LM3915 Issue

Discussion in 'The Projects Forum' started by dfigravity, Mar 3, 2012.

  1. dfigravity

    Thread Starter New Member

    Feb 9, 2011
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    Hello all,
    I have put together a basic VU meter circuit using the LM3915 as my LED driver. The input to the circuit is a little pre-amp that I picked up at Conrads to amplify the signal from an electret mic. Everything works well...the LED's bounce up and down to music/sound, however one LED (connected to pin 17 on the LM3915) is constantly lit.

    I have searched the forums and found a thread talking about using a resistor and capacitor on pin 5 (input ) to smooth everything out, but that didn't work....any suggestions!?

    EDIT:
    I also just noticed that LED 1 (LM3915 pin 1) is exceptionally dim
     
    Last edited: Mar 3, 2012
  2. dfigravity

    Thread Starter New Member

    Feb 9, 2011
    5
    0
    Let me add a little more information...the amplifier that I am using for the microphone input is based on an LN351.

    I included a schematic of my setup. Again, everything is working fine, except that the first LED is very dim (still flickers with the audio input), LED 9 is VERY bright and doesn't seem to be affected by the audio input, LED 10 never lights up.

    The LED supply is also 12V, not 3V.
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    If you haven't done a mistake like a solder bridge or a miswire, I'd have to say the chip is broken. The usual (sigh). Triple check everything you did before taking your money to the store.
     
  4. dfigravity

    Thread Starter New Member

    Feb 9, 2011
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    I was afraid of that...everything is laid out on a breadboard right now and I reconnected everything twice... :(
     
  5. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    I've built these before. Properly built, they work like a charm...unless the chip is bad.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    If this is the "lowest" LED staying on, it's possible noise or DC voltage offset from the pre amp stage is tickling it enough to glow a bit.
     
  7. dfigravity

    Thread Starter New Member

    Feb 9, 2011
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    --The "lowest" LED (LED 1, Pin 1 on the LM3915) is barely lit...still flickering with the other LED's above it, but very dim....

    --LED's 2-8 function properly, jumping around with the sound

    --LED 9 (pin 11 LM3915) is always on
    --LED 10 is not functional at all

    I am going to pick up another chip at Conrads tomorrow just in case...
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If your breadboard wiring is overly long or "loopy", you can be picking up electrical noise.

    Breadboards have lots of parasitics. They are generally OK for audio frequency circuits, but they can pick up noise.

    If you've used the breadboard with oversize wire or TO-220 type packages before, you may have damaged some of the sockets on the board. You might try re-building the circuit on a different area of the breadboard; but if the breadboard is new, disregard.
     
  9. dfigravity

    Thread Starter New Member

    Feb 9, 2011
    5
    0
    Thanks for the input SGTWookie, the breadboard is still pretty new, but I'll try rebuilding the circuit on another part of the board just in case.

    I did replace the LM3915 today and the issue with pin 1 cleared up...so now pins 1-8 function properly and look great. Pin 9 is still always on and nothing from pin 10.
     
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