Basically, the project involves modifying the neck of the guitar to add strings and frets to it. There are two ways of doing this: replacing the buttons with wood and frets, or replacing the neck with a real guitar neck. I did a button replacement, which is most likely somewhat cheaper but much more time intensive, whereas using a full real guitar neck would be more expensive, but give more professional results. I'll show both ways of doing it, though, so if it's easier to get your hands on a suitable guitar neck, I'd recommend that approach.
Step 1: Materials
1 guitar hero controller built out of a real guitar body - I'm using the Ashely International Rock Axe controller. -$80
1 Arduino microcontroller - $30
Guitar Strings - 6 is the usual number -$8
6 silicon diodes $3
If you're modifying the existing neck
1 block of wood big enough to fill in the area of the buttons - Ask for a piece of scrap walnut or maple at your lumber yard, it should only cost $2-4
Guitar Fret Wire - This can be purchased online for around $6
"If you're using a new neck"
1 guitar neck - Prices can range dramatically; I'd suggest looking for factory seconds or on craigslist
Lacquered wire - This is easiest and cheapest to find in the quantities we'll need in old headphones
Step 2: Overview and Disassembly
To start, we'll take apart the guitar. This means unscrewing the neck from the body and unscrewing the faceplate of the guitar so we have access to the body cavity. Once everything is unscrewed, unplug the wires leading to the neck of the guitar from the controller's circuit boards.
If you're just replacing the buttons on the existing neck, the next few steps will guide you through that. However, if you're using a new neck, jump ahead to step 6.
Step 3: Remove the Buttons and Measure
Unscrew the plastic pieces that hold all the buttons. You'll probably have to use a small screwdriver to gently pry them out of their holes. Next, using a piece of paper or your favorite way of measuring things, make an outline of the hole in the neck where the button used to be. I used a small scrap of paper, pushing it into the corners of the hole with my fingernail, to wind up with a piece of paper folded into the outline of the hole.
Trace the shape of this hole onto the block of wood you'll be using to fill in the hole.
Step 4: Cut the Wooden Plug
Step 5: Add Frets, Wire It
1st fret (already installed): 3.6 mm
2nd fret: 7 mm
3rd fret: 10.3 mm
4th fret: 13.2 mm
5th fret: 16.15 mm
6th fret: 18.95 mm
Insert the plug into the neck so that it fits flush with the face of the fretboard and then using the blade of a coping saw, cut thin grooves along the places where you marked. Your guitar frets will go in here.
Now, to wire up the frets. You can either just cut and hammer in the fret wire now and put the wires along the side of the guitar, or you can try to wire them from the middle of the fretboard. To wire them from the side, jump to the next step; otherwise, gently remove the wooden plug block and cut some notches in the side of it where you cut it for the frets. Cut lengths of fret wire as wide as the neck of the guitar, center them in the block, then using a piece of wood to protect the fret, hammer in the fret wire (the wood prevents the fret wire from being damaged by the hammer). Now, cut some long lengths of wire and where you cut the notches, solder one end of each wire to each fret.
If your guitar has a first fret already installed, you'll want to cut a channel from the button hole to the first fret, allowing you to solder in a wire to that fret as well. Now jump to step 7.
Step 6: Wire the Frets
Along the side of the neck, between the neck and the fretboard, use a circular cutting disk with the rotary tool to cut out a small groove, approximately 1/8" deep. Make sure you cut this on the side of the guitar that will face the ceiling; the model in the picture was accidentally cut on the wrong side. This will be used to carry the wires from the frets to the body of the guitar, so it's best to leave the side your fingers will be sliding along alone.
After the groove is cut, either with a drill bit and rotary tool or small screwdriver, dig away at the wood beneath the first 5 frets. The goal is to have access to each of these frets beneath the fretboard so you can solder wire to each fret without the solder or the wire sticking out from the neck of the guitar.
Now cut the lacquered wire into 5 strips, each long enough to reach at least the middle of the guitar body - it's good to have extra to make sure we can get everything in place. To remove the lacquer on the tip of the wire to be able to solder it, hold the tip of the wire over a cigarette lighter or match and burn off the lacquer, then scrape the ash off briefly with your fingernails. One at a time, solder the lacquered wires to the frets, making a tape flag at the other end of the wire identifying which fret each wire connects to. Finally, use tape or wood putty to cover up the wires.
Step 7: Attach Some Strings
Using a bit large enough for the rings at the end of the guitar strings, drill some holes into the back of the bottom of the guitar's neck. Make sure the holes are distinct; the strings need to not touch each other. Using a razor knife, cut some grooves leading to the bottom edge of the guitar for the strings themselves to lie in. In the proper order, put the strings into the holes, and then screw the neck back into the body. This might take a few tries, since the strings will probably want to pop out of their holes when the strings are bent upon insertion, but it can be done.
Now, cut a small piece of wood or acrylic to act as a bridge. It will sit between the bottom two frets and hold the strings off the frets. It should be about 1/4" tall. You can try and glue it in place, but the tension you'll be putting on the strings will hold it in just fine. Now, put the strings into the tuning pegs and tighten them up a bit. Don't do it too hard; if you break a string, you'll have to take the neck off and replace it.
Finally, tightly wrap some wires around each of the strings. Don't solder them, since the heat from soldering tends to weaken the string, making it snap a lot easier. However, also make sure that none of the wires touches any other string.
Step 8: Wire It to the Arduino
Next, connect the wires from the frets to pins 2-7, (although again, you'll probably wind up doing some software editing anyways).
Finally, take the ends of the wires coming off the strings and solder them to the diodes. Make sure you wire them so that the current can only flow away from the string side. Plug each of these diodes into the arduino on ports 13-19 (or whatever), again making sure that the diode is soldered such that current can only flow into the Arduino from the strings.
Finally, find the power switch and solder two wires from the power switch to the Arduino, making sure that the Arduino only gets power when the switch is turned on (Otherwise, your batteries will always be draining).
Step 9: Program the Arduino and Test
Once you've tested it in Arduino, plug the Arduino assembly into the controller circuitry with the cut-off cable you attached, and test it on Guitar Hero.
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