Light-Dependent Resistor (LDR)

A light-dependent resistor (LDR) is a component whose resistance changes with light intensity. In bright light, its resistance is low. In darkness, its resistance is high.

How an LDR works

An LDR is made from a semiconductor material (such as cadmium sulfide). In darkness, very few charge carriers are available and resistance is high (typically hundreds of thousands of ohms). When light falls on the LDR, photons give energy to electrons in the semiconductor, freeing them to carry current. More light means more free charge carriers and lower resistance (which can drop to a few hundred ohms).

LDR in a potential divider

An LDR is often used with a fixed resistor in a potential divider circuit to produce a voltage that changes with light level.

In bright light: LDR resistance is low, so it takes a smaller share of the total voltage. The voltage across the LDR is low and the voltage across the fixed resistor is high.

In darkness: LDR resistance is high, so it takes a larger share of the voltage. The voltage across the LDR is high and the voltage across the fixed resistor is low.

Applications

• Automatic streetlights: the circuit detects when light level drops and switches the lights on.
• Camera light meters: measure the light level to set the correct exposure.
• Security lighting: detects changes in ambient light.

LDR vs thermistor

Both are input sensors with variable resistance. The LDR responds to light intensity; the thermistor responds to temperature. In a thermistor (NTC type), resistance decreases as temperature increases.