Various Types Of Photoelectric Switches Are Skillfully Used: Tradition

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Clever use of photoelectric switchesSilicon photodiodes and silicon phototransistors are semiconductor devices that convert optical signals into electrical signals. Photoelectric switches composed of them are not only small in size, long in life and strong in anti-interference ability, but also have the advantages of non-contact input and output, so they are widely used in industrial and agricultural production, national defense and military, astronomy, geography and daily life.In practical applications, photoelectric switches include trigger type, delay type, light control type, dark control type, etc. It can be said that they are versatile and highly applicable, and are an indispensable and important device for realizing automatic control. Now we introduce a variety of practical circuits using photoelectric switches for readers' reference.1． How does a phototransistor work?In Figure 38-1, (a) is a light-controlled photoelectric switch, and (b) is a dark-controlled photoelectric switch.Figure 38-1In figure (a), the resistor R1 and the phototransistor BG1 form the bias circuit of the BG2 tube, and the relay J is the switching circuit load.When light of a certain wavelength irradiates the base of BG1, a photocurrent is generated, causing the base of the BG2 tube to be at a low potential and cut off, the collector of the BG2 tube to be at a high potential, the BG3 tube to conduct, and the relay J to be electrically activated, thereby realizing the light control of the photoelectric switch.In Figure (b), when there is no light, due to the large dark resistance of the phototransistor BG1, the base of the BG2 tube is turned on at a high potential, and the relay J is electrically activated, thus realizing the dark control of the photoelectric switch. Based on the above working principle, many practical photoelectric switch circuits can be developed, and various practical circuits are listed below.2. Automatic night flash indicator lightAt night, many places in the city (such as road construction sites) or units working in the field need to install indicator lights at the construction site to ensure the safety of pedestrians and vehicles at night. The self-controlled night flash indicator light has the function of automatically turning off during the day and automatically turning on the flash and indicator at night. The circuit is simple, easy to install and reliable in operation.The circuit of the self-controlled night flash indicator light is shown in Figure 38-2.Figure 38-2In the figure, BG1~BG3 form a light-controlled photoelectric switch circuit, and BG4 and BG5 form a self-excited multivibrator. Changing the capacity of capacitor C1 or C2 can change the frequency of the flash indicator light. BG6 and BG7 form a composite tube amplifier to drive a larger load with a smaller current.During the day, the BG3 tube is turned on, and the control electrode of the one-way thyristor SCR is turned off because there is no trigger voltage. The power supply of the self-excited multivibrator circuit is blocked, and the light bulb ZD does not flash. At night, the BG3 tube is turned off, and the thyristor SCR is turned on, allowing the multivibrator to work, and the indicator light shines flashingly, which is very eye-catching.The printed circuit of the automatic night flash indicator light is shown in Figure 38-3.Figure 38-33． Flash indicator light for use with the power gridAt present, in order to rectify and beautify the city's appearance, construction projects such as laying underground cables and underground drainage ditches across highways are being carried out. This requires trenching and digging on the road. If construction indicators are not installed, it will bring great danger to vehicle driving. Therefore, safety lights are installed at construction sites. Figure 38-4 is a nighttime photoelectric switch signal light circuit that is stepped down by a capacitor from the 220V mains power supply.Figure 38-4After the 220V mains power is stepped down by capacitor C1, bridge rectified by D1~D4 and stabilized by DW, it provides a stable 9V DC power supply to the photoelectric switch circuit. During the day, the photoelectric switch is blocked under the action of strong light. The oscillator composed of the time base circuit NE555 cannot work because the circuit has no power supply voltage. The triac SCR1 has no trigger current and is in the off state, and the red signal light ZD does not light up.When it comes to night, BG1 has no light, so the photoelectric switch is in the on state, and the oscillator composed of NE555 and others works. The ③ pin continuously outputs ultra-low-frequency rectangular pulses, triggering the bidirectional thyristor SCR1 to continuously turn on and off, making the signal light flashing.The printed circuit of the circuit is shown in Figure 38-5.Figure 38-54. Light control street lightFigure 38-6 shows the light-controlled street light circuit. The street lights are turned off during the day and automatically lit at night.Figure 38-6In the figure, capacitors C1 and C2, diode D1 and voltage regulator