Step 1: What It Does and How It Works
I didn't have a spare one to cut up, and they'd only just gotten rid of all the broken equipment in my music department, so I had to use a working cable, which I didn't want to destory.
All I needed to build then was a switch which connected the wires of a socket together, and I needed to put it in one of the 9V battery cases I had lying around.
The cable I chose was the one to my violin, which ha a much smaller, headphone-sized jack on the other end, which was better because I didn't actually have any bigger sockets around, nor would they fit in the little container. If you do have a broken lead you want to reuse, feel free to ditch having a socket on the pedal and just attach the wires directly to the switch.
So yeah, you'll need:
A used 9V battery
A push switch (I had a few left over from an old, failed project)
A socket if you want to not destory your cable (I desoldered it from a piece of TV equipment, they're pretty easy to find)
Tape, wire, etc.
Step 2: Make Case
I'd already done that and used the useful end for another project, so I'll be using tape to close it up, but if you hold onto both ends you can have it still look like a battery when you're done.
Step 3: Build Circuit
Solder wires onto the switch and fold out the legs so they don't touch the case when inside. Solder one wire to the gound (that's the pin that touches the outside of the socket) and the other to the rest of the pins.
Or if you're just going straight to the lead, chop the lead's head off and seperate the two wires which'll usually be bunches of fine copper thread. Twist the both bunches and solder to the pins on the switch.
Plug in your device to your keyboard and see if it works.
Step 4: Fit Switch to Case...
So drill a hole in the case on top, and make a cut so you can pull the flaps back to get the switch in place and then close the flaps again.
Once you can get the switch in place, you can make a hole in the cap and put the socket through, and glue that in place.
Step 5: Stick It All Together
Alright, so it's a piece of crap. But it's functional though, and that's what matters. I'd like to think that at least SOMEONE learnt something from this instructable, even if it's only that you only need to connect the wires to activate the pedal. I'd originally thought it sent data or something.
You could attach it to two pieces of board and a hinge to make it more pedal shaped, but I didn't bother.