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toggle on/off

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faceless105

n00b
Joined
Jun 26, 2006
Messages
37
i was talkin in another forum and someone mentioned using a hard on/off switch as a power switch for their computer and using a relay to do it. I've never heard of that before so i was wondering if anyone could elaborate on this? I was under the assumption that you would always need to use a momentary switch but if not i'm pretty curious to how this would work.
 
You can replace the momentary switch with a DPDT toggle switch, a 5V relay, and a capacitor. The down side of this is the switch can become confused whether "UP" is on or off if you pull the PC power cord or have a power outage.

I haven't done this before, but it theoretically it will work. The relay must not have a protection diode on the coil, and the capacitor will probably be several hundred microfarads (uF) and should be a non-polar type. The relay coil resistance and desired contact closure time will determine capacitor size.

Picture the diagram below as being the six contacts on a DPDT switch (I don't have my CAD program running):
1-O O-4
2-O O-5
3-O O-6

You would wire 1&6 to +5VSB (standby) on your PSU, and 3&4 to Ground. Wire 2 to one side of the capacitor, and 5 to one side of the relay coil. Wire the other end of the capacitor and relay coil together. And connect the normally open relay contacts to your MOBO power switch pins.

Theory of Operation: Starting with the switch in the "up" position when you plug in the power cord, the capacitor will quickly charge through the relay coil causing the relay contact to momentarily close, turning on the PC. When you flip the switch "down", the capacitor will quickly discharge and then charge in the other direction, again causing the relay contact to momentarily close, turning off the PC.

Nothing better to do at 4:30 am. ;)

Edit: You need to use the "Instant Off" power button option in BIOS for this to work.
 
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