In the earlier article, we discussed the circuit which used LDR (Light Dependent Resistance) to make Good morning alarm. With slight modification in the circuit, we can use an LDR to make automatic night lamp. The LDR senses the intensity of sound in the surrounding and changes resistance accordingly.

This circuit can also be used as emergency light in case of power cut.

Circuit Description

When the resistance of LDR is low (in daylight), the positive supply gets grounded through the LDR. Transistor BC 548 is not activated and rest of the circuit does not work.
When the resistance of LDR is high (in night), the voltage across it increases and the positive supply is given to base of transistor BC548. It acts as pre-amplifier. It drives transistor BC 558. The output at the collector lets the LED glow.

LED is a power saver and acts as a night lamp.