If you regularly hit up thrift shops or garage sales, then you know that used electronics often don't come with the right power supplies or batteries. Make Magazine shows off how to build a simple pocket-sized power supply so you can generate power on the go.
You don't need much in the way of materials to get this to work, just a few capacitors, resistors, a perf board and a 9V battery connector. When you're done, you'll have a tiny little power supply that you can fit in your pocket that you can use to test all the weird electronics you find on your shopping adventures.
Pocket-Size Power Supply [Make Magazine]