This is a simple circuit that will turn on an ATX power supply using a momentary switch. I made this specifically to use an AT style motherboard in an ATX case, however, you could use this generically in any case that you want to turn an ATX PSU on/off using a momentary switch.

The mounting hole is sized to fit a 6-32 screw so that you can mount the board using a standard motherboard standoff.

If using one of those common AT-to-ATX power adapters, you will most likely have to add a pin/wire for the +5VSB signal.

A parts list is silkscreened on the back of the PCB. Resistors and capacitors are 0805 size. You can use standard 2.54mm single row pin headers for the switch and PSU connections.

This has been tested using an AT motherboard and ATX power supply for several hours and found to work as intended. But as always, use at your own risk.

This circuit was taken from a post found on Stack Exchange. More info can be found here: https://electronics.stackexchange.com/questions/186841/momentary-button-as-switch