# 4.2.4 The Light-Dependent Resistor (LDR)

### The Light-Dependent Resistor (LDR)

• A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
• Its resistance automatically changes depending on the light energy falling onto it (illumination)
• As the light intensity increases, the resistance of an LDR decreases
• This is shown by the following graph:
• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
• In the dark, its resistance is very large (millions of ohms)
• In bright light, its resistance is small (tens of ohms) Resistance of an LDR depends on the light intensity falling on it

#### Worked Example

Which graph best represents the way in which the current I through an LDR depends upon the potential difference V across it?  • As the potential difference across the LDR increases, the light intensity increases causing its resistance to decrease
• Ohm’s law states that V = IR
• The resistance is equal to V/I or 1/R = I/V = gradient of the graph
• Since R decreases, the value of 1/R increases, so the gradient must increase
• Therefore, I increases with the p.d with an increasing gradient
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