LM3915 Sensitivity

Discussion in 'General Electronics Chat' started by denno, Apr 10, 2009.

  1. denno

    Thread Starter Active Member

    Jul 1, 2008
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    Hi all, I have just built a circuit from the LM3915's datasheet, the Audio Power Meter, however the lights never make it up past the 5th LED, and that's when I have the music volume cranked up as high as it will go.
    I was wondering how I go about increasing the sensitivity of the circuit? I'm sure it's pretty simple, like a resistor change or something, but I'm not sure which one so hopefully someone can suggest what to change?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the datasheet there is a drawing with a formula to calculate the reference voltage.
    For the audio power meter calculate a new value for R2.(you will need a lower value).

    [​IMG]

    Greetings,
    Bertus
     
  3. denno

    Thread Starter Active Member

    Jul 1, 2008
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    Thanks for the reply..
    So what do I use for Vref in that formula? Do I need to measure that from my speaker outputs? Also, I didn't use that circuit, you know that right? I'm pretty sure you do but I just wanna make certain.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The sensitivity in the circuit is set for 100 Watt full scale.
    You say it will never pass led 5 , so about 3 Watts.
    At 100 Watts the reference is set at 10 Volts.
    For 3 Watts you need a reference of about 1.8 Volts.
    Try a resistor of 150 Ohm for resistor R2.

    Greetings,
    Bertus
     
  5. denno

    Thread Starter Active Member

    Jul 1, 2008
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    Ok thankyou :). Sorry for sounding a bit silly, but yeah, thanks alot :). I guess the best bet would be to just solder a variable resistor in wouldn't it? Which I'm pretty sure is called a rheostat right?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It is OK to take a potentiometer of 1K and adjust it to scale your amplifier.

    Here is some information on resistors and potentiometers:
    http://www.educypedia.be/electronics/resistors.htm

    There are pages for other components too.

    Passive semiconductors technology: Batteries Diodes Photovoltaic systems Switches Buzzers Diode types Quartz crystals Thermoelectric Capacitors General overview Relays Transformers Capacitor types Heat Sinks Resistors Transformers-RF Coils Photo diode Resistors-Nonlinear Transformer types

    Greetings,
    Bertus
     
  7. denno

    Thread Starter Active Member

    Jul 1, 2008
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    thankyou very much for the help :). I'll let you know how it does :).
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Ahhh, note that changing the value for one of the resistors will also require changing the value of the other resistor in order to keep the LED current the same.

    See the formulas in the schematic that Bertus posted.

    Elliot Sound Products has a free LM3915 calculator that you can download somewhere from this page:
    http://sound.westhost.com/site-map.htm
    Scroll most of the way down the page, and look under the "Downloads" section; about 1/3 to 1/2 you'll see "LED VU Design (ZIP)" - download that.

    Running the numbers really quickly: for a Vref of 1.81V and LED current of 15mA, use 880 Ohms for R1 and 370 Ohms for R2.
     
  9. denno

    Thread Starter Active Member

    Jul 1, 2008
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    Can you please explain how you got those two resistance numbers? I've just tried it by hand, substituting those two values into the Vref equation, but I didn't get 1.81V. I downloaded the program you suggested but it doesn't work on my computer, says it can't find a dll.

    Thanks
     
  10. denno

    Thread Starter Active Member

    Jul 1, 2008
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    Ok I've just changed the two resistors to resistors that equate to 880 and 370 (I didn't have each resistance value explicitly so I had to add a couple in series to add up to the resistance value). It works well now. Although the sound meter does tend to stay nearer the top and only move between around the 6th and 9th LED's, but I guess that could just be because of the music. I haven't tested it too throuroghly with different music and stuff.

    Thanks for the help guys :).
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Which DLL's can't it find?

    You may not have the Visual Basic run-time DLL's installed; you can download them from Microsoft's site.
     
  12. denno

    Thread Starter Active Member

    Jul 1, 2008
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    VB40032 is the dll it can't find, which looks like it might have something to do with visual basic, which I don't have no. I'll look into getting them then.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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  14. rachaelfox

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    Jun 1, 2011
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