Sunday, July 22, 2007

Temperature Logging Part III: The Exciting Conclusion

Some pics of the final state: You might notice an extra button. I decided to write to EEPROM rather than an array after all. The reason was an unreliable power situation. A 9V battery lasts for a few hours and I didn't want to lose hours of data at the last second. So we have a switch for power on/off, a button for "send data" and a button for "start collecting data". The last one is I can control when the EEPROM starts getting overwritten the next time I turn it on.

That relatively minor change forced a paradigm shift in what processing was done where. Originally, I was just reading the voltage ratio from the pin and sending it to the PC that did the work to convert to degrees. The pin reports a value in 10 bits (i.e. 0-1023) but the EEPROM stores only bytes (i.e. 0-255). I could have renormalized to the smaller range, but only at the loss of a lot of precision. Instead, I now calculate the temp right on the Arduino. Surprisingly (to me, anyway), you can use the regular math library functions like log(). So then I end up with a temperature that practically speaking won't get above 255 or below 0, and even if it did I could add or subtract a constant to recenter it to my working range. I will describe what temperature I was logging in another post, but for now here's the pretty graph.


  1. 95 degrees Celsius?!? Where are you keeping this thing?

    (Nice idea, though.)

  2. It was in here.

    And come look at more recent information on the logger.