Back in Experimenting with Thermistors, I described the challenge I had with Bitcoin miners heating the warehouse area I use as a training space. Recently, we had three small upper windows installed that I can open. The original windows did not. In the Summer, I ran the exhaust fan in an ajoining room 24/7 to try to mitigate the heat. Now... cold weather is here and running the exhaust fan overnight with just the three small windows open nets me a space that is only a few degrees above ambient. In other words... it's cold! 55 degrees F is the lowest I've seen so far, but real Winter is coming. Turn off the exhaust fan and it could be 75 degrees F in a couple of hours. Too much swing and manually turning on and off the fan is just silly.
My first thought was to use one of the thermistors and control a VFD. The Facilities Manager had already gotten a quote to install one. $5,000 for the drive and installation and another $4,000 to wire it all up. STOP THE PRESSES. The VFD wouldn't work anyway because we have horizontal, gravity close louvers on the exhaust fan. We would never be able to take advantage of the variable speed. I told the Facilities Manager "I got this."
For the curious: 2.5HP, 3P 208/120v motor driving a 48" vertical discharge fan. Simple industrial Start/Stop contactor control.
After some deliberation, I opted to use a simple on/off thermostat and a Raspberry Pi Pico for control. The thermostat matches the other thermostats in the building so it's functional and asthetically matching. I went for the Pico because... all that power for $4. I did think about giving my DigiSpark a purpose but integrating the Pico was just going to be so much easier.
The system is simple. It translates the thermostat cool/off input to a start contact closure (in parallel to the Start button and sealing contacts) or a series, normally closed Stop contact. Each relay activates for three (3) seconds on state change. There is also a parallel 10 minute routine that executes Start/Run or Off based on the thermostat contact - just as a safety measure. It's been online for a week now and it holds the training bay to near 70 degrees F +/- 1 degree - from what I've seen.
I'm really happy with the outcome. There are (of course) upgrades that I want to do - like the inhibit switch - but for now... it's doing what it's suppoised to do. As a parting shot, huge kudos to the Pi Pico designers. I'm able to use the 5.08mm pitch terminals in an "every other" pin mode and get away with it on standard stripboard. Now to figure out what to do with the Pico W's that I bought.
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