LM 3915 Vu meter

Discussion in 'The Projects Forum' started by Kristaus_tevas, Feb 19, 2015.

  1. Kristaus_tevas

    Thread Starter New Member

    Feb 19, 2015
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    Hello,

    I made LM3915 Vu meter which is working fine, however i have some problems.

    I used this schematic:
    http://users.otenet.gr/~athsam/vu_meter_4.htm

    At R1 I used 10Kohm potentiometer and for input I connected 3.5mm jack.

    When I connect my VU meter to my laptop it lights up to 4 LEDs, so I have to connect it to my speakers (which gives higher volatage, as I understand) and only then I can light up all LED's, however I need to be able to light it up from laptop.

    Can anyone recommend me any amplifier circuit that would be easy to fit up into this schematic and would solve my problem?

    Also my dot-bar switch doesn't work, when I switch it on/off it changes only brightness of LED for some reason, but it still is in bar mode.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You can make the circuit more sensitive by reducing the value of R4 in the given schematic.
    Here is a picture taken from the datasheet how to calculate the reference voltage:

    [​IMG]

    Bertus
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You can reduce R4, this will lower the full scale setting,or reduce R1 to zero, otherwise you need an amplifier.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You are both talking about R4 but your schematic only shows R1 and R2.

    EDIT - I see, you are talking about R4 in the OP's drawing.
     
  5. Kristaus_tevas

    Thread Starter New Member

    Feb 19, 2015
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    What is Vref in that formula and how do I find it?
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You don't find Vref, you measure or estimate what the max peak output voltage from your laptop is and that should be Vref. Then you calculate what R4 in your schematic should be to make Vref on Pin 6 (R-Hi)
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I would make R4 a 4K7 preset, then set it to the middle and slowly turn it to zero whilst your laptop is connected with sound up full, then the leds will start lighting up.
     
  8. Kristaus_tevas

    Thread Starter New Member

    Feb 19, 2015
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    I have connected 50k ohm potentiometer parallel, and when I lowered his resistance it lit up, but now almost all the times leds 1-6 are lit up, 7-8 are moving and 9 sometimes lights up too. But it looks really not responsive, and if I increase resitance it just differates between which leds will light up. But it's always only 3-4 who moves nicely, what should I change to make it work more responsively in whole scale instead of 3-4 led's?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You say that when the R4 is shorted, led 7 and 8 work and 9 sometimes lights up.
    You can try the following: Short R4 and place the 50K pot between pin 6 and 7.
    Start with the lowest value and turn it up until the wanted response.

    Bertus
     
  10. Kristaus_tevas

    Thread Starter New Member

    Feb 19, 2015
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    And R3 should be left as it is?
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes, R3 is used to set the led current.

    Bertus
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The problem with an Lm3915 is its not linear, with each led step you need more voltage usually double the step, where as a Lm3914 is the same voltage per step.
     
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