LDR: Light level and Resistance

LDR: Light level and Resistance is a free interactive simulation of a light dependent resistor (LDR) for GCSE Physics (Edexcel IGCSE 4PH1). Students vary the light level and measure the LDR's resistance to investigate how the two are related. No login required — includes a worksheet and self-marking quiz.

Aim: To investigate how light level affects the resistance of an LDR.

Light source
LDR
OHMMETER
Ω
— Ω

Results table

Light level (%) Resistance (kΩ) Action
  1. Set the Light level slider to 0% (fully dark) and read the resistance shown on the ohmmeter.
  2. Increase the light level in steps of about 10% up to 100%, reading the resistance at each step and typing the light level (%) and resistance (kΩ) into a new row of the results table.
  3. Take at least 6 readings spread across the full range, then plot a line graph with light level on the horizontal axis and resistance on the vertical axis.
  4. Change the LDR type to a different option and repeat a couple of readings at a low and a high light level. Does the resistance range change, while the overall shape of the graph stays the same?
  5. Look at the shape of your graph. How does the resistance of the LDR change as the light level increases — is the relationship linear, or does resistance fall quickly at first and then level off?
  6. LDRs like this are used in circuits that switch street lights on automatically at dusk. Using your graph, explain why a large change in resistance between dark and bright conditions makes the LDR useful for this job.