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Light-dependent Resistor

A light-dependent resistor (LDR) is a special type of resistor. In the dark, it has a high resistance. When light falls on it, its resistance becomes small.

A light-dependent resistor (LDR) is made of a semiconductor like silicon. In the dark, silicon has fewer electrons that are mobile, so conductivity is lower. When light falls on it, light energy causes more electrons to move freely, so the resistor becomes more conducting. This explains why the resistance becomes smaller.

This behaviour is different from metals like copper or iron. For metals, the resistance is the same in light or in darkness. (The resistance of a metal does change if the temperature is increased.)

To see the effect of the change in resistance, think of a circuit in which the current through the light-dependent resistor (LDR) remains roughly the same even when it changes. We can make such a circuit by connecting a very high resistance R₁ in series with the LDR.

When light falls on the LDR, its resistance falls. We would expect the current to increase. However, because R₁ remains large, the total resistance may not decrease much, so the current stays roughly the same.

Connect a voltmeter across the LDR. If its resistance falls, the potential difference also drops because of Ohm's law: V = IR. So in this circuit, when light falls on the LDR, the potential difference across it decreases.

This behaviour makes light-dependent resistor very useful. It is often used in electronic devices as a light sensor. For example, it can be used as part of a device to switch on a streetlight automatically when sun sets.

A simple way to do this is to insert an electronic switch, called a transistor, in the streetlight circuit. A transistor switches on the circuit when it receives a high voltage, and switches off when it gets a low voltage.

Connect the voltage from the light-dependent resistor (LDR) to the transistor. When sunlight falls on the transistor, its resistance falls. So its potential difference falls, and the voltage going to the transistor also falls. Then the transistor switches off, and the streetlight is off. This would happen in the day.

At night when there is no light, the resistance of the LDR increases. Its potential difference increases, and the voltage to the transistor increases. The transistor switches on and the streetlight lights up. In this way, the streetlight comes on automatically at night.