Light Sensor to Switch On a Light or Any Device

Abstract

The Light Dependent Resistor (LDR) is made from a Cadmium-Sulfide compound (CdS). CdS is a semiconductor material. The resistance of the LDR changes as the light level changes. More light produces more electron — hole pairs in the material, making the material a better conductor in light. Resistance decreases (conductivity increases) as light levels increase because there are more charge carriers in the material. Note that R = 1/G (G is conductance).

The LDR controls the voltage on the transistor’s base. As it gets darker in the room, the voltage on the base will increase until the transistor is forward biased. This results in high current flowing from the transistor’s collector to the base. This collector current flows to the LED and is responsible for turning the LED on.