DW form a simple capacitor step-down rectifier circuit. After the power is turned on, a DC voltage of about 12V is obtained. Transistor BG, photodiode D2 and triac, etc., form a photoelectric switch circuit.During the day, the photosensitive diode D2 has low resistance due to exposure to light, and the base potential of the BG tube decreases, causing the BG to cut off. There is no voltage output on the resistor R1, so the triac SCR is in the off state and the bulb ZD does not light up. When night falls, the dark resistance of D2 increases due to the lack of light, causing the base potential of BG to increase and turn on. The voltage output on R1 triggers the thyristor SCR to turn on, and the street lamp is powered to light up. Adjusting the potentiometer W can adjust the BG base potential, that is, adjusting the sensitivity of the light control.During installation, the photodiode D2 should be equipped with a rainproof transparent glass cover and placed in a place where it can be exposed to sunlight all day long. D2 can also be used as a photoresistor or phototransistor.The printed circuit of the light-controlled street light is shown in Figure 38-7.Figure 38-75. Photoelectric automatic water-saving switchThe circuit of the photoelectric automatic water-saving switch is shown in Figure 38-8.Figure 38-8When illuminating, the current generated by BG1 is supplied to the base of BG2, causing BG2 to be turned on and BG3 to be turned off. The triac SCR is turned off because there is no trigger voltage. The solenoid valve that controls the water flow does not operate and no water flows out. When someone goes to wash hands to block the light, the dark resistance of BG1 increases, causing BG2 to cut off, the collector potential of BG2 increases, BG3 turns on, and the voltage output on resistor R2 triggers the thyristor SCR to turn on, and the solenoid valve coil is energized, causing the solenoid valve to open, and water automatically flows out of the water pipe. When the hand leaves the pool, the light shines directly on BG1. The thyristor has no trigger voltage output due to the cutoff of BG3, and turns off again, causing the solenoid valve to close and the water flow to stop.The installation diagram of the photoelectric automatic water-saving switch is shown in Figure 38-9.Figure 38-9The printed circuit of the photoelectric automatic water-saving switch is shown in Figure 38-10.Figure 38-106. Automatic lighting doorbellAfter dark, when there are no lights in the corridor, the doorbell button is not easy to find, especially for first-time visitors, who have to knock on the door. If a light-emitting diode is installed on the doorbell button switch, the light-emitting diode can automatically flash after dark and automatically go out after dawn, which provides convenience for visiting guests.Figure 38-11 is the circuit of the automatic lighting doorbell.Figure 38-11After turning on the power switch K1, the resistance of the phototransistor BG1 decreases due to exposure to light during the day, and the base potential of the BG2 tube decreases, causing both BG2 and BG3 to cut off, the circuit stops oscillating, and the light-emitting diode does not light up.At night, because the phototransistor BG1 is not exposed to light, the dark resistance increases, and the base potential of BG2 rises, causing the direct coupling amplifier composed of BG2 and BG3 to oscillate due to the positive feedback effect of the capacitor C1, and the light-emitting diode LED shines. At this time, press the button switch K2, the normally open contact of the relay J is closed, and the power of the musical doorbell circuit is turned on, which emits the "announcement" doorbell sound.Adjusting W1 can not only change the volume of the "ding-dong" doorbell, but also change the rhythm of the "ding-dong" doorbell; adjusting W2 can change the tone of the "ding-dong" doorbell.The printed circuit of the automatic lighting doorbell is shown in Figure 38-12.Figure 38-127． Photoelectric alarm switchThere are two ways of light-controlled alarm switch. One is that the light-controlled switch is in the alarm state and is always exposed to light. When the light is blocked, it will alarm immediately; the other is that the light-controlled circuit is not exposed to light during the alarm period, but once exposed to light, it will alarm immediately.The circuit of the photoelectric alarm switch is shown in Figure 38-13.Figure 38-13Normally, due to external light being blocked, the phototransistor is in a high-resistance state, BG2 and BG3 are conductive, and BG4 is turned off, and the relay J is in the released state. The alarm sound circuit at the rear cannot sound the alarm because there is no working power supply.When light shines on the photosensitive transistor BG1 for some reason, BG2 and BG3 are both cut off, and BG4 is turned on, the relay J is energized, and its normally open contact J1 is closed, and the power of the alarm circuit is turned on, and the alarm sounds immediately.The printed circuit of the photoelectric alarm switch is shown in Figure 38-14.Figure 38-148. Low power street light switchThis circuit is characterized by low power consumption. Its operating current is no more than 