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Fading LED

Discussion in 'Electronic Design' started by m7n, Dec 20, 2004.

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  1. m7n

    m7n Guest

    Hi there,

    I'm trying to figure out if I can design a circut that would cause a
    LED to fade in and out. I'm pretty sure that LED's are only on or off,
    but my computer has a LED that "breathes" while it's sleeping.

    Can someone please tell me if this is possible?


  2. Leon Heller

    Leon Heller Guest

    PWM is often used for LED brightness control. It's quite easy to implement
    with most MCUs.

  3. Sure. If you vary the current through a LED its brightness will vary.

    Now you need to find a circuit to vary the current. ;)

    If you want it to be logically controlled I guess you could do it with a
    cheap DAC followed by a buffer ... You'll need a resistor in series with
    your diode to limit the current.

  4. Yes. You must use current-drive. Start with 12 V and 1.2 kohms in series
    with your red, green or yellow diode. Now turn the 12 V down slowly. For
    blue or white, you may need to start with 15 V.
  5. Couldn't it also be done by varying the switch on/off time (frequency) to
    fool the eye into seeing fade in/out?

    If you switch a LED on and off rapidly, with more on than off, wouldn't the
    eye see it as lit solid? Start reversing the on/off to more off and it
    would "appear" to dim...


    Just an idea...

  6. Mark Jones

    Mark Jones Guest

    Yes, this is called PWM - "pulse-width modulation."

    Yes it is possible. LED's are not just on-off devices. They are
    kinda like a lightbulb. If your flashlight batteries wear out, the
    light gets dim - same with a LED. If you use less power, the LED will
    be less bright.

    In the case of your PC, there is a little circuit in there to vary
    the power to the LED, making it appear to "breathe."

    May I recommend you start your journey here:

  7. Thanks for all the info... I appreciate it.
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