The application of LDRs in circuits eg switching lights on when it gets…

The idea

The obvious use of an LDR is switching lights on when it gets dark. The LDR is placed in series with a fixed resistor across the supply, so as daylight fades the LDR resistance rises, the share of the supply potential difference across it rises, and once it passes a set value the control circuit switches the street lamp or security light on. At dawn the reverse happens and the lamp switches off. The same idea runs a garden light, a night light and the automatic headlights on a car. Other applications use the current directly: a camera adjusts its exposure from the light reaching an LDR, and a burglar alarm triggers when a beam falling on the LDR is broken.

What earns the marks

• Naming a suitable application such as automatic street lighting or a burglar alarm.
• Stating that the LDR resistance rises as the light level falls.
• Linking that to the potential difference across the LDR in a potential divider.
• Describing the switching action that results, such as turning the lamp on.

Common mistakes and how to put them right

• Saying the LDR turns the light on by itself rather than triggering a switching circuit.

• Getting the direction wrong, so claiming resistance falls as it gets dark.

• Describing the LDR as producing electricity when light falls on it, which is a solar cell.

• Naming an application with no mention of what changes inside the circuit.

Exam tips

• Trace the chain from the light level through the resistance to the potential difference and the switch.
• Name the component that does the switching, since the LDR only provides the signal.
• Use the same reasoning pattern as for a thermistor; only the trigger quantity changes.