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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Optoelectronic devices. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting Figure 1 shows a diagram of a device that can be used to automatically turn on and off the light depending on the illumination of the surrounding space.
In the proposed switch, a symmetrical thyristor VS1 is used as a non-contact switching element. As a result, it is possible to power the load during both half-cycles of the mains voltage without the use of powerful full-wave rectifiers. The electronic switch is a photo relay based on a Schmitt trigger (VT1, VT2). As a photosensitive element (light sensor), a photoresistor R1 of the FSK-1 type was used, which has a high resistance in the dark (about 3,3 MΩ), and when illuminated, its resistance decreases by 400 times. Photoresistor R1 together with resistors RP2 and R3 form a voltage divider that determines the base current of transistor VT1. Resistor R3 limits the current in the divider if R1 is exposed to direct sunlight. Resistor R7 determines the current in the control electrode of the triac VS1, and R6 serves to equalize the voltage at the control electrode and at the cathode VS1 when the transistor VT2 is closed. This ensures stable operation of the triac. The device works as follows. During the day, when it is light, the resistance of the photoresistor R1 is low, VT1 is open, and VT2 is closed. The collector current VT2 and, consequently, the current of the triac control electrode is almost zero. In this state, VS1 is closed and the HL1 light is off. With decreasing illumination, the resistance of the photoresistor increases. The base current VT1 begins to decrease. When a certain level is reached, VT1 closes, and VT2 opens, i.e. the trigger switches. The current of the control electrode VS1, flowing through the open transistor VT2 and resistors RP5 and R7, keeps VS1 open during both half-cycles of the mains voltage, and the HL1 light bulb glows at full power. Switching off is carried out in the reverse order. The response threshold is adjusted in the evening using trimmers RP2 and RP5. Photoresistor R1 must be installed so that during the day it is not exposed to direct sunlight, and at night - artificial lighting. Using such a device, you can control a load with a power of up to 400 W without the use of a cooling radiator. If it is necessary to increase the power to 1500 W, VS1 requires cooling. This requires a radiator with a cooling surface of approximately 260 cm2. In addition to the Zener diodes (zener diodes) indicated in the diagram, you can also use D816A, D816AP, KZ714, KS527A. The symmetrical thyristor can be replaced with KT729, KT784, BTW38, BTW42, BT853D, BT853E, TIC232D. Figure 2 shows a diagram of another device that can be used to determine the number of people passing through a certain place, or the number of parts moving in a certain direction (for example, on a conveyor).
The photoresistor FSK-1 serves as a photosensitive element in the device. Transistors VT1 and VT2 operate in key mode, and VT3 and VT4 are assembled as a composite transistor and act as a DC amplifier. For indication, an electromechanical counter (EMB) is used, which is included in the VT4 collector circuit. With the help of tuning potentiometers RP3 and RP4, such an operating mode of the transistor VT1 is selected so that it is closed when the photoresistor R1 is lit. Since no current flows through VT1, there will be a negative potential on its collector, which is fed to the base of VT2. As a result of this, VT2 is open, and the DC amplifier (VT3, VT4) blocks the zero potential from its collector. A very small current will flow in the VT4 collector circuit, which will not trigger the electromechanical meter. If at some point the luminous flux directed to the photoresistor is interrupted, its resistance increases sharply, and with it the negative potential at the base of VT1 also increases; transistor opens. As a result, the transistor VT2 closes, which ensures saturation of VT3 and VT4. The collector current VT4 increases and triggers an electromechanical counter that counts one visitor or one object that has passed through the control point. To focus the light flux and, thereby, increase the sensitivity of the counter, a focusing lens is installed in front of the photoresistor. The device uses an electromechanical counter from an automatic telephone exchange, having a coil resistance of 100 ohms and a response current of 30 mA. The device is powered by two flat batteries connected in series (2 x 4,5 V), but you can also use a small rectifier that gives an output of 9 V / 0,2 A. Instead of the transistors indicated in the diagram, you can use others with a similar amplifier coefficient (at least 80). The FSK-1 photoresistor can be replaced with FSK-1A, FSK-G1, FSK-2, SFH203, VRH-60. Author: G.Kuzev; Publication: radioradar.net
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