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Drive LEDs with a soundcard?

Discussion in 'Electronic Basics' started by dnrg, Aug 3, 2003.

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  1. dnrg

    dnrg Guest

    Any simple circuits out there for driving two LEDs from pulses of
    sound (separate left and right channels) coming out of a PC soundcard?
    I'd like to do this using the parallel port but am afraid I'll fry the


    - Dana
  2. Lord Garth

    Lord Garth Guest

    Is there a particular voltage or frequency that activates the LED?
  3. Are you talking about driving them using the power from the speakers? I am
    not sure how an LED in series with the speakers will affect sound quality...

    If you just want them to light up, there are lots of circuit schematics for
    sound detectors using electret microphones. There has to be one for lighting
    up an LED based on sound volume. For these, however, you'll need to use
    external batteries or a power supply of some kind.
  4. cpemma

    cpemma Guest

    There's only about a volt max on the line-out, a led needs over 2v and over
    5mA, so you need some modest amplification. The transformer route will put
    too much load on the output.
  5. dnrg

    dnrg Guest

    Using a radio shack audio transformer works; I attached the 8 ohm side
    to the output from a walkman (for a test) and the 1000 ohm side to an
    LED. Problem is that the LED doesn't light brightly enough. It's
    definitely responding to what it's being fed (at full volume), but how
    do I get the LED to light brighter? Presumably there's not enough
    amperage. I actually put up to 4 LEDs in series and they all lit at
    the same brightness; I'm a bit confused here. Help!


    - Dana
  6. Both LEDs and speakers are driven by current; however, LEDs only work with
    current going one way, and further have a minimum voltage requirement of
    around 1.5V.

    Running your sound card at full volume will cause it to put out more power,
    which will increase the LED brightness. However, transformers don't increase
    power, they just change voltage and current. Thus, since power is equal
    (ignoring losses in the transformer) you have I(in) X V(in) = I(out) X
    V(out). Therefore, increasing V with the transformer decreases current in

    If you are driving 4 LEDs with this, The voltage must be greater than 4 LED
    drops, which is about 6V. Since the voltage gain of your transformer is
    about 11, and you couldn't see the LEDs light before using it, I'm guessing
    that your sound card signal is between .6V and 1.5V peak to peak. Normal
    LEDs light up with about 1mA of current, so your 11 gain means your original
    speaker current might be around 11mA. Thus, if you got a transformer with a
    smaller turns ratio, (ie, 250 ohm to 8 ohm, for example,) the current would
    be twice as big, but your voltage would be 1/2. This would mean you could
    drive fewer LEDs, but you would drive them more brightly.

    Note that the current through the LEDs will be the same if they are in
    series, which is why they all light with the same intensity.

    You could also try a rectifier diode ($1 at RS) to put more of the voltage
    on the positive side... that will ensure that the current is travelling in
    the right direction for more of the cycle, thus causing the diodes to light
    up for twice as long (they are flashing now with the frequency of the sound,
    and only lighting when the voltage is positive). However, there is a voltage
    drop across these rectifier diodes of around 1.2V that you'll need to take
    into account. Put the AC leads (the ones with a tilde marking) across the
    speaker wire outputs, and connect the long lead of the LED to the + end of
    the rectifier, and the short lead to the - side.

    Disclaimer: I'm a hobbyist, not an EE. You are on your own if you follow
    this or any other advice I give you...

    Bob Monsen
  7. If you use a bridge, you can control the output using the volume control. it
    just allows you to get more of the current from the card through your LEDs.
    Once the voltage drops below 1.5V, you won't have any output, so the volume
    control can turn off the LEDs even with this case.

    As to where to buy the transformer, try digikey or mouser. You could also
    try the surplus shops like all electronics or goldmine. Take a look at

    for some ideas on where to buy parts.

  8. I would suggest using a powered circuit, maybe taking 5V from the
    parallel port (if you're feeling brave!) and using a pair of
    transistor drivers to light up your LEDs. This would be the best way
    of doing it without overloading your sound card's output.

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