It’s time to learn about a new component called a thermistor.
A thermistor is a type of variable resistor, whose resistance depends upon the temperature. This is the symbol for a thermistor and there are commonly two types of thermistors.
If the resistance increases as the temperature increases, then it is said to have a PTC or positive temperature coefficient. On the other hand, if the resistance decreases as the temperature increases, then it is said to have an NTC or negative temperature coefficient. BBox 1 contains an NTC thermistor.
The datasheet gives you the relationship between temperature and resistance and this is usually in the form of a table or a chart.
This means that you can determine the temperature if you know the resistance. The thermistor contained in BBox 1 has a resistance of 10K at 25 degrees Celsius.
Here’s what the circuit looks like. We have an NPN transistor that controls an LED. The thermistor and trimpot form a voltage divider circuit whose output is fed to the base of the transistor. We can adjust the value of R3 and use the voltage divider formula to set the threshold.
Let’s build the circuit and adjust R3 such that the LED is OFF. I’m using a candle lighter to simulate heat and the LED will start to glow as I hold the flame closer to the thermistor. The LED will continue to glow until the surface temperature of the thermistor drops below the set threshold.
This is how easy it is to build circuits that react to temperature. Let’s move on to the next project.