LM 3914 audio led vu-meter problem.

Discussion in 'The Projects Forum' started by simosihvonen, Sep 8, 2009.

  1. simosihvonen

    Thread Starter New Member

    Sep 7, 2009
    Hello AAC forum!

    I am seventeen year old teen that's currently studying for electronics engineer at Tampere's college in Finland. I am an beginner in electronics.

    I have a bit of problem with my audio LED VU-meter project. It uses the LM3914 (yes i should use lm3915, but my nearby store didn't have them), and draws the power from USB-port from my computer to both the circuit and LEDs (5V). Signal is from my comps speaker output, which produces ~2V at the very peak.

    I use this schematic:
    Resistor values are R1 1,5kΩ and R2 1kΩ which gives a ref out of 2,08V.

    When I first connect it to the USB while the signal cable is disconnected, all the LEDs light up and shut off when the signal cable is connected, but after few moments of playing music and stopping the playback, the couple first LEDs keep on burning (no.1 and 2). And after 10 minutes even more LEDs keep on burning :confused:.

    And that is my question. Why do those couple first LED keep on burning even after the playback has been cut ? I just need the reason. Thank you.

    PS. I'll post a video regarding this later in the afternoon. And sorry if I missed any information that you might need.
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    If your speakers are disconnected, the LM3914 input bias current will charge up the capacitor which is in series with the amplifier output. Add a 10k resistor from the LM3914 input to ground.
    If your speakers are connected, then I don't know what is going on (unless the speaker also has a capacitor in series).
  3. simosihvonen

    Thread Starter New Member

    Sep 7, 2009
    That did the trick! Thanks.