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Can I drive an LED via audio line out?

Discussion in 'Audio' started by Macrosaurus, Feb 17, 2013.

  1. Macrosaurus

    Macrosaurus

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    Feb 17, 2013
    I'm trying to build a (partial) reconstruction of John Logie Baird's early-20th-century mechanical television design.

    The instructions are here:-

    http://www.sptv.demon.co.uk/nbtv/

    They claim that one should be able to place an LED across the line out signal from a computer sound card and have it flicker in response to the signal. (In other words, the intended varying luminance of the LED is encoded via an audio signal).

    However, I'm getting nothing out. I'd wondered whether the problem was that I was testing it with ordinary audio files (which are effectively AC, but going through an LED, i.e. a diode). However, I've tried DC offsetting some of them so that they're all above zero, and downloaded some of their WAV-encoded files (which seem to be partially DC offset), and I'm still getting nothing out.

    So... should I be getting something by connecting an LED across line or headphone out? I've confirmed that everything else is okay by connecting a mini-speaker via a breadboard, and I can hear the audio.

    - Macrosaurus
     
  2. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    The line out signal doesn't have enough voltage to drive current through an LED without amplification.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,186
    2,692
    Jan 21, 2010
    The voltage across the headphone output is probably insufficient to tun the LED on.

    You will need an amplifier, preferably one which adjusts the output current to be proportional to the input voltage (since the LED is a current driven device)
     
  4. Macrosaurus

    Macrosaurus

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    Feb 17, 2013
    Thank you for both the responses- I suspected as much myself, but I didn't have enough experience to know whether I was correct or not.

    What I don't get is why the article implied that it *was* possible. Strange... :confused:

    - Macrosaurus
     
  5. BobK

    BobK

    7,624
    1,654
    Jan 5, 2010
    Did you look at the circuit? He has a 3 volt battery and a couple of resistors with the headphone out going across one of the resistors. So the the voltage is adding or subtracting from the voltage seen by the LED, with most of the voltage supplied by the battery.

    Bob
     

    Attached Files:

    Last edited: Feb 18, 2013
  6. Macrosaurus

    Macrosaurus

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    Feb 17, 2013
    Yes, I saw that circuit. The text says that's an improved version (which I didn't initially have much luck with). The basic version (which they claim should work) is

    * "Get 10 or more hyper bright LED's (any colour)."
    * "Connect them all in parallel."
    * "Connect the group of LED's directly to the Loudspeaker output of your computer's sound card."

    I tried with my other computer, which has a proper headphone out (i.e. not just line out) with greater volume, and lo and behold... it *did* actually work.

    The LEDs are lighting, and there's some patterning seen through the Nipkow disc. No picture, but I haven't added the potentiometer to adjust the speed of (and hence synchronise) the disc, so I'll have a go at that tomorrow. :)

    Thanks,

    - Macrosaurus
     
  7. Starbuckin

    Starbuckin

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    Jan 22, 2013
    BIG difference between "line out" and the computer sound card's output to speaker. I'd be VERY careful if I were you, hooking an L.E.D.(s) directly across speaker output!

    Once the signal exceeds the Vf (forward Voltage drop) of the L.E.D. there will be nothing to limit current. The L.E.D.(s) could quickly be destroyed. I would make a simple transistor L.E.D. driver with a resistor that limits the maximum current and drive the base of the transistor from true line out with a pot in series.
     
    Last edited by a moderator: Feb 19, 2013
  8. Macrosaurus

    Macrosaurus

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    Feb 17, 2013
    To be honest, I'd be more worried about potentially damaging the sound card (which is quite old now, but still decent and was expensive) than a few very cheap LEDs. But I do take your point.

    Even at the maximum output, the LEDs still seem to be okay, so it's probably fine in this case, though I do appreciate that maybe an (even) more powerful output might fry them.
     
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