Hi everyone,
We wanted to install a small walk-in cold room in our house, and I looked around at options for cooling. Commercial solutions are $$$, and the only reasonable choice for a small personal system is a window AC unit and a CoolBot, basically a small microcontroller that hijacks the AC unit's temp sensor by placing a small heater next to it that forces the unit to cool lower than normal—down to about 34°.
CoolBots run $330-$400US, but I found a few DIY builds posted here; mainly this one from 2015:
https://forum.arduino.cc/index.php?topic=328770.0
There are others too, but I couldn't find anything less than 3-4 years old; so I made my own. It borrows heavily from those other projects, but I added a set of fans that can be run if the outside air drops cold enough to warrant it. Basically there's a 4" duct that runs under the floor and a 4" exterior vent at the top of the room; each has a small fan attached to push air in and out (1 fan wasn't strong enough). You can set a differential temperature in the software so that, for example, if it's 20°F colder outside than inside, the system will run the low-wattage fans instead of the high-wattage AC unit.
My system is based on a NodeMCU ESP8266-E12 board and communicates with ThingSpeak. Eventually I want to keep my data internal, so I will probably implement an MQTT solution and OpenHab on my own server. All install instructions and advice on building a cool room can be found on the web or on the CoolBot website. My cool room is about 4' x 6' x 7.5', so I chose an LG 8000 BTU unit that runs on 120VAC.
The system has been running for a week, and it's great. Everything is working fine. I'm not an EE, and don't know much about transistors; I understand some of the power switching can be done that way instead of using relays. Also I guessed at the power resistor for the heater element; a single ceramic 100 Ohm 5W resistor with my 5VDC power supply would only get to about 90°F in a 70°F room; so I wired two in parallel. They're bulky. Someone with more experience in electronics could definitely spec something more elegant here.
I'll attach pics, my sketch and a Fritzing file and sample output from ThingSpeak.
Happy cooling!
CloneBotCooler_06_WiFi.ino (16.9 KB)