Bandpass Filter signal into an LM3914

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

  1. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
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    Hey guys...I'm building a 100 LED matrix Spectrum analyzer using 10 bandpass filters built to certain frequencies , an LM3914 Bar Graph driver and a 4017 counter.....I'm undecided if I need to tie all the outputs from the bandpass filters together and then have them for the input of the LM3914.....Because the LM3914 Outputs for the LEDs are low...and at First I was measuring highs......the highs are suppose to come from the 4017 counter which is clocked by 2 nand gates formed together to produce a clock.....my question is....does the LM3914 input pin 5 just need all 10 output signals just loaded into it?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    LM3914 are bar graph LED driver and can be driven on bar or dot mode.
    It can accept low level audio or full wave rectified audio for precision measurement of the signal.
    There are ways to built low level AC rectifies using OP amps, to eliminate diode voltage drop losses so that the IC can receive the actual audio signal strength without conversion loss.
    A typical LM3914 can drive 10 LED's and it gives a linear reading of the decibel level provided that the input audio rectifier is build properly.
    It cannot alone designate the difference between the frequencies.
    Now a spectrum analyzer is build on different levels or octaves of the audio frequency.
    Any given spectrum will indicate a typical range of frequencies. The no. of spectrum indicators depend on your need, say it can be a five band, 10 band or even a 20 band spectrum analyzer. Each band has it's own dedicated LED drivers and input filters. Input filter is (notch Filter) designed for that specific band and filters out the rest so that the driver can reliably indicate that band of frequency strength.

    This is how a spectrum analyzer is made.
    But your case might be a bit different and cheap but not that accurate.
    I can only comment if you post a schematic of your design

    Rifaa
     
  3. dyeraaron

    Thread Starter Active Member

    Oct 27, 2008
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    0
    I realize what functions the LM3914 and 4017 have..thats why I chose them....I'm just trying to figure out this.....you have 10 bandpass filters....lets say..............does each output of the filters go into the input pin of the lm3914?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I replied to your other thread you started about this same project.

    The LM3914 has a linear display. You need the LM3915, which has a log response.

    You will need one LM3915 per frequency band.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Yes . One IC (linear or log) per spectrum.
    One filter specific to each spectrum and probably rectifiers too.
    Building Spectrum analyzer with discrete parts is not easy, but I like to see me proven wrong.
    On the other hand a powerful PIC might me better approach which is one of my future project that will someday become a reality.
    I will post it on my blog as soon as it is done.
    Till then you can post ur shema so we can improve and comment on it

    Rifaa
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    R!f@@,
    Member Mik3 has done just that.

    Look in Mik3's blogs. I posted a link to his blog in the other thread on this subject that our OP started.
    Here is the link to his blog: http://forum.allaboutcircuits.com/blog.php?b=116
    You can see it in action in a YouTube clip.
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    that was the Idea, but mine will be far more bigger and finer.
    Like 20 Octaves.
    I like to think big.

    Rifaa
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    If you look around on YouTube, you'll find a video of someone's analyzer who used the FFT function in the library for a PIC30 to perform the signal analysis, rather than have a series of bandpass filters. That is the way I'd go; you'd eliminate virtually all of the analog circuitry.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Hopefully you're joking. 20 octaves is a frequency range of a million to one.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I'm sure he really meant 20 frequency bands rather than octaves. ;)
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Then this is a good opportunity for him to learn the meaning of "octave".:)
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ooooops! my bad. :p

    I meant 20 indicators.

    Rifaa
     
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