and output. This completes the electro-optical-electrical conversion, thereby functioning as input, output, and isolation. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, and thus has good electrical insulation capability and anti-interference ability. Moreover, since the input end of the optocoupler belongs to a low-resistance component of current type operation, it has strong common mode suppression capability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation component in long-line transmission information. As a signal isolation interface device in computer digital communication and real-time control, the reliability of computer work can be greatly increased.
The main advantages of the optocoupler are: one-way transmission of the signal, the input end and the output end fully realize electrical isolation and isolation, the output signal has no influence on the input end, strong anti-interference ability, stable operation, no contact, long service life, transmission efficient. Optocouplers were developed in the 1970s and are widely used in electrical insulation, level shifting, interstage coupling, drive circuits, switching circuits, choppers, multivibrators, signal isolation, interstage isolation, Pulse amplifier circuit, digital instrumentation, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrumentation, communication equipment and computer interface. In the monolithic switching power supply, the optocoupler feedback circuit can be constructed by using a linear optical coupler, and the duty ratio is changed by adjusting the current of the control terminal to achieve the purpose of precision voltage regulation.
Optocoupler working principle
The light-emitting device of the optical coupler for transmitting the analog signal is a diode, and the light receiver is a photo-transistor. When a current passes through the light emitting diode, a light source is formed, which is irradiated onto the surface of the phototransistor, so that the phototransistor generates a collector current, the magnitude of the current and the intensity of the illumination, that is, the forward current flowing through the diode. The size is proportional. Since the input and output ends of the optocoupler are transmitted by optical signals, the two parts are completely electrically isolated, and there is no feedback and interference of electrical signals, so the performance is stable and the anti-interference ability is strong. The coupling capacitance between the light-emitting tube and the photosensitive tube is small (about 2pf) and the withstand voltage is high (about 2.5KV), so the common mode rejection ratio is high. The electrical isolation between the input and output depends on the insulation resistance between the two parts of the power supply. In addition, because of its small input resistance (about 10 Ω), the noise to the high internal resistance source is equivalent to being shorted. Therefore, the analog signal isolation circuit composed of the optical coupler has excellent electrical performance.
Three non-contact measurement speed
The torque sensor is equipped with 60 tooth-toothed speed measuring wheels on the rotating shaft. A slot-type photoelectric switch frame consisting of a light-emitting diode and a photo-transistor is mounted on the sensor housing. Each tooth of the speed measuring wheel will light the LED. When blocked, the phototransistor outputs a high level. When the light is incident on the window of the photosensitive tube through the tooth gap, the photosensitive tube outputs a low level, and each rotation of the rotating shaft can obtain 60 pulses. Therefore, The number of pulses detected per second is exactly equal to the speed per minute.
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