A light sensitive switch is a specialized kind of switch whose characteristics change depending on the amount of light that penetrates it. Here is a light sensitive switch that is uncomplicated, easy to operate, and does not cost very much. The detection of light is the driving force behind the operation of this circuit; more specifically, it will switch on or off automatically depending on how much light is falling on it.

Because this is a switching circuit, we can break it up into two sections—the power supply and the switching circuit—in order to get a better understanding of how it works.

In this part of the power supply, the function of the step-down transformer is performed by register R1, and the zener diode ZD1 is responsible for the additional rectification necessary to change the voltage into 10 V dc. The output voltage that is measured across the zener diode is subjected to an additional filtering process by capacitor C1.

Another part of the circuit is the switching section, which is constructed around the light-dependent register LDR1 with the assistance of the operational amplifier IC 741. The LDR serves as the sensor for the switching circuit. Simply by moving the switch to a different position, we are able to make this circuit sensitive to both light and dark (i.e. turn on in light and turn in dark respectively). LDR detect the light and alter their resistance corresponding to the light, and the result is delivered to pin 2, where it is regulated further by preset VR1. The output is received from pin 6 of IC1, and it is then delivered to the base of transistor T1 by way of resistor R6. Resistor R7 serves the purpose of acting as a current limiter. The output from the collector of the transistor T1 is transferred to the gate of the TRIAC1, which then causes the light to turn on. It is possible to make the circuit function as a light sensor by connecting points 1 and 2 of switch SW2 to points 2 and 3 of switch SW1 (i.e. turn on the bulb when light fall on it). As dark Sensor The function of the circuit as a dark sensor can be activated by connecting points 2 and 3 of switch SW2 to points 1 and 2 of switch SW1 (i.e. turn on the bulb in absence of light).

Resistors (all of them are 14 watt, less than 5% carbon). R1 = 100 KΩ/1W; R2, R3 = 100 KΩ; R4 = 4.7 KΩ; R5, R6 = 220 KΩ; R7 = 68 KΩ; R8 = 33 KΩ; VR1 = 100 KΩ (preset) Capacitors C1 = 100 µF/16 Semiconductors IC1 equals LM741 (an operational amplifier), T1 equals BC547, TR1 equals 10GD (a triac), D1–D4 equals 1N4004, and ZD1 equals 10V. /100 mW Miscellaneous LDR1 = Light – Dependent – Resistor; B1 = 200W bulb