A street lighting system uses light dependent resistors (LDRs) to automatically switch lights on at night and off during the day. Explain how the resistance of an LDR changes between daytime and night-time, and explain why this makes LDRs suitable for use in automatic street lighting systems.

OCR A-Level Physics A (H556) — 3.2 Forces in action · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Modern street lights often use sensors rather than timers to control when lights turn on and off. One common type of sensor relies on how certain components respond to changes in light intensity.

Model answer (5 marks)

The resistance of an LDR falls as the intensity of light incident on it rises – during the day the resistance is low. At night, when the light level is very low, the resistance is very high. This change in resistance alters the current flowing through the circuit in which the LDR is placed. The varying current (or the voltage across a resistor in series with the LDR) can be used as a signal to trigger a relay or transistor that turns the street light on when the current falls below a set threshold. Because the LDR responds directly and automatically to the ambient light level, no human intervention is required, making it an ideal sensor for automatic street lighting.

Examiner tips

  • Show the LDR’s resistance–light relationship first, then explain the circuit effect, and finally state the automatic control benefit
  • Use the exact terms ‘decreases with light’, ‘high resistance in darkness’, ‘current change triggers switch’

Common mistakes

  • Confusing the direction of resistance change (stating it increases with light)
  • Failing to link the resistance change to a change in current or voltage that triggers the light
  • Not mentioning the automatic nature of the system

Mark scheme (5 marks)

  1. The resistance of an LDR decreases as light intensity increases (daytime = lower resistance)
  2. The resistance of an LDR is greatest when it is dark / at night-time
  3. The change in resistance of the LDR causes a change in current in the circuit
  4. This changing current (or signal) can be used to trigger the lights to switch on when it gets dark
  5. The process is automatic / the LDR responds directly to light intensity without human input, making it a suitable sensor for this application

Key terms in this question

resistance

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