What Is an LDR?
A light‑dependent resistor (LDR)—also known as a photocell, CdS cell, cadmium sulfide resistor, or photoresistor—is a passive component whose resistance varies according to the amount of light falling on it. In darkness, its resistance is very high; under bright illumination, its resistance drops dramatically.
How It Works
- Dark Conditions: High resistance (often in the megaohm range), limiting current flow.
- Bright Conditions: Low resistance (down to a few hundred ohms), allowing more current to pass.
This characteristic makes LDRs ideal for sensing ambient light levels without requiring a power source to measure the light itself.
Common Applications
- Automatic Lighting
LDRs can trigger streetlights or outdoor lamps to switch on at dusk and off at dawn. - Light Meters
Photographic light meters and exposure devices use LDRs to measure scene brightness. - Security Systems
Detect changes in light when a door or window is opened, activating alarms or cameras. - Consumer Electronics
Adjust screen backlighting on televisions, smartphones, and laptops based on room lighting.
Example Usage
In a simple dusk‑to‑dawn circuit, the LDR forms part of a voltage divider that feeds into a comparator or microcontroller input. As evening falls and the LDR’s resistance rises, the voltage at the divider’s midpoint passes a threshold, signaling the system to turn lights on. Conversely, increasing daylight lowers the resistance, bringing the voltage below the threshold to switch the lights off.
