Tinkering with my 3D printer has taught me a lot about small electronics, but it's all been very specific to this one application. I wanted a better understanding of circuits, resistors, capacitors, etc... and how they work on a very basic level. So I picked up an Arduino and a bunch of components, and that's kept me well occupied for the last few weeks.
So far the Arduino's been tethered to a computer to get power from the USB port. That's been ok for now, but I have a few projects in mind where I'll want a battery-powered stand-alone rig. I had been able to wire a 9v battery directly to pins on the board, but using the barrel jack would be a better solution. I salvaged parts from a few other things and it looked like I could make the component I needed.
It was hard to tell which wires were which, so I tested for continuity between the ends of the wires and the inside of the barrel (positive) and outside (negative).
After soldering this to the battery clip I could attach the battery and test the voltage at the jack before connecting it to the board.
Seems to work, the next steps will be add an inline switch to turn it on and off more easily, and 3D print a battery case of course.