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to the other pin.
If you put a capacitor across these pins, as in Figure 4-10, when the motherboard first gets standby
power (the trickle of 5V power from the power supply when the computer is off but plugged into a wall
socket), the power switch will also have power. The capacitor will start to fill up, and during this filling
period the other power pin will feel a surge in voltage, turning on the computer. Once the capacitor is
filled (within a second or less) it doesn't change the voltage, so the computer will remain on until it is
manually shut down or the power to the unit is cut.
In my experience, a 10uF capacitor works well with the EPIA M2 motherboards. Increasing that
number increases the length of time for which the button is "pushed"; you may need to experiment
with a couple of different capacitors for your particular motherboard. The capacitor has a little + and
- diagram on its label, and these should correspond to the + and - pins on the power header on the
motherboard.
This capacitor trick can be used for other soft power devices, too. For instance, some video displays
have power switches and don't automatically turn on when power is supplied. To solve this problem,
you simply solder a capacitor across the power switch. Now, instead of having to hit the switch, the
unit will always turn on when it receives power.
4.4.4.4 Shutting down.
Although both the relay and capacitor solutions get the computer turned on, you also need some way
to turn it off.
Manually powering down in software is one approach. If you choose "shut down" in your OS, the
computer will turn off. If you used a capacitor to power up, it won't power on again until you turn the
car off and on again. If you used a relay circuit and the car is still running, the computer will shut
down and the WOL relays will cause it to power right back up, as mentioned earlier.
Figure 4-10. A 10uF capacitor acting as a power switch