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SCHEME OF DEVICE FOR AUTOMATIC INCLUSION OR OFF OF LIGHT
DEPENDING ON LIGHTING

Author: G. Kuzev

The figure shows the scheme of the device, which can be used to automatically turn on and off the light, depending on the ambient light.

The proposed switch to a symmetrical thyristor VS1 used as a contactless switching element. As a result, it is possible to power the load during both half-periods of the mains voltage without using powerful full-wave rectifiers.

The electronic switch is a photo relay based on a Schmitt trigger (VT1, VT2) . As a photosensitive element (light sensor), we used a photoresistor R1 of the type FSK-1 , which in the dark has a high resistance (about 3.3 MΩ ), and under illumination its resistance decreases by 400 times. The photoresistor R1 together with the resistors RP2 and R3 form a voltage divider that determines the base current of the transistor VT1 . Resistor R3 limits the current in the divider with a possible hit on R1 of direct sunlight. Resistor R7 determines the current in the control electrode of the triac VS1, a R6 serves to equalize the voltage on the control electrode and the cathode VS1, when transistor VT2 is closed. This ensures a stable operation of the triac.

The device works as follows. In the afternoon, when it's light, the resistance of the photoconductor R1 is slightly, VT1 is open, and VT2 is closed. The collector current VT2 and, consequently, the current of the control electrode of the triac is almost equal to zero. In this state, VS1 is closed and the HL1 light is off. As the illumination decreases, the resistance of the photoresistor increases. The current of the base VT1 begins to decrease. When you reach a certain level VT1 is closed, a VT2 opened, ie The trigger is switched. The current of the control electrode VS1 flowing through the open transistor VT2 and the resistors RP5 and R7 , maintains VS1 in the open state during both half-periods of the mains voltage, and the lamp HL1 glows at full power. Switching off in the reverse order.

The regulation threshold is carried out in the evening with the help of trimmer potentiometers RP2 and RP5. The photoresistor R1 must be installed so that during the day it does not get direct sunlight, and at night - artificial lighting. With this device you can control the load with a power of up to 400 W without using 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 cm ² . In addition to the Zener diodes (zener diodes) indicated on the diagram, D816A, D816AP, KZ714, KS527A can also be used . The symmetrical thyristor can be replaced with KT729, KT784, BTW38, BTW42, BT853D, VT853E, TIC232D .

Author: G. Kuzev
Source: Radio, Television, Electronics, 7/99.
Translation: A. Belsky

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