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 situation that you want to turn an ATX PSU on/off using a momentary switch.

The mounting holes are sized to fit a 6-32 screw so that you can mount the board using a standard motherboard standoff. There’s a stop mask ring around the mounting holes so that a ground connection can be made between this PCB and a chassis.

I’m using one of those common AT-to-ATX power adapters, which already breaks out the COM and PS_ON wires. If using one of these adapters, you will most likely have to add a pin/wire for the +5VSB signal.

This is an updated version of the PCB found here: https://oshpark.com/shared_projects/h6LgDD3k

This has been bench tested and found to work as intended. It has also been used with a baby AT motherboard and ATX power supply for several hours and found to work as intended. However, as always, use at your own risk.

The latching 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

All components have a footprint to use either a through hole or surface mount part. Use either through hole or surface mount for any one part, not both!

• U1 - 74HC04
• R1 - 1 k ohm
• R2 - 100 k ohm
• R3 - 10 k ohm
• C1, C3 - 100 nF
• C2 - 10 nF

All resistors have footprints for through hole (10mm lead spacing) or 0805 chips. 1/4w resistors should fit fine. All caps have footprints for through hole (7.62mm/300mil lead spacing) or 0805 chips. Sorry for the mismatch, those are just the footprints I like to use that are included with Eagle.

R4 and R5 are current limiting resistors for LED1 and LED2 respectively, be sure to chose values that respect that max current output of the 74HC04. For reference, because LEDs are so blindingly bright, I’m using 1k5 for a red LED and 4k7 for a green LED.

LED1 and LED2 have pads for 5mm through hole (3mm should also fit) or 0805 surface mount parts. You can simply omit R4, R5 and the LEDs if you do not wish to use them.

LED1 will illuminate when the PSU is in standby. LED2 will illuminate when the PSU is turned on.

J1 controls how LED1 will operate. If you close pads 1&2 then LED1 will illuminate when the PSU is in standby but shut off when the PSU is powered on. If you close pads 2&3 then LED1 will stay illuminated when the PSU is in standby as well as powered on.

Use standard 2.54mm single row pin headers for PWR_SW, this is what an ATX style computer case will use for its power switch connection.

You can also use 2.54mm pin headers for the +5VSB, COM and PS_ON signals, but I found it to be annoying due to the weight of those wires. I ended up using a 3-pin JST-XH connector here, they are also 2.54mm but more secure since they snap it. Or you could just hardwire in these signals.

A smaller SMT-only version of this PCB can be found here: https://oshpark.com/shared_projects/f07pnuO2