LDR / Photoresistors

1. Definition

An LDR (Light Dependent Resistor) or Photoresistor is a special type of resistor that changes its resistance based on the amount of light falling on it.

  • In bright light, its resistance goes down.
  • In darkness, its resistance goes up.

They are often used in night-lights, solar garden lamps, light meters, and streetlights that turn on automatically at night.

ldr photoresistor

LDR Symbol

ldr symbol

2. Features

  • Light Sensitive: Resistance changes depending on light intensity.
  • Non-Polarized: Can be connected in any direction.
  • Slow Response: Works for detecting gradual changes in light, not fast flashes.
  • Low Cost & Simple: Easy to use in beginner projects.

4. How to Use

4.1. Identify the LDR

  • Looks like a small round or square disc with zig-zag lines on top.
  • Two metal legs for connecting into a circuit.

ldr identify

4.2. Choose the Right LDR

  • Light Range: Some LDRs are more sensitive to dim light, others to bright light.
  • Size: Larger ones generally have a bigger light-sensitive area.
  • Resistance Range: Common LDRs range from a few hundred ohms in bright light to several megaohms in darkness.

4.3. Connect It Correctly

  • LDRs are non-polarized, so you can connect either leg to positive or negative.
  • Usually used in a voltage divider with another resistor to turn light levels into measurable voltage for a microcontroller (like Arduino).
  • Keep the light-sensitive face uncovered and pointed towards the light you want to measure.

4.4. Test It

  • Use a multimeter in resistance mode:
    • Shine a light on it → resistance drops.
    • Cover it with your hand → resistance rises.
  • If using with Arduino or similar, read the analog value to see changes in light level.

4.5. Mount Securely

  • On breadboard: Push the legs into the holes.
  • On PCB: Solder the legs into the pads.
  • Avoid bending the legs too close to the body to prevent damage.

5. Video Explanation

video coming soon