I had an old ( no idea what year) chest freezer in my basement from when I bought my house and decided to convert it into a beer fridge.
Step 1: Get a Thermostat
I bought a cheap one off eBay. But I before I did that I set up a DHT21 with an Arduino and relay card that worked fine... It was just a little too bulky for the area I had.
Step 2: Cut the Wires
My freezer from the Mid 1990's had ac power going through a lamp into a thermostat/ potentiometer. From there it sent a signal into the compressor relay... So I have two power wires, 1 signal wire.
I cut them all.
Step 3: Install the New One
The ac wires (blue/brown) here in Sweden are the power white is my signal wire and that's that. Except the signal wire goes via a switch. This means you need to bridge power over to it too. I just made a jumper from one of the AC wires.
The other two black wires in the picture are the new temp sensor.
Step 4: Finish It Up
Now when I plugged in the freezer nothing happened... I needed to program the thermostat to react when it's too warm it's default was to do nothing despite I had set the desired temp.
The hole from the old electronics was too large so I just measured the hole and the new thermostat and printed a cover. It needs to be at least 3 or 4mm to withstand pushing of the buttons.
Step 5: Conclusion
I have this running several weeks at 10 degrees c and have not had a drop of condensation, it has held the temperature great ;however, when it turns off the compressor the temp continues to drop nearly 3 degrees so if you are sensitive to that it's important to know.