Light Dependent Resistor

Key Stage 4

Meaning

The symbol for a light dependent resistor.

A light dependent resistor (LDR) is a resistor which changes resistance depending on the intensity of light that it is exposed to.

About Light Dependent Resistors

Light dependent resistors decrease resistance as the intensity of light increases.

A light dependent resistor can be used to control the current passing through a circuit. If the potential difference is constant then the current decreases as the light intensity decreases.

A light dependent resistor can be used to control the potential difference of another component in series with it. If the light intensity on the light dependent resistor is decreased then the potential difference across other components will decrease.

Light dependent resistors can be used in security lights and street lamps which activate depending on how dark it is.

IV Graph

In this circuit diagram the light intensity is low, which causes the resistance of the LDR to be high. As a result the potential difference across the lamp will be high (5.9V) and the current through the lamp will be large. So in dim light the lamp will remain lit.	In this circuit diagram the light intensity is high, which causes the resistance of the LDR to be low. As a result the potential difference across the lamp will be low (0.1V) and the current through the lamp will be low. So in bright light the lamp will remain unlit.

IV Graph

Description

The IV Graph for a light dependent resistor shows that:

• At a high light intensity the current increases rapidly with the potential difference
• At a low light intensity the current increases slowly with the potential difference.

Explanation

The resistance of an LDR increases as the light intensity decreases.

Obtaining the IV Graph

Connect an ammeter in series with the LDR to measure current through the LDR. Connect a voltmeter in parallel with the LDR to measure the potential difference across it. Use a variable resistor in series with the LDR to vary the potential difference across the LDR. Cover the LDR so that very little light reaches its surface. Start with a potential difference of zero and increase the potential difference by an interval of 0.2V up to 2V. Recording the reading on the voltmeter and ammeter. Reverse the connections on the battery and repeat steps 4 and 5 to find the I-V relationship for negative potential difference and current. Repeat steps 5-7 while shining a constant bright light on the LDR

References

AQA

LDR (light dependent resistor), pages 52, 53, 64-5, GCSE Physics; Student Book, Collins, AQA

LDRs, page 27, GCSE Physics; The Revision Guide, CGP, AQA

LDRs, pages 180, 184, GCSE Combined Science; The Revision Guide, CGP, AQA

Light dependent resistor, pages 52, 53, 64-5, GCSE Physics; Student Book, Collins, AQA

Light-dependent resistors (LDRs), pages 293, 299-300, GCSE Combined Science Trilogy 1, Hodder, AQA

Light-dependent resistors (LDRs), pages 44-5, GCSE Physics, Hodder, AQA

Light-dependent resistors (LDRs), pages 60, 80, 81, GCSE Combined Science Trilogy; Physics, CGP, AQA

Light-dependent resistors (LDRs), pages 62, 82, 83, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA

Light-dependent resistors (LDRs); circuit symbol, page 38, GCSE Physics, Hodder, AQA

Light-dependent resistors, page 57, GCSE Physics; Third Edition, Oxford University Press, AQA

Edexcel

LDRs, page 187, GCSE Combined Science; The Revision Guide, CGP, Edexcel

LDRs, page 74, GCSE Physics; The Revision Guide, CGP, Edexcel

LDRS, pages 225, 226, 229, GCSE Physics, CGP, Edexcel

Light-dependent resistors (LDRs), page 148, GCSE Physics, Pearson Edexcel

Light-dependent resistors (LDRs), page 388, GCSE Combined Science, Pearson Edexcel

OCR

Light-dependent resistors (LDRs), page 179, Gateway GCSE Combined Science; The Revision Guide, CGP, OCR

Light-dependent resistors (LDRs), page 47, Gateway GCSE Physics; The Revision Guide, CGP, OCR

Light-dependent resistors (LDRs), pages 109, 113, Gateway GCSE Physics, Oxford, OCR