hmmm...little problem with a LM3915 IC

Discussion in 'The Projects Forum' started by JoeBro391, Sep 21, 2010.

  1. JoeBro391

    Thread Starter Member

    May 15, 2010
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    Okay, so I have lots of experience with LM3915 IC, it was the first IC I ever used and have made a bunch of gifts and displays with them. Now, the cool thing about this IC is the ability to display in either Bar-Graph-mode or in Dot-mode. I usually hardwire Pin9 to Pin3, which keeps it in Bar-Graph-mode, but yesterday I built a little pocket-version VU meter and this time I hooked a switch up to Pin9 so that I could toggle between Bar-Graph-mode and Dot-mode. Bar-Graph-mode works perfectly, but Dot-mode doesn't. When I flip the switch, it goes SORT OF into dot-mode; where there's always about 4 LED's on, but not the single LED. This is weird because I had dot-mode working perfectly over christmas break (was making a present for my brother) but right now, i can't seem to get it to work.

    Here's the circuit I've been using:
    LM3915.jpg

    And here's the datasheet of the IC:
    http://www.national.com/ds/LM/LM3915.pdf

    I've been using 6V with the circuit, but have tested up to 12V without different results. And for the record, IN THIS PARTICULAR BUILD, instead of using individual resistors for each LED, I used a single 100-ohm resistor, hooked up to the Positive strip that runs down the right side of the diagram, but I don't think that's the problem, because Pin3 (Vcc) and Pin2 (Gnd) are still getting direct power from my power supply.

    Thanks to anyone that can offer an insight. -Joe
     
  2. JoeBro391

    Thread Starter Member

    May 15, 2010
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    okay, nevermind, I think I figured out the problem. I think It's because I didn't rectify the audio signal and the Inputs of the LM3915 is meant for DC in dot-mode (but runs fine in bar mode off of AC). Makes sense, cause when I DID have it working, the input was coming from a transistor's emitter, and hence, from a DC supply. -Joe
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The LM3915 automatically rectifies its input.
    Your circuit is missing the important supply bypass capacitor that every IC needs. It should be a minimum of 2.2uF but use 10uF.

    Maybe your wiring is picking up mains hum because you did not use shielded audio cable for its input.
     
  4. JoeBro391

    Thread Starter Member

    May 15, 2010
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    Is this like a smoothing cap? cause I usually use 100uF caps for smoothing. Does this 10uF cap go between Pin2 and Pin3 or between the positive and negative supply connections?? -Joe
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Pin 2 is ground. Pin 3 is the positive supply. The supply bypass capacitor goes between them to provide a low impedance. 10uF to 100uF is fine.
    The LM3915 does not have a negative supply.
     
  6. JoeBro391

    Thread Starter Member

    May 15, 2010
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    hmf, i put in the 100uF cap and the results are no different. -Joe
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    You said that you placed a 100 ohm resistor from +12V to the anodes of the LEDs (the positive strip on the right side of your diagram).
    Then the datasheet says and shows that you must also bypass the anodes to ground with a 2.2uF tantalum or 10uF electrolytic capacitor to avoid oscillations.

    What are the values for your R1 and R2?
    What is the level of your input signal?
     
  8. JoeBro391

    Thread Starter Member

    May 15, 2010
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    just to be clear, i'm only using 6V with this circuit.

    i'll try that

    R1= 1k-ohm
    R2= 150-ohm
    The input is just the headphone jack from my laptop, but I put an audio-input jack on the circuit so I can adapt just about anything to be the input (light circuit, microphone, temperature, etc).
     
  9. Audioguru

    New Member

    Dec 20, 2007
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