The circuit diagram given here is a photo switch which switches on a relay when the intensity of light falling on its sensor changes from a preset limit. The central part of this circuit is the IC NE 555 and two transistors Q1 (BC 548) and Q2 (SL 100). The transistors Q1 and Q2 are used here as driving elements for relay. The voltage across LDR is kept at 1/3 of the supply voltage by adjusting the potentiometer R2 (100K). The output of IC is high at this stage. This makes transistor Q1 turn on and Q2 will be in off state since its base is connected to the collector of Q1. As a result of this, relay will be put into inactive state. The voltage across LDR increases when light falls on it. When this voltage crosses 2/3 of the supply voltage, the output of IC goes high. By the effect of this, relay will get activated. That is relay works with respect to the intensity of light falling on LDR.
- Use a good quality PCB or common board to assemble this circuit.
- This switch is suitable for home as well as industrial purposes.
- Use a 12V DC supply for powering this circuit.