V. Ryan 2010


A thermistor is another analogue device, that can be used as an input to a microcontroller circuit . A thermistor’s resistance varies, determined by temperature. A 30R @ 25oC thermistor, will have a range of resistance, from 37.13 ohms to 3.26 kilo ohms.

The animation below shows the range of resistance of a 30R @ 25oC thermistor. The thermistor can have numerous resistance values, depending on the temperature applied to it.
Using Circuit Wizard software, the resistance value / temperature of a thermistor can be altered.

The 30R @ 25oC thermistor has a resistance of  37.13R resistance at 80 oC.

The 30R @ 25oC thermistor has  resistance of  3.26K resistance at -20 oC.

The resistance is altered by using the computer’s mouse, to raise or lower the thermistor’s resistance/temperature.
The circuit below, shows a thermistor being used as an input device, connected to input A/D 1. When the resistance /temperature of the thermistor falls within a certain range, the LED illuminates and the solenoid energises.

Diagram ‘C’ shows the complete circuit, the resistance value and temperature of the potentiometer, analogue sensor, as the LED illuminates.
The 3D circuit (below) is a GENIE E18 PIC microcontroller circuit, similar to the circuit diagram, shown above. The thermistor is the only input. When the temperature rises and falls, its resistance varies immensely. This means that a thermistor can have a large range of settings, making it ideal as an analogue sensor.
Circuit Wizard ensures that programming an analogue input is easy and straightforward. The flow chart below, has been constructed to control the GENIE E18 microcontroller circuit (above).

The thermistor analogue input, is connected to A/D1. This input is continually monitored by the GENIE E18 (see Analogue box of the flow chart). When it’s resistance falls below 950R ohms (2oC), the microcontroller outputs current at Q0 and Q7. This means that the LED illuminates and the solenoid energises.