A little help needed designing a power button circuit for my PC

[Marmotta]

I'm currently planning a HTPC using an Xbox One case and am planning to keep as many original features as possible, including the Xbox One power supply and front power/eject/sync assembly.

I have built the touch sensitive momentary/latching circuit for the power button, but I need some advice on the startup/shutdown circuitry.

The Xbox One PSU outputs 12V, so I'm connecting the motherboard using a PicoPSU and will wire the 12V directly to the graphics card. Powering on is not an issue - pressing the power button will latch together the 5V standby and Sense outputs on the PSU and switch on the 12V; I will connect the motherboard power pins either with a slight timed delay directly to the power button's momentary circuit or to the 12V source via a relay.

However, I'm not sure about how to go about shutting down the system - I need to create enough of a delay between pressing the power button and the 12V cutting out for the PC to safely power down. I think maybe I could do something to affect the behaviour of the button depending on whether 12V is detected (i.e. the console is clearly in a powered on state) or not, but not sure how, maybe with a CMOS analog switch?

Thanks in advance for any help.

[wraper]

However, I'm not sure about how to go about shutting down the system - I need to create enough of a delay between pressing the power button and the 12V cutting out for the PC to safely power down. I think maybe I could do something to affect the behaviour of the button depending on whether 12V is detected (i.e. the console is clearly in a powered on state) or not, but not sure how, maybe with a CMOS analog switch?

Why don't you just do the right thing and use the PS-ON signal from the motherboard? No fixed delay (unless completely ridiculous) will predict when it is safe to shut down the power, as OS (guess windows) may decide installing updates or something else. Also, if power other than 5VSB is present before starting the motherboard, it may happen to glitch in various strange ways.

[Marmotta]

I was thinking about this and it's slightly complicated by the fact that the PicoPSU's only intake is 12V and I really wanted to avoid having to solder to the PSU or motherboard at all. What I might do though is design a circuit that will unlatch the Xbox One PSU's SENSE when a low signal is detected from the motherboard's 5V USB port connections, as it would mean the motherboard had safely gone into standby.

[sparx]

Why not find the "Power on" led - on the motherboard header and use that as a "power on" sense? It might go wacky in standby, but you could maybe sort that out also.

5V usb on all of my computers here remains on, even in complete shutdown.