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Electronics VU meter (20 LED verson) Need help!

Discussion in 'Modding' started by Smilodon, 10 Jun 2007.

  1. Smilodon

    Smilodon The Antagonist

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    Hello all.

    I'm planning to make a stereo VU meter to measure line signals (doesn't have to be accurate, just look nice ;) )

    however, i have a slight problem with the design. I was thinking of using the LM3915 for this. This one is for 10 LEDs, but can be cascaded to several units. I don't quite understand how to cascade two, and still have the option to use both dot and graph-mode.

    Also, will i need a preamp on the input to get increased resolution? (all designs i have seen so far is 10 volts max in. Line signals is 1v p-p, isn't is?

    I'm trying to avoid using opamps, as this will require dual-rail power to work properly. I'm trying to keep this as simple as possible.


    The second option i have is to make a 5.1 version for the speaker output on my home "cinema" system. However, i'm a bit unsure how the circuit will work on subwoofer frequencies. Anyone know anything about this?

    (if i go for the 5.1 option, it will be a 6x 10 LED VU meters. 5 of them for speaker outputs, and one for Line signals (subwoofer).


    Any input on this is welcome :) (including opinions on the design of the casing)
     
  2. ch424

    ch424 Design Warrior

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    Just a thought: these LM3915 chips are £1+ and you're going to need two of them, while a PIC16F72 with 22 I/O pins and analogue inputs can be had for £1.74... do you have access to a programmer?
     
  3. Smilodon

    Smilodon The Antagonist

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    I have a programmer. (and even some PICs), but i never got the time to learn programming.

    if I could program, PIC would defeneatly be the best choice. Cheaper, and more customizable. (could integrate peak hold)

    I made a VU meter a while ago using a special stereo VU meter chip. Can't remember it's number, but it doesn't seem to be around anymore. (it had multiplexed graphs and 12 (or was it 14?) LEDs per channel, programable peak hold. Didn't have dot mode, though). Too bad i can't find it. :sigh:
     
  4. cpemma

    cpemma Ecky thump

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    I'm not sure you can; the system with dot-mode (and a cascade of 2) is to turn off the bottom section display once the signal strength has got past the half-way point. The simple dot/bar switch won't then work. Pick bar mode, it's easier. ;)
    Running an op-amp on single-rail is no problem, just a few more passive components for audio (AC) signals. A cheap LF353 or similar is fine.

    The most basic LM391x circuit is 1.2V full-scale, but you can make it less (or more). The main problem is resolution; 1V over 20 LEDs is only 50mV per step. For a bling device it may not matter. The other problem may arise if you need some signal shaping to slow down fast level changes. Datasheet explains.
    The column will bounce with the beat, simple harmonic motion at very low frequencies. :rock:

    I think I've posted the cascade method here somewhere.
    http://forums.bit-tech.net/showthread.php?t=122503&highlight=cascade may help.
     
    Last edited: 10 Jun 2007
  5. Smilodon

    Smilodon The Antagonist

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    What if I go for 19 LEDs? then i could just let the last LED on the first controller to whatever it wants to, and just set the second controller to start from where the 10th. LED on the first controller was supposed to light up.

    of course the inputs needs to be calibrated, but that shouldn't be any problem, right? this would in practice just be two separate VU meters.
     
  6. LuitvD

    LuitvD What's a Dremel?

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    it's easier to link two LM3915 together... though they can only do this in dot mode.
     
  7. cpemma

    cpemma Ecky thump

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