2.6 LDRs and thermistors

Sensing components

Some components change their resistance when their surroundings change. They are used as sensors in circuits that switch things on and off automatically.

Light-dependent resistors (LDRs)

• In bright light the resistance of an LDR is low.
• In the dark the resistance of an LDR is very high.

So as the light intensity increases, the resistance of an LDR decreases.

Thermistors

• When the temperature is low, the resistance of the thermistor is high.
• When the temperature is high, the resistance of the thermistor is low.

So as the temperature increases, the resistance of this type of thermistor decreases.

Why the current changes

The potential difference of the supply is fixed. Using I = \frac{V}{R}:

• When the resistance of a component falls, the current through it rises.
• When the resistance rises, the current falls.

Worked example. A thermistor is connected to a 6.0 V supply. At 20 °C the current is 0.0030 A, so its resistance is R = \frac{6.0}{0.0030} = 2000 \text{ Ω}. At 60 °C the current is 0.012 A, so R = \frac{6.0}{0.012} = 500 \text{ Ω}. The resistance fell as the temperature rose.

Uses

For example, in an automatic night light the LDR's resistance becomes high in the dark. This changes the current in the circuit so that the lamp switches on.

Why does the resistance change?

LDRs and thermistors are made of semiconductor materials. In the dark, or when cold, very few electrons are free to move, so the resistance is high. When light falls on an LDR, or when a thermistor is heated, more electrons gain enough energy to become free, so more charge carriers are available and the resistance falls. The type of thermistor that you need to know about is called an NTC thermistor (negative temperature coefficient), because its resistance decreases as its temperature increases.

Calculations with R = V/I

You can find the resistance of an LDR or a thermistor by measuring the current and the potential difference.

Example. An LDR is connected to a 9.0 V supply. In the dark, the current is 0.10 mA, so R = \frac{9.0}{0.000\,10} = 90\,000 Ω (90 kΩ). In bright light the current is 5.0 mA, so R = \frac{9.0}{0.0050} = 1800 Ω.

Everyday uses of LDRs and thermistors

Reading the graphs

For a thermistor, the graph of resistance against temperature is a curve that falls, steeply at first and then more gradually. It never reaches zero. The graph for an LDR is a similar curve against the light intensity. In an exam, read values from the graph and use them with R = \frac{V}{I} to work out the current or the potential difference.