1mA during the day or at night. If 6 AA batteries are used, it can be used for one year.The low-power street light electric switch circuit is shown in Figure 38-15. It consists of four parts: photoelectric conversion part, NAND gate, differential circuit and control execution circuit.Figure 38-15When there is light, the phototransistor BG1 is turned on, the potential at point a reaches the door-opening level of NAND gate YF1, and the output point b is at low potential, then YF2 outputs a high level, through the differential circuit composed of capacitor C1 and resistor R5 , a positive spike pulse is output at point C, causing BG2 to conduct, relay J1 is powered on, normally closed contact J1-1 is disconnected, AC contactor CJ is powered off and released, its self-protecting contact CJ1-1 is disconnected, and the street light goes out.When there is no light at night, point a is at a low level and point b is at a high level. Through another differential circuit composed of NAND gates YF3 and YF4, capacitor C2, and resistor R6, a positive sharp pulse is generated at point d, causing the transistor BG3 to conduct, and the relay J2 is instantly attracted (and then released). Normally open contact J2-1 is closed, AC contactor CJ is electrically activated, CJ1-1 resumes self-protection, and the street light power is turned on and lit. Since both relays J1 and J2 adopt the method of instantaneous closing and releasing, the power consumption of the circuit is extremely small.The NAND gate is a CMOS integrated circuit CO36, with 2 inputs and 4 NAND gates.Low-power street light electric switch printed circuit, as shown in Figure 38-16.Figure 38-169. Train light control switchIn our country's trains, the lights in each carriage are manually controlled by the crew to turn on and off. When there are many sections of tunnels, the crew needs to turn on the switch one by one, which is very inconvenient. If a photoelectric switch is used, it will light up automatically when there is a tunnel or enter the night, and go out automatically when exiting the tunnel or entering the daytime.The train light control switch circuit is shown in Figure 38-17.Figure 38-17The circuit is very simple. Each carriage is equipped with a photoelectric control switch as shown in the figure to achieve the purpose of automatic control of train lights.The printed circuit of the train light control switch is shown in Figure 38-18.Figure 38-1810． Photoelectric metering switchOften there is insufficient light in some workplaces (such as workshops, operating sites, etc.), but it is impossible to measure. With a photosensitive element, a good photometer can be formed by using a piece of CMOS and a small number of components.The circuit of the photoelectric photometric switch is shown in Figure 38-19.Figure 38-19In the figure, YF1-YF4 are CO36 type two-input NAND gate integrated circuits. Among them, YF1 and YF2, resistors R1, R3, capacitor C1, etc. form a beat pulse generator with a control terminal; YF3, YF4, resistors R6, R, capacitor C2 and piezoelectric buzzer HTD, etc., form an audio pulse oscillator with a control terminal.After the power switch K is turned on, the beat pulse generator and the audio pulse oscillator start to work. In the beat generator part, the pulse polarities output by YF1 and YF2 are opposite. With the output of the oscillation pulse, the light-emitting diodes LED1 and LED2 light up alternately. The flash period is determined by the values of R3 and C1. In the audio oscillation part, the oscillation voltage is taken out from the output end of YF4, causing the HTD to emit a single-frequency sound, and its audio pulse frequency is determined by the values of R7 and C2.YF1 and YF3 each have an input terminal used as a control terminal. The high or low input level of the control terminal will determine whether the two pulse generators oscillate. The level of the input level, in addition to adjusting the resistance of W, mainly determines the change of the phototransistor BG. In this way, the purpose of photometry can be achieved by dividing the potentiometer W into scales.In order to facilitate readers to choose photodiodes and phototransistors, the following is a brief introduction to photodiodes and phototransistors.1． Two working states of photodiodesPhotodiode, also known as photodiode, is a photoelectric conversion device. Its basic principle is that when light shines on the P-N junction, it absorbs light energy and converts it into electrical energy. It has two working states:(1) When a reverse voltage is applied to the photodiode, the reverse current in the tube changes with the change of light intensity. The greater the light intensity, the greater the reverse current. Most of them work in this state.(2) No voltage is applied to the photodiode, and the P-N junction generates a forward voltage when exposed to light, and is used as a micro photovoltaic cell. In this working state, it is generally used as a photoelectric detector.Photodiodes are divided into P-N junction type, PIN junction type, avalanche type and Schottky junction type. Among them, the P-N junction type is the most used and is cheap. …

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