Problem with output 1 always being lit on a LM3915

Discussion in 'The Projects Forum' started by WVU Engineer, Feb 2, 2008.

  1. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    I am having a bit of trouble with an LM3915. I am working on a project which will take in an audio signal (via an electret mic) amplify it, then using a low pass filter to discard sound levels above 1K (looking for bass only). i want to display these levels on a bar graph style meter with the LM3915. My problem is that when i just turn on the power supply it already lights up the first LED. Any ideas as to why, or has anyone had this problem before? Also i read on this forum somewhere that someone recommended a "schematic drawing tool" but now i cant find it again. Can anyone help me out, i can give a schematic once i know where to find the tool.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    If you have access to a scanner you can draw the circuit and scan it to a format like .BMP, .PNG, or JPG.

    In a crunch, you can take a digital picture of your sketch and that will give you a .jpg format that you can post.

  3. SgtWookie


    Jul 17, 2007
    This might be caused due to the offset voltage of the op amp that you're using to amplify the output of the electret mic, depending on whether you're using the op amp as an inverting or noninverting amplifier. If it's an inverting amp, increasing the voltage on the + input slightly should drive the output of the amp more negative. You could do this using a simple voltage divider network consisting of a precision pot. De-coupling the input to ground with a small cap would help to eliminate resistor noise.

    You can generate a rudimentary schematic using Windows Paint, although it's not a lot of fun.

    Circuitmaker Student is no longer supported, and doesn't have any of the LM391x IC's in the library, and you can't add new IC's in the student version.

    Oh, I just remembered about ExpressPCB:
    It DOES have the LM3915 in it's library, has a schematic generator and board layout editor, and it's free.
  4. WVU Engineer

    Thread Starter Member

    Jan 18, 2008
    ok, i dont think my microphone wants to couperate (spelling error). Anyway, what if i took the speaker wires that feed the speaker, from the stereo, and wired them into my circuit. Could this be done instead? I could then use a low pass filter to ensure that only lower frequencies pass through it, into the LM3915. Also, when lower frequencies output from the stereo (bass), is there more voltage being output from the stereo? (i tried using a multimedter and it looked like the voltage was pulled negative, in this case i guess i would need an inverting amplifier). Please tell me if my thinking is logical. i do not want to blow this thing up, thanks again
  5. Audioguru


    Dec 20, 2007
    An LM3915 works much better if one of the "peak detector" circuits that are shown in the datasheet feed it. Without the peak detector then the LEDs are just a dim blur because they turn on and off (half wave) much quicker than you can see.
    The peak detector holds the highest input just long enough for you to see it then it drops until the next peak.

    I have a Sound Level Indicator project that has an electret microphone with a very sensitive preamp. If I hold my breath and the furnace is off then all the LEDs turn off. It shows a pin dropped on the floor of the next room. I added 20dB of automatic-gain-control so it displays loud sounds.
    I usually leave it in the BAR mode.