This is inspired by Realities' excellent idea, but I used a PC controller instead of a keyboard circuit board, eliminating the need for an elaborate flipper build.
Step 1: Parts!
I used a cheap $10 electronic guitar (you can find them just about anywhere... Dollar General has this particular double fret model =) and a PC game controller. I used an old Xbox controller that I had modded to use as a PC controller, but any PC controller should work. Dual stick models are great!
Step 2: Prep Work...
Take apart the guitar and gut the electronics. Lots of good stuff in there for other projects! Remove the frets and cut the wires from the frets as close to the guitar's PC board as possible. Go ahead and take apart your controller as well. If your controller has Force Feedback motors, you can simply snip those off without effecting how the controller works.
Step 3: Map Out the Fret Buttons
Use the continuity setting on your voltmeter to map out which wires go to which fret. Removing the top of the fret helps greatly, as you can also follow the fretboards PC traces. One wire will be a common ground (different colored), and the rest will correspond to each fret.
Step 4: Fit the PC Controller to the Guitar Shell.
Trim plastic on the guitar body shell as necessary until you can nicely flush fit your controller PCB into the guitar body shell. You'll want to fit it at an angle with a multiple of 45 degrees (i.e., 0, 45, 90, 135, 180, 225, 270, etc.). This makes use of the eight way position of your controller's joysticks, so that two of the positions will be "up" and "down" for the "flipper" picking used by the Frets on Fire game.
Step 5: Solder Time!
Solder the wires from the fretboard that you mapped out earlier to the gamepad's button pads on the controller's PCB. It doesn't matter which controller buttons you solder to, because Frets on Fire setup will automatically map the buttons you use in the game's setup!
Step 6: Final Check!
Plug in the controller cable, and go to the Game Controller Control panel in your PC. Press the five fret buttons you soldered to your controller to make sure they light up in the control panel.
Step 7: Hot Glue Alert!
Now that everything has checked out, Reattach the fret to the guitar body, then use copious amounts (lots) of hot glue to firmly attach the controller board in your guitar shell.
Step 8: Almost Done!
Use a circular file or Xacto knife to make a whole for the controller cord. I found that using some hot glue around the cord just inside where it fits in the guitar body makes a nice "stopper" to protect the cord from being accidently yanked out, just like the rubber stoppers used on regular controllers. Now, reassemble the guitar. Almost done!
Step 9: Cosmetic Time!
Your controller is perfectly usable at this point, but you can do some cosmetic work if you want your controller to have a "finished" look. Thin 1/16th or 3/32nd aluminum sheeting can be used, as it is easily cut, even with scissors. Plastic cut from coffee can lids, cheap tupperware, etc., also works great! I used colored tape on the frets to match the color scheme used in Frets on Fire, and hot glued two extra thin guitar picks to the flipper controller. The real beauty of this is one joystick becomes the flipper, and the other stick allows you to navigate through all the game's menus from your guitar!
Step 10: Thrash Away!
Have fun with your new controller. If you'd like to support this Instructable, and see my controller in action, then please watch the video! Thanks and have fun!
Watch the Video!