Here's a video of the belt buckle in glorious action:
Step 1: Materials
Super Mario Bros. FX Keychain (available online, $5-10, I got mine at Hot Topic for $10)
On/off switch (I got 2 SPDT switches from Radio Shack for $2.50)
Belt buckle, belt buckle blank (available from craft stores), or wire hanger
Soldering Iron (& flux, solder, etc.)
Hot Glue gun
Dremel or some other tool for cutting the controller board
Wire strippers & scissors
Small phillips scredriver
Drill w/ 1/16" bit
Step 2: Theory
Since both circuits work on the same principle (using conductive buttons to complete circuits), we can use one board to operate the other. All we have to do is solder connections between the hot wire and the out wires for each button. (see pic) Then when a button is pressed on the controller, the keychain is bypassed and the controller button completes the circuit and sends a signal to the microchip on the keychain board, creating a sound effect. All of the buttons work the same way, so any sound can be wired to any button.
The second picture shows the sounds each of the keychain button makes and how I wired them to the controller buttons, FYI.
Step 3: Dismantlification
For the NES controller, cut off the cord connections at the board, but save the cord (we'll use the colored wires later).
The keychain is a bit trickier. Take out the external screws holding the housing together and the two smaller screws securing the circuit board in place. Cut the red and black wires leading to the batteries and then (carefully) pry out the speaker, which is held in place with a small amount of glue. This should free up the circuit board completely. Also salvage the batteries and the battery connections from the keychain. The connections can be removed by prying off the inside cover and using a screwdriver to pry them free.
Step 4: Attaching the Buckle Blank
Personally, I wanted to make a belt buckle that would fit on any belt. I went down to the local thrift store where they have hundreds of belts for $1.50. I found one with a buckle, and stripped it off of the belt. I had to clean it up a little (grind it to death) to make it flush with the back of the buckle. Then I just used some glue to attach it to the back of the controller and waited for the glue to dry (24 hrs....)
If you use my method, just be sure to not block any of the screw holes, as you may eventually have to take the buckle apart to replace the batteries.
Step 5: Trimming the Controller Board
Step 6: Preparing for Soldering
Finally, once all of your holes have been drilled, use a screwdriver to scratch the coating off the circuit, revealing nice shiny copper. Take the time to make a nice clean surface to make a good solder connection..
Step 7: Soldering the Button Connections
I won't go into the details of soldering here, but I will tell you a couple of things I learned during this project.
1.) You will need to cut wires from the cord that are about 3-4" long.
2.) Be careful when placing wires on the backside of the controller board, as there are supports from the other side of the controller housing that make contact with the back of the board to keep it in place. See the pics for details.
Step 8: Installing the Speaker
First, I trimmed the back cover of the controller a little bit. I removed a plastic piece that was in the way and drilled some 1/16" holes where the speaker was going to be to allow the sound to escape (even though the thing's pretty loud to begin with!).
Now superglue the speaker in place. It should fit snugly in the upper right hand corner, as shown below. We'll fit the rest of the components in next.
Step 9: Making the On/off Circuit
Using the red and black wires which come off of the keychain board, solder the black wire to one of the switch connections. Next, solder another black wire to the other connection on the switch. This wire will lead to the negative terminal of the battery. I duct-taped the batteries together to complete and test the circuit at this point.
Step 10: Installing the Battery Holder
I cut three plastic pieces to house the batteries, then glued the battery connectors to 2 of them. Once the connectors were in place, I soldered the red and black wires to them. Using the duct-taped batteries as a template, I finally glued the three pieces in place. I also used a small piece of foam to prevent any other movement of the batteries.
The important thing here is to make a sturdy structure to restrict the movement of the batteries and maintain a solid consistent connection. I apologize for the 2 MP pictures from my phone, but I hope you get the general idea.