LM3915 Sensitivity

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

  1. denno

    Thread Starter Active Member

    Jul 1, 2008
    30
    0
    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
    18,587
    3,600
    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
    30
    0
    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
    18,587
    3,600
    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
    30
    0
    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
    18,587
    3,600
  7. denno

    Thread Starter Active Member

    Jul 1, 2008
    30
    0
    thankyou very much for the help :). I'll let you know how it does :).
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,202
    1,793
    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.whsites.net/download.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.
    http://sound.whsites.net/download.htm#lm3915

    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.

    Moderators note : corrected links
     
    Last edited by a moderator: Oct 21, 2018
  9. denno

    Thread Starter Active Member

    Jul 1, 2008
    30
    0
    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
    30
    0
    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
    22,202
    1,793
    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
    30
    0
    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
    22,202
    1,793
  14. rachaelfox

    New Member

    Jun 1, 2011
    1
    0
  15. John_2016

    Member

    Nov 23, 2016
    55
    5
    potentiometers are prone to failure, their MTTF is really low compared to for instance a swich.

    If you are going to sell this, the chances are that sooner than later a customer will turn 'too hard' or to 'the other left'
    and the pote will go south, without the operator noticing.

    If the visual reading of the signal is used for something else, the error may unadvertently propagate.

    It's recommended to use a switch instead, with just a couple of resistors behind, and label it high/low sensitivity.

    Another option would be to use expensive linear potentiometers like in sound equalizing boards, expensive compare to the typical
    round potentiometer.

    John BG
     
Loading...