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School-college Physics Notes: Electricity 4.3 Use of an LDR device

Electricity section 4: 4.3 LDR devices - light dependent resistors - how they work and what they are used for

[Author © Dr WP Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics, electricity page updated Feb 11th 2026 *]

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  4.3 LDR devices - light dependent resistors - how they work and what they are used for

electrical circuit to investigate the behaviour characteristics of an LDRCircuit 44 shows how you can investigate the resistance of an LDR in varying light conditions.

The voltmeter is wired in parallel with the LDR, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

You must decide on the initial p.d. and see how the current varies.

You calculate the resistance of the thermistor from Ohm's Law equation: V = IR, so R = V/I

Somehow you need to vary the light intensity shining on the LDR resistor e.g. using a lamp working of variable resistor and taking a reading with a light meter as well as the p.d. and current readings.

 

graph of electrical resistance versus light intensity for an LDRSince an LDR is a light dependent resistor and you should find ....

The higher the light intensity, the lower an LDR's resistance, the greater the current flow for a fixed p.d..

i.e. high resistance in darkness and low resistance in bright light.

You can see this trend clearly in the resistance - temperature graph for an LDR.

Therefore an LDR can respond to changes in light intensity e.g. daylight/night time.

 

 At constant temperature and constant light intensity, the current voltage graph for an LDR is linear, the same as for a fixed resistor (left graph 1), so it is an ohmic resistor at constant temperature.

Since the circuit system will sense the presence of light, that is the basis of an LDR's applications.

This means with increase in light intensity, the current flow will increase.

 

Uses of an LDR

LDR resistors are used automatic control of lights at night - outdoor lighting, burglar alarm circuits, light intensity meters.

 

LDR electrical circuit diagram for controlling a lamp in household lightingCircuit 33 shows in principle how to control the output of a lamp bulb.

(real LDR circuits are more complicated)

In this case the 'active' component, the bulb, is wired in parallel with the LDR response resistor.

In this case the p.d. across the LDR and the lamp bulb is the same, though the LDR is a variable resistor.

In dim light or darkness, the p.d. across the LDR and bulb is high because LDR's resistance is high.

The greater the p.d. across the lamp the greater the power output (P = IV), so the bulb lights up - glows more brightly as the surroundings get darker.

If the surroundings e.g. a room or a garden path gets brighter, the LDR's resistance decreases, the p.d. decreases, so the power output decreases and the lamp glows dimmer or 'goes out'.

 

As well as automatic night lights, an LDR can be used in a burglar alarm circuit.

A small and constant beam of light is shone on an LDR (it can be from an invisible infrared emitting LED). If the shadow (of the burglar) crosses the light beam, the intensity of light falling on the LDR is reduced. Therefore the resistance of the LDR is reduced and this triggers an alarm.

 

A simple light meter can be made by connecting an LDR in series with a battery and an ammeter.

The brighter the light, the lower the resistance of the LDR.

The lower the resistance, the greater the current flow, so the ammeter reading is a measure of the light intensity.

 

INDEX for physics notes on testing circuit devices and how they used

Key points about electric circuits in physics courses - function and use of an LDR

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations.

A structured, exam-board-aligned summary on Light Dependent Resistors (LDRs), tailored for GCSE/IGCSE Physics students:

Light Dependent Resistors (LDRs) – Summary Revision Notes

What is an LDR?

An LDR (Light Dependent Resistor) is a light-sensitive component whose resistance decreases as light intensity increases.

It is a non-ohmic component, meaning it does not obey Ohm’s Law.

  • In bright light → low resistance → more current flows
  • In darkness → high resistance → less current flows

Circuit Symbol of an LDR

A rectangle with arrows pointing towards it, indicating light

How an LDR Works

Light Level Resistance Explanation
Bright light Low More photons → more free electrons → more current
Darkness High Fewer photons → fewer free electrons → less current

This makes LDRs ideal for automatic light-sensing circuits.

Uses of LDRs

LDRs are used in circuits that respond to changes in light:

  • Street lighting (turns on at night)
  • Automatic night lights
  • Burglar alarms
  • Camera light meters
  • Solar garden lights

Required Practical (All Boards)

Aim: Investigate how resistance of an LDR varies with light intensity

Method Overview:

  • Connect LDR in series with ammeter and voltmeter
  • Shine light at varying intensities (e.g. using a lamp or covering with paper)
  • Measure voltage and current → calculate resistance using ( R = V/I )
  • Plot light intensity versus resistance graph

Expected Result: Resistance decreases as light intensity increases

Typical Exam Board Specification Content involving an LDR

Key Focus
LDRs as non-ohmic components; practical applications in sensing circuits
Emphasis on interpreting resistance–light graphs and real-world uses
Includes LDRs in component behaviour and control systems
Focus on circuit diagrams and LDRs in automatic systems
Investigates LDRs in control circuits and practical applications
Strong emphasis on definitions, graphs, and light-dependent behaviour

Student Tips for questions involving an LDR

  • Memorise: LDR = Light Dependent Resistor
  • Sketch graphs: Resistance versus light intensity is a downward curve
  • Use correct terms: “Non-ohmic”, “light-sensitive”, “automatic control”
  • Explain behaviour: More light → more free electrons → lower resistance
  • Revise practicals: Be confident describing method, apparatus, and expected results
  • Think real-world: Link to devices like night lights and alarm systems

Keywords, phrases and learning objectives for an LDR device in an electrical circuit

Be able to explain and describe with a diagram how an LDR circuit works.

Know the shape of the graph of resistance versus light intensity at constant temperature i.e. as the light intensity increases, the resistance decreases and the current increases.

Be able to show how an LDR light dependent resistor circuit device works.

Be able to describe what an LDR is used for.

Know the current-voltage curve I-V graph for an LDR is linear i.e. it is an ohmic conductor at constant temperature.

Know how an LDR is used to control the output of lights i.e. acts as on off light meter.

Be able to describe and explain how an LDR circuit can be used to control the output light from visible light lamps, as a burglar alarm and a simple light intensity meter.

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