Running some RGB LEDs had me thinking about alternate sources of power. Specifically I need a mix of +5v and +12v to power different types of LEDs. Old ATX computer power supplies are a good/cheap source of power because they are easy to harvest from old PCs. They're also usually rated for high wattage.

After some research I found that you need to do a little work to turn on a PC power supply that's not hooked up to a motherboard. Specifically you need to short the green PS-ON pin to ground. This simulates pressing the power button on your PC and triggers the supply to turn on. You can tell the power supply is on when the fan starts to spin. This can be accomplished by cutting the green and (any) black wire out of the ATX connector and connecting them together.

Alternately the power supply will supply +5v of power even in off or standby mode if you use the purple +5vSB line. If all you need is a small amount (less than 2 amps) of 5v power using the +5vSB line is a great alternative and will be quieter because the fan will not be on.

Once the power supply is on you can use the other wires as follows:

Other pins are less useful but available:

Be sure to check the amperage ratings for each voltage on the side of your power supply before use.