Light dependent resistors (LDRs)

A light dependent resistor (LDR) is a resistor whose resistance decreases as the light intensity on it increases. In the dark its resistance is high, so little current flows; in bright light its resistance is low, so more current flows. That makes an LDR a simple light sensor, used in circuits such as street lights that switch on automatically when it gets dark.

How does an LDR work?

An LDR is made from a semiconductor. In the dark, most of its electrons are held in place, so there are few free charge carriers and the resistance is high. When light falls on it, the light gives some electrons enough energy to break free and carry current. The brighter the light, the more free charge carriers, so the resistance falls.

AQA GCSE Physics puts it simply: the resistance of an LDR decreases as light intensity increases. It lists LDRs with filament lamps, diodes and thermistors as components whose resistance is not constant.

LDR symbol

In circuit diagrams an LDR is drawn as a resistor (a rectangle in UK diagrams) with two arrows pointing towards it, showing light falling on it. Many diagrams also draw a circle around the rectangle. Arrows pointing away from a component mean it gives out light, as on an LED, so check the direction.

LDR graphs

Resistance against light intensity

Here x is the light intensity and y is the resistance, both in arbitrary units. The curve falls steeply as the light goes from dark to dim, then levels off in bright light. It never reaches zero: even in very bright light an LDR still has some resistance.

I-V graph of an LDR

At a fixed light level, an LDR behaves like an ordinary resistor: its I-V graph is straight and passes through the origin. Make the light brighter and the line gets steeper, because the resistance is lower. See I-V characteristics for the graphs of resistors, lamps and diodes.

Practise finding resistance from a graph in this question from the Electricity course.

Example 1: current through an LDR

An LDR with 6.0 V across it has a resistance of 4.0 kΩ in dim light and 300 Ω in bright light. Find the current in each case.

The current is about 13 times larger in bright light. Remember to change kΩ to Ω (× 1000) before you divide.

Uses of LDRs

AQA GCSE Physics names switching lights on when it gets dark as the application of LDRs you need to know. Common uses include:

• Street lights and garden lights that switch on at dusk and off at dawn.
• Automatic night lights that come on when a room goes dark.
• Security systems that detect when a beam of light is blocked.
• Light meters that measure how bright it is.

LDRs in a potential divider

An LDR on its own changes a resistance, but most circuits need a changing voltage to switch something on. So the LDR is put in series with a fixed resistor, making a potential divider. The supply voltage is shared between them in proportion to their resistances:

Example 2: a dark-activated light

A 9.0 V supply is connected across a 10 kΩ fixed resistor in series with an LDR. The LDR has a resistance of 100 kΩ in the dark and 1.0 kΩ in daylight. Find the voltage across the LDR in each case.

The voltage across the LDR is high in the dark and low in daylight. Connect the input of a switching circuit across the LDR, and the light comes on when it gets dark. Connect it across the fixed resistor instead, and it switches on when it gets light. Replacing the fixed resistor with a variable resistor lets you set the light level at which it switches.

AQA A-level Physics names LDRs, thermistors and variable resistors as examples in potential dividers. Try your own values with the voltage divider calculator, or build the circuit in the Circuit Builder.

Check your understanding.

LDR vs thermistor

Common mistakes

• Getting the direction backwards. More light means less resistance, not more.
• Saying an LDR produces electricity. It does not generate a current; it only changes its resistance. A solar cell is what turns light into electricity.
• Mixing up the symbol arrows. Arrows pointing in mean light is detected; arrows pointing out mean light is given out.
• Forgetting the units. LDR resistances are often in kΩ: convert to Ω before using I = V ÷ R.
• Putting the output across the wrong resistor in a potential divider. For "on when dark", take the output across the LDR.

Practise with a short timed paper

LDRs come up in GCSE Physics and in potential dividers in the first year of A-level Physics. The daily GCSE Physics Higher and A-level Physics: Year 1 papers are short timed papers of 8 questions, with a new one every day.

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