Thursday, 5 May 2011

Frankenstein's Thermometer

Not much more complicated than the simplest thing you can do with an Arduino is measure and display temperature. The picture shows such a device at the embryo stage.

The parts' list for this is:
  • 3x10k
  • 10k NTC thermistor (beta = 3977)
  • 100nF ceramic capacitor
  • 7x470R
  • Pushbutton switch
  • 7-segment LED display (common cathode)
The measurement is performed by a 10k resistor and the thermistor at the top of the picture. This is read by the Arduino on A5 and converted using the thermistor equation.

Each digit of the result is output on digital pins 2-8 in turn. A resistor is required for each segment of the display because otherwise the total current through the display would have to be shared between all of the lit segments; use whichever resistor-decade you have most of.

Finally, digital pin 9 is used to sense the state of the pushbutton. This allows cycling between centigrade, fahrenheit and the absolute temperature scale.

This circuit can be powered by 3v and draws between 15 and 30mA, depending on the digit displayed, indicating that the display is consuming the majority of the power. Thus, a couple of alkaline AAA cells would power this circuit for about 3 days.

If the thermistor is disconnected entirely, it displays -273C, which is nice.

Fritzing is here.

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