Help with LM 3915 Dot/Bar Driver

Discussion in 'The Projects Forum' started by joe426, Aug 11, 2009.

  1. joe426

    Thread Starter Active Member

    Aug 11, 2009
    39
    0
    I'm trying to make a VU meter using a lm3915 dot/bar driver using this circuit:

    [​IMG]
    A few questions:

    • How can the 12V VCC be connected to the anode of each LED and not blow them up? Because if only 1 is on...then isn't the LED getting the full 12 volts? Or is there something I'm not seeing?
    • This schematic is reccomended by NI, but it appears some people modify it:
    • This meter looked like it works well: http://www.youtube.com/watch?v=EDZFk5os5Pw but the schematic is quite different... http://bit.ly/yVF3V If I followed that schematic, do you think everything would still work?
    Thank you for your help!

    Joe Green
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Have you got the data sheet? - www.national.com/ds/LM/LM3915.pdf

    The features section says the outputs are current regulated.
     
  3. joe426

    Thread Starter Active Member

    Aug 11, 2009
    39
    0
    I'm a bit of a noob, so bear with me. So current limiting means that although they have 12v coming in, they only receive the needed amount of current so they won't explode or anything?

    Also, still looking for an answer on the second question please! Thanks!
     
  4. joe426

    Thread Starter Active Member

    Aug 11, 2009
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    Do I need to amplify the signal coming from the PC? It doesn't appear so..but just wondering?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    The features section of the data sheet - you need this - indicates operation with 1.2 volt FS input. That is probably pretty close to the audio level out of your sound card (is that your PC source?).
     
  6. joe426

    Thread Starter Active Member

    Aug 11, 2009
    39
    0
    Yes, audio is from the soundcard. The datasheet says the current is adjustable from 1 to 30 mA. What if I want to run 2 LEDs on each output. Do I have to use a relay to access another power source or...is there a way to get more current for an additional LED or two?

    Thanks for your help.
     
  7. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    If the current is adjustable, you can run 2 LED's easily running them in series and current adjusted to 20mA. If you need to use more LED's, you can use transistors. Using relays will make it like a typewriter noisy thing with a sound input.

    You can also try KA2281 which is an IC used in Audio level indicators with minimum components. It can have stereo inputs.
     
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