11.14 Light-Dependent Resistor (LDR)

A Light-Dependent Resistor (LDR), also known as a photoresistor, is an electronic component whose resistance changes in response to the intensity of light falling on it. This property allows LDRs to be used as light sensors in applications such as automatic streetlights and security systems.

The fundamental working principle of an LDR is photoconductivity. In a photoconductive material, the absorption of light photons frees charge carriers (electrons and holes), which increases the material's electrical conductivity and therefore decreases its resistance.

Resistance vs Light Intensity

LDRs exhibit an inverse relationship between resistance and light intensity.

• In Darkness: An LDR has a very high resistance, often in the mega-ohm ($M\Omega$) range. This is known as the Dark Resistance.
• In Bright Light: An LDR's resistance drops significantly, often to just a few hundred ohms ($\Omega$).

This relationship is represented by a declining curve on a resistance vs. light intensity graph.

Types of LDRs

1. Intrinsic LDRs: Made from pure semiconductor materials such as Silicon (Si) or Germanium (Ge). They are sensitive to visible light.
2. Extrinsic LDRs: Made from semiconductor materials doped with impurities (e.g., Cadmium Sulfide, CdS). Doping makes them sensitive to specific, often longer, wavelengths such as infrared. CdS is the most common material for visible-light LDRs because its spectral response closely matches the human eye.

Mechanism of Photoconductivity

When photons with sufficient energy strike the semiconductor material:

1. Photon energy is absorbed by electrons in the valence band.
2. Electrons are promoted to the conduction band, leaving behind holes.
3. Both electrons and holes act as charge carriers.
4. More carriers → higher conductivity → lower resistance.

The greater the light intensity, the more photons are absorbed, and the more charge carriers are generated.

Characteristics of LDRs

Use in a Potential Divider Circuit

A common application of an LDR is in a potential divider circuit. The LDR is connected in series with a fixed resistor $R_{fixed}$. As light level changes, the LDR's resistance changes, altering the voltage distribution:

$V_{out}=V_{in}\times \frac{R_{fixed}}{R_{LDR}+R_{fixed}}$

• More light → $R_{LDR}$ decreases → $V_{out}$ across $R_{fixed}$ increases
• Less light → $R_{LDR}$ increases → $V_{out}$ across $R_{fixed}$ decreases

This output voltage can trigger a transistor or relay to switch a circuit on or off.

Applications

LDRs are widely used in devices that need to react to the presence or absence of light:

• Automatic Street Lighting: Turns lights on at dusk and off at dawn.
• Security Systems: Burglar alarms that detect when a light beam is broken.
• Cameras: Controls automatic shutter speed and aperture based on ambient light.
• Robotics: Line-following robots and light-source detection.
• Industrial Automation: Counting items on a conveyor belt as they interrupt a light beam.