Momentary power then off ?

[Slider2732]

Bit of a perplexer for someone.

I have a mobo that doesn't have a 'Last State' entry in the BIOS.
The last one used with the project did and there were no problems.
The wish, is to have the mobo start up with other components at the flick of a mains switch. On other motherboards the setting means that when power is lost the mobo starts right back up again as soon as power is restored. As in, you can turn the PC off at the wall and then turn it back on along with everything else with the same one switch.

Things are complicated by the 4 seconds rubbish on most mobo's. It means that if the connection on the mobo for the on operation is shorted for more than 4 seconds the mobo powers down again.
So that defeats merely soldering the power on switch to on !

(the ATX power supply in the project already has its green sense wire shorted so that power runs directly and should power the mobo you'd think. But it doesn't, the mobo only powers up if the power switch is pressed while switching the mains power on!)

How would you do it ?

I seem to need a momentary on, that switches to off and stays off.
Is there a chip that can do that ?
A relay would flutter itself on and off or just stay on.

[Slider2732]

Got it!
Possibly.

A 555 timer can run at say 1 pulse per 2 seconds. In that way, it will power the power on, then not give a pulse long enough for the mobo to switch off again!
Might need to bring up the TTL output but it seems workable.

Will post if it works, in case anyone else has something similar as a problem.

[Slider2732]

YES!
Works great. Took some time to get a momentary on and then a long off, but here's the component values.
Pin 8 to 7 = 20k (red, black, orange)
Pin 7 to 6 = 10ohms (brown, black, black)
Pin 1 to 2 capacitor ended up being a 4.7uF, plus a 1uF piggybacked.
Pin 3 = output
Pin 1 = Ground
Pin 8 = +5v
Power is taken from the ATX connector block but underneath the mobo.

End result, as tested with an LED, is that the circuit shorts the power pins on the mobo for about a second and then never seems to retrigger. The time gap is huge enough. When it does finally trigger the capacitor release stage will just give that 1 second short and back to off again.
Took the LED off and fitted the circuit.

This was where I met some troubles. The mobo booted, but, reset every 4 seconds *groan*
The fix, was to reverse the polarity of the originally fitted LED, except I used a diode. It was found that the mobo powered with just the Pin 3 output of the 555 connected to one of the power on pins. So, by connecting the diode across the two pins, so that whatever times the 4 seconds was permanently denied, the circuit worked !

[ceweberdasi]

Wow you post everything yourself.

[Slider2732]

Yep, in the hope someone found the idea useful :)

There's a simpler way, as discovered later.
If you put a 10uF capacitor across the start up switch pins on the motherboard !
A capacitor is usually in an uncharged state, as in, it will be like a closed switch to the motherboard sensor. The mobo will boot. The capacitor will quickly charge up and therefore conduct electricity through it, which won't short the circuit back to powering down after 4 seconds.

The method works quite well and is very simple, but needs sometimes a few minutes for the capacitor to drain, before attempting powering up again.