If you need a simple PSU (power supply unit) for free (or almost) without worrying about burning it, the best choice is to modify an old AT computer PSU. You can use an ATX or an AT PSU, but AT's are easier to get for free and very simple to modify.

Opening its case

When opening it will be full of dust. It's easy to remove using a brush and a vacuum cleaner.

All AT PSUs have four wires connected to a big switch usually placed at the front panel of the computer case. 120/220VAC flows through those wires and when you push it all the current switches off, because of that you should make those wires shorter and screw that switch to the PSU case so you can press it comfortably.

Holes and switch

You need to make holes for the capscrews that fixes the strip to the case, for the power-on LED and a bigger hole for the switch.

Power-on LED

A 5mm red LED is used as a power-on indicator. It is connected in series with a 2.7kΩ resistor and both of them in parallel with the fan powered by the 12V line.

The resistor used is a bit bigger than usual because we just need the LED as indicator and we don't require a lot of brightness.

Output lines

Output lines given by a bunch of wires are connected to a strip. Each head of the strip has two screws: one of them presses the wire and the other will be used when connecting external circuits being powered by this PSU.

Finished PSU