Today I was setting up my old G3 Tower when I realized that where I had to put it would make it hard to reach the power button. So, instead of hauling out the crappy original USB keyboard, I decided to make a separate button.
Step 1: Materials and Tools
I used one of those USB Smart Buttons to do this, but you could use anything with a momentary switch, even make your own if you wanted. You will need a screwdriver to take the button apart and some standard soldering supplies. I also used another USB cable because the one already in the button wasn't color coded properly and didn't have the shield wire running though it.
Step 2: Gutting
Now you want to take the button apart, being careful not to loose the springs that are around the screw posts. This is pretty easy. Now, with the button apart, cut the four wires coming from the USB cable and remove it.
Step 3: Preparing the Cable
If the cable isn't shielded, you will need to get another cable like I did. For the other cable, cut the end off and run it through the hole the old cable ran though. Strip the insulation off and get some of the shielding together and twist it. Strip the white wire and cut off the rest of them, we won't need them.
Step 4: Soldering
Ok, now that you have the USB cable ran through the hole in the bottom and have the wires prepared, solder them to the switch, with the shield being on one side and the white wire on the other. After this you might want to add some electrical tape under the wires like I did, if you have too much excess wire.
Step 5: Reassembly
Now, put the switch back together. Run the USB cable back through the clip on the back and out the hole in the side of the base. Now plug it in to your tray loading iMac G3, early slot loading iMac G3, or PowerMac G3 Blue & White and push it! If all went well the machine should power on.
NOTE: While highly unlikely, it is possible that you could fry your machine's USB controller if you hooked this up wrong. This will only work to power up the computer, and only these models. The early Yikes! G4 machines might be able to use this. This is the exact same way the Apple USB keyboard turns the machines on, but still, if you mess your computer up I am not responsible.