I can think of lots of exciting reasons for monitoring the temperature of something. It might be the sea temperature at your local surf spot, the swimming pool or perhaps your hot tub. Or, something mundane like the temperature of your house or hot water tank.
This time, it’s the mundane one. Some of the buildings at our offices are heated by an eco-friendly, but rather unreliable wood pellet boiler. We installed the pellet boiler five years ago, ignoring warnings from our plumber Tim, because we couldn’t stomach installing a nasty old oil boiler. Anyway, we still kind of love the wood pellet boiler, even though Tim was right: it’s kind of a high-maintenance relationship.
The Datacase logger is connected up to a very bog standard type of temperature sensor called an LM35. In this case I lazily bought the LM35 sensor pre-assembled onto a little circuit board for under a fiver on eBay. Like most analogue sensors, the sensor has three cables. One for giving it some voltage (+4.2v in our case), one for the ground (negative), and the third one which spits out a voltage on a scale between 0 and 4.2v depending on the temperature, which is what we want to measure.
The Datacase has a power take off for a sensor on a header pin (see instructions), as well as a ground, so that bit is fine. Then the output voltage reading cable is shoved into one of the analogue terminals. You can test it’s working with any multimeter on the voltage setting.
Then the LM35 sensor was fixed onto the outside of the box with a very elegant strip of gaffer tape, and then the whole box taped against a hot pipe coming out of our boiler (the sensor facing the copper pipe). And the Datacase’s 12v power supply plugged into a wall socket. Like so:
Now, logged into the Datacase app, to make the chart show Celcius rather than millivolts, the transformation formula is raw * (420 / 1024).
Lo and behold, we have a live chart of what our troublesome boiler is up to. The chart should update every 15 minutes or so.
It would be nice if, rather than hitting refresh all the time, we had an alerting feature so that if the temperature falls below a certain threshold, it would send me an SMS. But we haven’t built that bit yet.