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. The mounting hole is sized to fit a 6-32 screw so that you can mount the board on an unused standoff.

I’m using one of those common AT-to-ATX power adapters, which already breaks out the ground and PS_ON wires. If using one of these 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 bench tested and found to work as intended. The only noteworthy thing is that when the power is cut the circuit will stay latched to ground for about 8 seconds.

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