Simple Dark Sensor Circuit

Preview

If you need a bulb to light up automatically at night, then you can use this circuit.
This is a dark sensor and you can control any device by connecting it through the relay. The relay was activated when the nearby is dark or when we covered the surface of  the LDR.The circuit is very simple and you can use a dot board to assemble it and use any power supply to give power.

If you have a 9v or 6v relay then decrease the input voltage according to the voltage of relay. Use RV1 to change the sensitivity of this circuit.

Dark Sensor Circuit

Operation

In normal condition impedance of LDR is low. Q1 transistor is in off state. because bias voltage coming through R2 and RV1 was grounded through the LDR.

At dark state impedance of LDR is increase and therefore voltage of Q1 base goes high and after it reach ~0.5-0.7v Q1 activated and bias voltage for Q2 is start to going through R1 and Q1. then Q2 activated and also relay was activated.

If you connect you device across the NC and Com pin of relay then you can also use this as light activated switch.