So there's a couple other instructables already about converting a chest freezer to a kegerator (aka keezer), but most of them use the expensive plug and play temperature controllers. If you're willing to get your hands a little dirty, cheap controllers to hardwire in can be found on eBay for ~$20. Not only do you get a nice digital readout with the current temperature and the ability to set to specific temperatures, you also have the option of adding a heat source as well to turn the freezer into a temperature controlled fermenter.
I purchased mine for $20 (Link removed due to seller jacking price up $100). eBay links don't last forever, but you can search for "digital temperature controller 110V." Many of them are designed to run on 220V, so make sure you get the proper voltage controller (110V in the US). This particular one allows for many customized settings include compressor delay, which is important as rapid on/off cycling can kill the compressor.
Dremel/rotary tool or jigsaw
Extra power cord
Step 1: Preparing the Freezer
Foreword: You need to be very careful about cutting through the walls of the freezer as most of the walls contain the cooling element. If you puncture a cooling element, all the coolant will leak out and you'll be left with a fancy box. In my freezer, the was an area in the bottom right side that contained the compressor, enough room to install the controller, and a thin wall obviously devoid of any cooling parts.
Measure the size hole you need to cut and trace it out on the wall. Double check you're not drilling through the thick part. Drill a hole in each corner and cut between them with either a dremel tool or jigsaw. If using a jigsaw, make sure the blade won't poke through far enough to hit and of the compressor lines. Make sure the controller will fit in the hole you cut.
An alternative to mounting the controller on the freeser is to use a plastic junction box and put the controller in the middle of the existing power cord. A blank faceplate provides a suitable surface to mount the controller. The finished results may not be as neat, but it gets the job done.
Step 2: Wiring in the Controller
The first thing is to cut the plug end from the existing cable (unless you're using the junction box, then cut somewhere in between). It's important to note which line is the hot and which is the neutral. The ground is usually smaller and pretty easy to identify. After cutting the cable, the wiring schematic shows the hot going to the thermostat which is in the panel I pulled out. I looked for the wire coming in from the power supply and after disconnecting it from thermostat, tested for continuity with the multimeter. Since the end connected to the thermostat is the hot, you should get no resistance on the other end of that same wire. Mark it with electrical tape or black marker.
The way the temperature controller work is by cutting on and off the flow from the hot to either a heating or cooling element. Both circuits are normally open. When the temperature gets too hot, it will close the circuit and allow electricity to flow to the cooling element (the freezer). If I was using the freezer as a fermenter I might also have a heating element like a small ceramic heater or dehydrator connected that would be turned on when the temperature dropped too much.
To wire it up, the incoming (black cord in my case) hot and neutral get connected to the appropriate posts. Here the hot went to post 5 and the neutral to post 6, as shown on the diagram. The hot also needs to be connected to the post to switching portion before reaching the heating/cooling elements. Because post 1 and 5 both need to be connected to the incoming hot, you need to either pigtail a split in or do what I did an jumper a second short wire between the two. Next you need to connect the outgoing post shown going to the cooling element to the line that goes to the freezer (white cord here). Although the diagram shown this to be post 2, I found this to be a misprint and that post 3 actually is for the cooling element. Initially the freezer came on when the temperature was too low, which could have had disastrous implications if I didn't catch the mistake. Post 7 and 8 get connected to the included temperature probe, it doesn't matter which leg connects to which post. The neutral going to the freezer should get attached to post 6 along with the neutral coming from the outlet. The grounds don't actually connect to the controller and should be joined directly to each other.
Before sealing everything up, plug in the freezer to make sure it is all working as intended. To set the temperature, press set, use the up/down arrows to select the temp, then press reset. Test both when the probe it too hot or too cold. You can hold it against an ice cube if you need to cool it down.
Step 3: Finishing Up
Run the probe up the back of the freezer and snake it under the lid. Duct taping it to the freezer is a good way to keep the cables managed. Fit the controller through the hole and use the orange clips to hold it against the wall. They can be a little tricky to get on, but keep at it. My controller operates in Celsius, so fridge temp is 4 degrees. If yours is in Fahrenheit you probably want to set it to 39 or so. This particular freezer was tall enough to kit kegs and carboys right off the bat, but I'll be installing a wooden collar and faucets as detailed in other instructables soon. In the mean time a picnic tap will get the job done. Good luck and happy brewing!
rpeterika made it!