A student is designing an automatic security light for a garden shed. The light must switch on when it gets dark and switch off when it is light. The student decides to use a light dependent resistor (LDR) in the circuit. Explain how the resistance of an LDR changes with light intensity, and explain why this makes the LDR suitable for use in an automatic security light.

OCR A-Level Physics B: Advancing Physics (H557) — 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).

An automatic security light is designed to turn on at night and turn off during the day. The circuit uses a light dependent resistor (LDR) connected to a control unit that switches the lamp on when the resistance in the circuit exceeds a set threshold value.

Model answer (5 marks)

The resistance of an LDR falls as the light intensity rises (it is highest in the dark and lowest in bright light). In the dark the LDR therefore has a very high resistance. When this high resistance exceeds the preset threshold in the control unit, the circuit is opened and the lamp is switched on. In daylight the LDR’s resistance is low, falling below the threshold, which closes the circuit and turns the lamp off.

Examiner tips

  • Use the phrase ‘resistance decreases as light intensity increases’ to show understanding of the LDR behaviour.
  • Explain the two states (high resistance at night, low at day) and link each to the control unit’s threshold action.
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.
  • Use UK spelling (e.g. ‘behaviour’).

Common mistakes

  • Confusing the direction of change – saying resistance increases with light instead of decreases.
  • Failing to mention the threshold comparison in the control unit.
  • Using vague terms like ‘light sensor’ instead of ‘LDR’ and ‘resistance’.

Mark scheme (5 marks)

  1. The resistance of an LDR decreases as light intensity increases (or: resistance increases as light intensity decreases / is greatest in the dark)
  2. At night / in the dark, the LDR has a high resistance
  3. This high resistance (at night) exceeds the threshold value, so the control unit switches the lamp on
  4. During the day / in bright light, the LDR has a low resistance
  5. This low resistance (in daylight) is below the threshold value, so the control unit switches the lamp off

Key terms in this question

light dependent resistor (LDR) · resistance · light intensity

Related

More Forces in action questions

▶ Try answering this question with AI marking (free) →