# 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

> 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.

## 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

- [light dependent resistor (LDR)](https://www.gradenine.co.uk/glossary/light-dependent-resistor-ldr)
- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [light intensity](https://www.gradenine.co.uk/glossary/light-intensity)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-designing-an-automatic-626afcbd) · Published by Druglandscape Ltd.