Light-dependent resistors (LDR) – fundamentals

A light-dependent resistor (LDR) is a semiconductor component with a resistance that depends on the intensity of the light that is incident on it. The greater the intensity, the lower the resistance of the LDR.


How LDR works

Light provides the energy to release charge carriers and decrease the resistivity of the material. Higher intensity means that more photons are hitting the LDR each second, so more charge carriers are released each second. Thus higher intensity of light results in lower resistance.

Circuit symbol for an LDR

circuit symbol of LDR
figure 1

A resistance vs light intensity graph for an LDR

Higher intensity of light results in lower resistance.

figure 2: A resistance vs light intensity graph for an LDR

Investigating the resistance of an LDR

figure 3:

You can use the circuit in Figure 3 to investigate the resistance of an LDR.

  • Switch on the power supply, and record its potential difference — this should stay constant.
  • Measure the current through the LDR.
  • And then cover up part of the LDR’s surface with a piece of thick paper. This will decrease the light intensity on the LDR.
  • Again, measure the current through the LDR.
  • Record the area of the uncovered surface.
  • Gradually cover more of the surface with the paper, taking new current readings each time you do.
  • Record the area of the uncovered surface for each current measurement.
  • You can use the current and p.d. data to calculate the resistance of the LDR for different uncovered areas (i.e. light intensities).
  • Then plot a graph of the uncovered area against resistance.
  • Your results should match the relationship shown in Figure 2. (resistance vs light intensity graph for LDR)
Light-dependent resistors (LDR) – fundamentals
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