Low Voltage CircuitPower Supplies

Solid-state light-sensitive switch

This electronic switch is designed to be connected directly to the mains, which obviates a low-voltage supply and so keeps the cost and space requirement to a minimum. The circuit switches a lamp on when it gets dark. and switches it off again when it gets light. The switching is done without a relay, avoiding problems with sparks and mains pollution caused by the contacts and the coil inductance.

The switch is powered by the mains via R10, C4. D3, D2, and C3. A voltage reference. D1 supplies 8.2 V to a light measuring network, R2-P1. As the light intensity drops, the resistance of the LDR (light-dependent resistor), R2, increases. Consequently, the voltage across P1 drops, so that the gate-source voltage of FL’T’ T1 drops also. When switch S1 is closed, the time constant R3-C2 causes the gate voltage of T1 to change more slowly than the resistance of R2. This is necessary to prevent the circuit responding to quick changes in the ambient light intensity.

Components T1. T2. R.4, R5, Re), and R8 form a Schmitt trigger. Normally, T1 conducts so that T2 is off. When the gate voltage of the VET drops below a certain level, T2 is switched on. Consequently, T3 starts to conduct and supplies the gate current necessary to trigger triac Trii. The load, lamp Lai, is then switched on. When the light intensity increases above the level set with P1. T1 is switched on so that the load is switched off.

Switch S1 is included to disable the time constant during adjustment. Resistor R9 serves to discharge C4 after the circuit has been disconnected from the mains.
WARNING. Since the circuit carries dangerous voltages at a number of points, it is essential that proper electrical insulation is applied. Never work on the circuit when the mains is connected to it. Make sure that no part of the circuit can be touched when it is being set, adjusted or used.

Solid-state light-sensitive switch Schematic diagram

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