LM3914 with 741 opamp

Discussion in 'The Projects Forum' started by diablo666bass, Mar 8, 2009.

  1. diablo666bass

    Thread Starter New Member

    Mar 8, 2009
    Last edited: Sep 21, 2009
  2. SgtWookie


    Jul 17, 2007
    My first question is:
    Why aren't you using an LM3916? It's designed as a VU meter, which is precisely what you want. An LM3914 has a linear response, which will perform very poorly for your application.

    Why are you using an LM741? It's so outdated that practically anything else is better.

    You're trying to use a dual-supply opamp with only a single supply, but you're not offsetting the input. This means a great deal of clipping for the output, if you get any at all; the LM741 can't get within about 2v of the supply rails; the range is less under load.

    Mixing battery types is a bad idea.
  3. Audioguru

    New Member

    Dec 20, 2007
    The LM3916 has big steps and little steps. I use the LM3915 that has 23dB between each of its steps.

    The lousy old 741 opamp is properly biased at half the supply voltage but its output swing might not light the 10th LED on the display. But its gain is way too high at 100.

    The LM39xx LED driver does not have a peak detector circuit so the LEDs will look like a flickering dim blur. It lights the LEDs momentarily for only half the time then turns them off for the other half of the time for each cycle of sound.
    A peak detector circuit holds the highest level for a moment that is long enough to see and does not turn off each half cycle.
  4. SgtWookie


    Jul 17, 2007
    You hot-linked to an image on a site that doesn't approve of such things.

    I did manage to view it, but there are elements missing from the LM3916's controls.

    Velleman sells a stereo VU meter kit, with all necessary parts included (except for the stereo and power supply).
    Manual: http://www.vellemanprojects.com/downloads/0/manual_k4305.pdf
    Google search for suppliers: http://www.google.com/search?hl=en&q=K4305

    Note that it's not for use with high-power car audio systems; many of them have isolated output amplifiers, and will burn up this kit.

    Funny, Velleman used a 741 opamp - go figure. :rolleyes:
  5. SgtWookie


    Jul 17, 2007
    Not offhand, and not after spending 10 minutes searching, either.

    I'll leave the rest of the bargain hunt up to you :)