I decided to abandon my own plans mainly due to time and cost and ordered the parts from SparkFun. What follows is essentially the same information you'll find on their guide but with a few of my own notes. I hope that you find my insights helpful and it will get your project off the ground without a hitch.
Step 1: Parts and Safety
Now a brief note about safety. Every time you work with electrical lines you may be risking your life if you don't use the right precautions. In general you should always employ a certified electrician but you can do this project on your own if you're careful. Absolutely ensure the plug is not connected to a live electrical socket when working on the relay, the outlet, or the extension cord at any point. Also, it's probably good practice to enclose any wires before testing. With that you should probably do just fine.
Step 2: Assemble the circuit
Assembling the circuit only takes a few steps. I've included pictures of them below and a list of how I built things, which was based upon the height of all the parts.
Put the relay on last because it gets in the way if you don't. You will find it to be a little tight against the two pin screw terminal, but that is ok because it still fits. You don't have to use the two pin screw terminal either and can opt to solder the extension cord directly to the board, but I decided against that for useability.
Step 3: Splicing the Wires
When you splice the extension cord wires you are likely to see one of two things. Either your cord has three different color wires or it doesnt, but there ought to be three or this project won't work. The three cords are as follows:
Your going to cut the extension cord about a foot from the end of the female plug. Then split the three wires about 6 inches down. Cut the black wire five inches from the end. This should give you about one inch attached to the cord and a 5 inch extension that will go from your relay board to the outlet.
Next strip and tin the end of all the wires. It's likely your wires are a collection of smaller wires, twisting them before tinning is a big help. Then lay out everything and check it before moving on.
Step 4: Assemble the Relay and Outlet
You're almost there! You need to attach the relay and the outlet to the extension cord you just prepared. Something to remember here is to thread the extension cord through the nail mount housing before you attach it to the relay and the plug. This is expecially important if you intend to solder the hot wire to the relay board. Remember, I decided against this in case I wanted to re-use the relays at a later time and instead used the screw terminals.
The GFCI outlet is the most important part of this entire apparatus. The reason you're using this instead of a different outlet is that it may protect your life in the case of an over-voltage event. For this reason I recommend that you actually take the time to read the manual that came with your outlet before connecting the wires.
I was fortunate that my outlet had color coded screw terminals on it. On mine the ground plug screw was green (for ground), the hot wire attached to the brass screw, and the neutral wire to the silver screw. Also, my wires connected through holes in the back of my outlet plug, not the exterior. I can't help you with your outlet, so again read the instructions.
Finally, cut three six inch pieces of 22-guage wire. I chose three different colors so I could distinguish them when hooking them up to my microcontroller. I suggest you do the same thing. Also, don't get them backward. I made two of these and accidentally hooked up the ground and +5V lines in the opposite position. It didn't hurt anything but I had to take the whole thing apart just to reconect the lines properly.
Step 5: Complete the outlet box
Step 6: Test with an Arduino
You are now done with your project. If you've wired everything up then you're ready to test out the box. I tested mine with my arduino. Below is some code you can use to test yours out too. In this case I connected up the red wire to +5V, the black to ground, and the green to digital pin 12.
Here's the code I used:
I hope this project worked for you. You'll find this useful in a number of great projects, so go out and make something fun!