An example of a thermistor is seen to the left. They are made up of a mixture of sulphides or oxides or sometimes metals such as copper, iron or cobalt. They tend to be formed into a disc or a bead sealed with plastic or glass.

They have great resistance at low temperatures but when they warm up their resistance decreases rapidly. Current can then flow through them. This makes them ideal as one of the components for a temperature sensor.

Build the simple thermistor circuit below. When the thermistor is cool or cold the LED should not light because of the high resistance.

However, warm up the thermistor by blowing warm air from a hair drier across it. This should warm it sufficiently that in a few seconds the resistance will drop and the LED will light.

WHEN THE THERMISTOR IS COLD RESISTANCE IS HIGH AND CURRENT CANNOT PASS THROUGH. WHEN WARMED, RESISTANCE FALLS AND CURRENT PASSES THROUGH.

Circuit explanation in detail:

When the thermistor is warmed up by the hair drier its resistance drops, this will take a few seconds. As its resistance drops current begins to flow from positive 9 volts to negative 0 volts. Current flows into the base of the transistors allowing the LED to light.

The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.

2. What is the role of the preset resistor ?

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