Photo Switch Circuit using 555 IC

Here’s a design circuit for simple photo switch suitable for home and industrial purpose. The circuit switches on a relay when the light intensity falling on the sensor crosses the set limit. The circuit is based on a NE 555 and two transistors for driving the relay. The resistor R1 is adjusted such a way that under normal condition the voltage across the LDR is less than 1/3 the supply voltage. In this condition output of IC is high.

This makes transistor Q1 on. Transistor Q2′s base is connected to collector of Q1. So Q2 will be off keeping the relay de energized. When light falls the voltage across LDR rises above 2/3 Vcc. This makes the output of IC go high making the relay to energized. The result we get a relay that operates according to the intensity of light falling on the LDR. To setup the circuit switch on power supply, connect a multi meter across LDR and adjust R1 so that voltage across   LDR is just below 1/3 supply voltage. Ensure that relay is off in this condition. If not reduce voltage across LDR further more by adjusting R1 to make relay off.

Now cover the top of LDR using a black paper and see the relay gets activated. If not, make further adjustments with R1.Nothing to care after some trial and error you will get the correct set point of R1 where the circuit works. This circuit  is nothing great, but some simple basics. I have tested this and got good results. So no problem .Just proceed. If there is some chattering in the relay , that may be because off the difference between the turning on and turning off voltages. This can be avoided by connecting a resistor whose value is equal to 1/2 the resistance of LDR at illuminated condition.