I posted this in a reddit comment, but I wanted to have it on this blog for when that thread fades into oblivion.
Hands down, this is the most useful thing I have created in my time brewing.
I set up a ESP8266 chip with DS18B20 temperature probes. I use the MQTT protocol, which allows for internet connected devices to communicate with each other, to send temperature data to a RPi that is running an Apache server which displays a graph of the temperature over time. I can access this data at any time on my local network.
I had done basic Arduino projects up until this point but I had very little trouble setting it up. You can wire up multiple temp sensors in parallel if you have multiple fermentations going on. I use stainless steel thermowells to get the temp inside the vessel. I also use this set up while I am mashing to monitor my temps from a different room.
Here’s some of the links I found useful while researching all this.
A basic wiring guide
I can provide you with my code if you do decide to go down this route.
Here’s what one of my graphs looks like for a mash. The temperature jumps around due to me moving the probe and stirring.
There are definitely a lot of improvements I could make. For one thing, the RPi doesn’t save a history of the temperatures it receives, so it will only display data while I have the page up. I have some ideas about how to save this data, just haven’t attempted anything yet.