Hi everyone, this is my first walkthrough, hope You'll enjoy ;-)
___Where the story begins___
I wanted to use a PC PSU as power supply for my works and I was looking for a clever power switch circuit: a circuit which works like most of PCs where if You click once it powers up, then if You hold it again for a while it turns off. After a long time projecting the circuit, I received the actual idea which seems to work fine.
___How does it works___
I started putting a NPN transistor between the PS-ON pin, the green one which number is 14 or 16 depending on ATX connector format, and the ground pin.
The transistor base is driven by a PNP one, trough a 47k resistor. While the power is down, the only source is the Stand-By 5V purple pin, the 9th, which will saturate Q1 as Q2 is pulled down by the non-charged capacitor C1.
Now the circuit is steady powered by the standard 5V red pin until we hold again the button: the R-C circuit charges C1 which will positively polarize Q2, breaking the power to Q1, powering down the whole circuit.
Obviously D1 prevents the 5V source charging the condenser.
The overall current drainage is ONLY 100uA! ;-)
Hope this is helpful ;-) Please, tell me if the explanation is too complicated ;-)