I couldn't come up with a better title...but here's the explanation.

Basically, I need to create a small circuit for my PC to attach to the power LED. The way my motherboard works is that if the system is powered-on then the LED is ON. But when the system goes to standby the LED starts blinking.

I want to do two things:

  1. Have two separate color LEDs for each state i.e. powered-on(green) vs standby (amber).
  2. I want the standby LED to slowly fade-in/out (not fully out though, half-way).

I figured that I can make the blinking signal fade by attaching a capacitor and a diode (to prevent current going back to the motherboard). That should take care of standby light.

However, I can't figure out how to switch LEDs when the current goes from continous to intermittent. So when the system goes to standby the LED switches from green to amber. Any ideas? Btw, I don't want a expensive or complex circuit. I need something that will Not require any outside power-source and will work just from the current coming for the power-LED. Someone suggested me to use two RC circuits as such:

Explanation here: http://forum.allaboutcircuits.com/showpost.php?p=411691&postcount=8

  • \$\begingroup\$ It's really hard, because different colors require also different voltage...the red will be stronger than the green, even if it will fade after... \$\endgroup\$
    – clabacchio
    Feb 4, 2012 at 9:19
  • 1
    \$\begingroup\$ I've heard that fade-blinking effect called "breathing". \$\endgroup\$
    – jdh
    Feb 8, 2012 at 4:03

1 Answer 1


First you need to detect the blinking/not blinking.

An RC lowpass filter followed by a comparator would do this.

Then you need to use the comparator output to switch between the LEDs.

I have thrown together a quick circuit that should work (sorry for the mess but I'm rushing at the moment):

PC LED power

And the simulation:

PC LED power sim

The pink line is the PC power indicator in, you can see it starts to toggle after around 2ms (I forgot to expand the time realistically, sorry - depending on the frequency of the flashing you will need to adjust R1 and C1 - probably 10k and 100uF are better values)

When it starts to toggle the voltage after the RC filter (RC_FILT) drops below V_REF and the comparator output switches (not shown)
Depending on the state of the output (5V or 0V) either the NPN or PNP transistor is on, and the LED in series with it is lit.
The botton graph is of the current through each LED - you can see one drops to 0 and the other turns on when the toggle starts/stops.

Hope this helps - ask if you don't understand anything and I will try to add some more later if needed.

EDIT - here is another version that does not use a separate power supply. It's a quick hack so I make no guarantees - the components shown are guidelines, you can use any small signal schottky and pretty much any small N-channel MOSFETs. This is about as simple as I think you can make something to do what you want:

PC LED Power 2

Here's the sim:

PC LED Power sim 2

  • \$\begingroup\$ What about using a bigger limiting resistor for the red LED, that has a lower drop? \$\endgroup\$
    – clabacchio
    Feb 4, 2012 at 10:33
  • \$\begingroup\$ @clabacchio - yes good point, the values for the resistors would need to be adjusted too - I just put two of the same random LEDs in there from those available in LTSPICE. The circuit is more a rough concept, the components shown (e.g. 2N2222) are not recommendations (could use FETs fro example) \$\endgroup\$
    – Oli Glaser
    Feb 4, 2012 at 10:49
  • \$\begingroup\$ Also re-reading the question I realise this doesn't cover all requirements, such as the standby not turning fullyoff (didn't notice that first time - could easily add a voltage divider to cover that though) Also the no other power requirement - this would be a bit more difficult as we would be wanting to power 2 LEDs instead of one, it would depend on what is available on the original LED output and the circuit would need to be adjusted. It would be easier just to "borrow" 5V from the PC supply. \$\endgroup\$
    – Oli Glaser
    Feb 4, 2012 at 10:53
  • \$\begingroup\$ @OliGlaser thanks for the detailed response. Actually both don't have to be powered on at the same time. Green should turn on when signal is continous and amber, when the signal is intermittent. Did you look at the link I referenced? Can you confirm if that guy's logic makes any sense? \$\endgroup\$
    – tunafish24
    Feb 4, 2012 at 18:53
  • 1
    \$\begingroup\$ @tunafish24 - If the LED pin is current limited, then the worst that will happen is that the voltage may sag a bit, it shouldn't damage anything. If you want to though, you can compensate for this by sizing R5 and R6 as necessary (these probably need adjusting anyway - if you are not sure there are answers on here that deal with selecting the series resistor for an LED - I'd start with 300 ohms as a guideline figure for ~10mA each) to take less current (e.g. if the original LED took 20mA, then set each LED to 10mA) I would use caps rated for twice the voltage, so for 5V pick >10V rated. \$\endgroup\$
    – Oli Glaser
    Feb 8, 2012 at 21:38

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