Last night I began dusting off my solar project a bit. It still seems like we are at the bottom of the atmospheric river, so I don’t think it will be going outside any time in the near future, but I wanted to make some software changes.

It uses a pair of temperature sensors and records it to an MQTT server. It revealed that my garage seldom gets below 55 degrees or so (not too surprising, since the weather here seldom gets that cold, but we did have a couple of snaps down around freezing). Over the last 24 hours it’s recorded the following:

This data comes from a Microchip MCP9808 board, and the temperature reported by an MPL3115A2 barometric pressure sensor. I haven’t bothered doing a calibration to see which one is more accurate, but you can see that they seem to report the same values up to an offset. Both are more accurate than I really need.

Several years ago I didn’t have such sensors in my junk box, and tried just measuring the forward voltage drop across a very cheap 1N4148 diode. That “worked” after a fashion. It wasn’t especially sensitive, since each bit of the value returned by analogRead() turned out to be about 2 degrees C. The post generated some comments however, and pointed out that I could use either the internal 1.1V reference or the actual internal temperature sensor to get more accurate values. It was fun, but soon after I got real temperature sensors which promised still better accuracy, and I didn’t revisit the issue.

But today I found an interesting and different approach on the Cave Perl Project Blog. The ATmega328p used on the Arduino uses an internal RC oscillator to run the watch dog timer. It runs at around 110Khz, but isn’t especially accurate. It is… you guessed it… highly temperature dependent.  So, you can time the amount of time it takes the WDT to overflow, and with some math determine what the temperature might be. And it turns out that the results are likely sufficiently more accurate, perhaps down to around 0.1 degrees C, which is better than a lot of inexpensive sensors. There are some details which are interesting to read and think about, but I might have to try experimenting with this.

In any case, the website is actually pretty cool for other projects as well. It was founded to develop “a submersible data logger system for long term environmental monitoring projects” and has lots of other good data and ideas. I’ll probably be killing a few hours reading it tonight. I think you all could do worse than do the same. 🙂

Have a good one.