2.1.8 LDRs

• LDR

  Light Dependent Resistor, a type of resistor whose resistance decreases with increasing light intensity
• Structure of LDR

  • Made of semiconductor materials like cadmium sulfide (CdS) or cadmium selenide (CdSe)
  • Changes electrical resistance when exposed to light
• Principle of LDRs

  Resistance of an LDR is inversely proportional to the light intensity
• As light intensity increases

  Resistance decreases, allowing more current to flow through the LDR
• Behavior of LDRs

  • Low Light Intensity: High resistance, less current flows
  • High Light Intensity: Low resistance, more current flows
• 1. I Characteristics of LDRs

  • Non-linear relationship between voltage and current
  • Resistance decreases as light intensity increases
• At Low Light
• At High Light
• Applications of LDRs

  • Automatic Lighting Systems
  • Light Meters
  • Alarm Systems
  • Solar Garden Lights
• Automatic Lighting Systems

  • Streetlights that turn on automatically at dusk and off at dawn
• Light Meters


  • Devices that measure light intensity for photography and other applications
• Alarm Systems

  • LDRs used in security systems to detect changes in light levels
• Solar Garden Lights

  • Solar-powered garden lights that turn on when it’s dark
• Practical Experiment with LDRs

  1. Setup

  2. Procedure

  3. Analysis
• Setup

  Connect the LDR in a simple circuit with a power supply, a variable resistor, and a voltmeter or ammeter
• Procedure

  1. Measure resistance at different light intensities

  2. Record voltage and current

  3. Plot graph of resistance vs. light intensity or current vs. light intensity
• Analysis

  1. Resistance decreases as light intensity increases

  2. Current increases with increasing light intensity for a given voltage
• Understanding LDR behavior and characteristics helps in designing systems that react to changes in light
• LDRs provide varying resistance with light intensity, making them versatile for many practical applications