Here it is the simplified way to turn on an ATX PSU, If you jumper the pin13 and pin 14 the PSU will turn on.
For this purpose I use an MCU from Microchip the famous PIC16F84A (PIC16F84A-04/P).
The schematic
JP2 = Pin9 from PSU
JP3 = Pin7 form PSU
JP4 = Pin14 from PSU
The Source Code...