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How to fade cycle PC powerled when it's in sleep mode

Discussion in 'LEDs and Optoelectronics' started by TR7, Apr 24, 2015.

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  1. TR7


    Apr 24, 2015
    Beware, beginnersquestion..
    I had to buy some electro stuff for a school assignment, so i decided to put them to good use.
    Normally the PC's powerled has a duty cycle with a period of 1s and a lenght of 0,5s, but it'd be cool if it fades in and out instead of blinking. I made this (attached) circuit for it tot work, but it doesn't work.
    Maybe that's because the current is already biased by the PC's mobo itself, I don't know.
    (Values for resistors and caps aren't right yet, obviously.)
    Can someone help me with this?

    Attached Files:

  2. kpatz


    Feb 24, 2014
    The concept is sound, but to make the LED fade on and off you'll need a much larger capacitor. I would eliminate R4 as well, as R1 will control the charging rate (fade-in) and R2 will control the discharging rate as well as limiting LED current.

    Also, depending on how the mobo drives the LED, if it is using a logic circuit that can source and sink current the mobo will "forcefully" discharge the capacitor on each off-cycle. A diode in series with R1 would take care of this.

    Lastly, the LED won't light up as brightly using this circuit due to the additional resistances on top of the LED current limiting already built into the motherboard. If this is an issue, a transistor circuit could be used to drive the LED.
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