5mm LDR Light Sensors

Operating Principle

The 5mm LDR operates on the principle of photoconductivity:

1. Material: It is typically made from a semiconductor material, such as Cadmium Sulfide ($\text{CdS}$), which is sensitive to visible light (often peaking around $540\text{ nm}$, similar to the human eye).
2. In Darkness: In the absence of light, the material has very few free charge carriers (electrons), resulting in very high electrical resistance (the Dark Resistance), often in the range of $1\text{ M}\Omega$ to $20\text{ M}\Omega$.
3. In Light: When photons from incident light strike the semiconductor material, they transfer energy, knocking electrons from the valence band into the conduction band. This creates electron-hole pairs, which are free charge carriers.
4. Resistance Change: The increase in free charge carriers leads to a large increase in the material’s conductivity and a sharp decrease in electrical resistance. In bright light (e.g., $10\text{ Lux}$), the resistance drops significantly, typically to $10\text{ k}\Omega$ to $20\text{ k}\Omega$.

Key Relationship: $\text{Higher Light Intensity} \implies \text{Lower Resistance}$

Typical Specifications

The 5528 model is one of the most common 5mm LDR types in hobbyist electronics.

Interfacing with Microcontrollers (Voltage Divider)

Since a microcontroller’s Analog-to-Digital Converter (ADC) can only measure voltage, not resistance directly, the LDR is typically integrated into a voltage divider circuit to convert the resistance change into a measurable voltage change.

1. The LDR is placed in series with a fixed resistor (often $10\text{ k}\Omega$).
2. The series combination is connected across the supply voltage (e.g., $5\text{V}$ from Arduino).
3. The analog output voltage ($\text{V}_{\text{OUT}}$) is read at the junction between the LDR and the fixed resistor.

• Dark Conditions: LDR resistance is high, pulling $\text{V}_{\text{OUT}}$ low (close to $0\text{V}$).
• Bright Conditions: LDR resistance is low, pulling $\text{V}_{\text{OUT}}$ high (close to $5\text{V}$).

Common Applications

• Automatic Streetlights: Used to turn lights on at dusk and off at dawn.
• Security Systems: Used as a simple beam-breaker sensor for burglar alarms.
• Camera Exposure Control: Measures ambient light for automatic shutter speed adjustment.
• DIY Projects: Simple light meters, ambient light-controlled toys, and nightlight