Signal source for this schematic ( LED Spectrum Analyzer)

Discussion in 'The Projects Forum' started by dyeraaron, Jan 4, 2010.

  1. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
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    Hello all.....I just can't seem to get a straight answer from anayone...can anyone please let me know ..............ok...with this schematic or a similar one like it.....the "signal" going into pin 5 of the LM3914 ......in the design i built...I have 10 Bandpass filters all tying into pin 5 is this correct? this schematic is somehwat like I have...I think this is a 10 x 15 matrix...mine is a 10 x 10 with only one 3914 and one 4017


    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Looks to me like you're trying to use an LED oscilloscope display as a spectrum analyzer.

    It's not going to work like you want it to.

    You'll need a separate LM3914 for each frequency range that you wish to display.
    However, the LM3914 is not the IC to use for sound as it's linear; you want to use an LM3915 which is logarithmic. The display will be much closer to how humans perceive sound.
     
  3. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
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    Sgt,

    I have plenty of the LM3915s as well.....are you saying I'd still have to use 10 of them to produce the analyzer output.....the LM3915 has 10 buffers to distinguish which frequency will pass.......like I said once before though.....I have 10 bandpass filters in parallel whose outputs all tie back into series going into the lm3914 or lm3915 signal input.....is this correct?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.
    This is not correct.
    The LM3914/15/16 contain 10 comparator circuits which give output indications relative to the voltage at the input. These LED drivers don't respond to frequency; they react to the voltage level at the input.

    No.

    I don't know why you've gone to the trouble of creating 10 bandpass filters, and then mixed them all together. The net output will be the average of the 10 filters.
    [eta]
    As I said before, you will need to use one LM3915 per bandpass filter.
     
  5. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
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    I see all these you tube videos on how cool their led spectrum analyzers are...and not one person ever posts a schematic......the schematics you do find are garbage or incomplete.....I don't understand why you would need 10 chips for 10 bandpass filters...would you happen to know where to find a schematic that may be some help?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Mik3, one of our members posted a project awhile back where he used (I think) a PIC18 series uC to create a large audio spectrum analyzer display.
    It's in his blog, here:
    http://forum.allaboutcircuits.com/blog.php?b=116

    If you wanted to use just one LM3915 IC, you would have to do a lot of multiplexing and latching. It wouldn't be easy.
     
    Last edited: Jan 4, 2010
  7. Audioguru

    New Member

    Dec 20, 2007
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    If you multiplex the filters into a single LM3915 then the resulting display will be very dim.
     
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