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September 15, 2006

Bottle Cap PC Thermometer

Based on a circuit I found in this website, I built a very simple thermometer. The major thing I added to the original design is a nice case.

  • 1 Diode
  • 1 Thermistor
  • 1 Capacitor
  • 1 Serial cable (or plug)
  • 2 Plastic bottle caps (one slightly bigger than the other)

The CircuitI salvaged all the parts from old electronics (found in garbage) so they were free. All the parts are very easy to find except for the thermistor. I did not get precisely a thermistor but a very close approximation to it. In fact, I’m not very sure of what it is that I used.

I just soldered the components together, no need of PCB.

My Thermistor

I got the thermistor form a broken drinking water dispenser /cooler.

The relationship between the temperature and resistance in an usual thermistor is mostly linear. For my thermistor, the lower bound for the linear region of the resistance-temperature relation is 11°C. Below this point, its resistance goes to infinity (is doesn’t allow any current to pass). I suppose this is used to stop cooling the water when it gets at the desired 11°C temperature. I haven’t yet found an upper bound for the liner region.

Anyways, the only problem is that it can’t measure temperatures below 11°C.

The Case

To build a nice looking case, I simply used two plastic bottle cap (from a Propel and Powerade bottles) that my girlfriend brought me from the recycling bin at her job (a gym).

These two caps happen to fit perfectly one inside the other. So I simply cut a hole for the cable and drilled another for the probe (MT). I placed the circuit (properly insulated with tape, of course) inside, closed it and the thermometer was done.

The Software

In order to run use the thermometer you need a small program. I used the program provided in the original project but it wouldn’t be hard to right a new one or to improve the existing one.

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