This is a typical pin layout for the ATX, although not all are the same. By jumping one of the com pins to the PS On pin the ATX powers on. I made this connection with an on/off switch and used one of the 5Vdc pins for a standby LED indicator. The PWR OK pin is used for a second LED indicator. The end result is an extremely useful, although bulky bench power supply that inludes terminals for 3.3Vdc, 5Vdc, 12Vdc, 24VAC, 80-120VAC.
A Better Power Supply
Inspired by _http://www.instructables.com/id/DIY-Small-Bench-Power-Supply/