On one chipset I looked at, they use a GPIO signal and a GPblink
control bit causes the LED to flash. Presumably this is set
during the power down sequence, making it unlikely you could get
programmatic control of the hardware yourself.
You can make an adapter cable, to drive the power LED on your computer
case. You would need a disk drive power cable (like one of those
"Y" cables you can buy at Radio Shack, to take one disk drive
connector, and make power for two drives). By taking power from
the +5V and ground on the cable, and inserting a series limiting
resistor, to set the current level to the LED, you can get the
effect you are after. (LED lit when computer is running, and LED
extinguished when in standby.) The Radio Shack 278-767 cable
will save a bit on the cost of the cable.
Cut the "Y" cable, so you have an end that mates to a connector
on your power supply. Insert a resistor in series with one
of the leads - I like to do this on the positive rail.
X---Yellow=+12---
X---Black=GND----
X---Black=GND-------------------------------------> Power_LED_minus
X---Red=+5----------330_ohm_1/4_watt_resistor-----> Power_LED_plus
How you work out the resistor value, depends on the forward bias
voltage Vfb of the LED. A red LED is about 1.8 volts . A blue Gallium
Nitride LED is around 4 volts (values taken from HP LED datasheet).
The equation is: LED_current = ( 5 - Vfb )/ resistor_in_ohms
For the suggested values, (5-1.8)/330 = 9.7 milliamps, which should
be bright enough. Reducing the resistor value increases the brightness.
(Note - I guessed at 330 ohms as a starting point from experience.
Normally, you start with the value of current desired, and work out
the resistor from that. It just means rearranging the equation a bit.)
The power dissipated in the resistor is related to the voltage across
the resistor. Power = (V*V)/R = (5-1.8)*(5-1.8)/330 = 0.031W and
a quarter watt resistor has plenty of power rating for the job. The
resistors at Radio Shack could be 5% or 10% tolerance, and the
tolerance doesn't matter for this application.
Now, one of the nice things about using +5V from the drive connector,
is if you accidently reverse the + and - leads on the LED on the
computer case, you won't damage the LED. If we were to use +12V and
GND as a power source, if the power source got reversed to the LED,
then in theory the LED could be damaged. It seems the average LED has
a 5V reverse bias rating, and so by using the +5V supply and a
limiting resistor, there is no danger to the LED on the computer
case.
You may find that the leg of a 1/4 watt resistor, will fit nicely
into the hole on the Power_LED cable. In which case, you can draw
the circuit like this:
X---Yellow=+12---
X---Black=GND----
X---Black=GND-------220_ohm_1/4_watt_resistor-----> Power_LED_minus
X---Red=+5----------220_ohm_1/4_watt_resistor-----> Power_LED_plus
In this case, the current to the power led is (5-1.8)/440 = 7.3ma
The advantage of using two resistors, is if you select the
resistors carefully, the legs on the end of the resistor will
plug right into the Power_LED cable. (Snip off excess lead length,
so about 1/4" is left on the end of the resistor, enough to plug
into the Power_LED cable.) I selected the resistor value, based on
what I could find in the Radio Shack online catalog.
(Part number 271-1313). When you are at the store, examine the
diameter of the legs on the resistor, to make sure they are small
enough to fit into the holes on the Power_LED cable. (1/2 watt
resistor legs may be too fat to fit.)
Make sure everything is wrapped with electrical tape. If you want
a real professional look to the project, get two sizes of heat
shrink tubing. Smaller tubing can be slid over each individual resistor,
to prevent the wire ends from being exposed. A larger tubing can be slid
over both resistors, to hold them together and make them more
rigid and easier to plug into the Power_LED. Apply a gentle source
of heat to the heat shrink, to get it to conform to the objects
underneath. Never touch the source of heat directly to the plastic,
as it could get burned. You have to hold heat shrink tubing pretty
close to a soldering iron, to get enough heat to do the job, and
it is difficult to do it without touching the plastic to the iron.
It takes a bit of practice to get good at it. A heat gun with a
concentrator tip on it, might be a better way to do it, but I
don't own one of those. (Radio Shack 278-1627 is an example of
heat shrink A.K.A polyolefin tubing. I prefer the transparent
tubing type, as it is easier to see what is going on.)
Under no circumstances allow +5V to touch GND. An ATX power supply
can pump a lot of current, and you could either get burned or
start a fire. That is why I've enphasised the use of insulation
in the above paragraph. Make sure there are no exposed wires when
you are finished. You can use Ty-Wrap nylon retainers, to hold
the finished cables in place inside the computer, and arranged
the right way, they might even be used to prevent the resistor
legs from falling out of the Power_LED connector. Radio Shack
278-1656 is an example of some cable ties you can get.
I think just unplugging the cable is easier
Have fun,
Paul
