A security light uses a light dependent resistor (LDR) connected in a circuit to switch a lamp on automatically at night. Explain how the LDR causes the lamp to switch on as it gets dark.

OCR A-Level Physics A (H556) — 3.1 Motion · Explain · 5 marks · View as Markdown

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

Security lights that switch on automatically at dusk are common in homes. They rely on an LDR to detect changes in light intensity and trigger the lamp.

Model answer (5 marks)

As it gets dark the light intensity on the LDR falls, so its resistance rises. The higher resistance increases the total resistance of the circuit, reducing the current. The drop in current (or the change in voltage across the LDR) is detected by a switch or transistor, which then turns the lamp on when the light level is low enough.

Examiner tips

  • Mention the LDR’s inverse light–resistance relationship first. Show the chain: light ↓ → resistance ↑ → current ↓ → trigger activates. Use the exact wording “light intensity falling” and “resistance increases”.
  • Explain the role of the change in current or voltage as the trigger signal. This shows understanding of the circuit’s operation.

Common mistakes

  • Saying the resistance decreases instead of increases. Students often mix up the LDR behaviour. They may also forget to link the current drop to the trigger mechanism. Some write “the lamp turns on because the current increases”, which is incorrect.

Mark scheme (5 marks)

  1. As it gets dark, the light intensity falling on the LDR decreases
  2. The resistance of the LDR increases (as light intensity decreases)
  3. This increases the total resistance of the circuit
  4. So the current in the circuit decreases
  5. The change in current (or potential difference across the LDR) is used to trigger / switch on the lamp when it gets dark enough

Key terms in this question

light dependent resistor (LDR)

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