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LED Spectrum Analyzer schematics

  1. Nov 26, 2006 #1
    I am looking to build a basic spectrum analyzer (if that is what you would call it) out of some LED lights for a car stereo.

    Basically, I want about 8 - 10 bars (for different frequency bands), and I want them to jump up and down to the music, giving it a direct feed off of the car stereo head unit.

    Looking for advice on components, any schematics that I may be able to use for the various parts (I figure I will need to build several cheap bandpass filters for each of the frequency bands, or use some sort of multi-band filter that I might find in a basic cheap equilizer circuit or something like that.

    Still trying to get some ideas.
  2. jcsd
  3. Nov 26, 2006 #2


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    Staff: Mentor

    I googled stereo led bargraph, and got lots of great hits. Here's the 2nd one on the list:

    http://www.talkingelectronics.com/Projects/StereoVUmeter/StereoVU.html [Broken]

    Welcome to PF, Bob.
    Last edited by a moderator: May 2, 2017
  4. Nov 27, 2006 #3
    Thanks for the info (and the welcome)

    I have already been doing a bit of searching....

    These two most closely match the basis of what I am trying to accomplish:
    and most specifically this:
    http://www.rose-hulman.edu/~sharpjm/led/led.html [Broken]

    The second link, is almost exactly what I am trying to accomplish, however, I am relatively new to circuit design, and I was hoping to find something that was a bit more complete, but I don't think I'm going to .

    The best I can tell, I am going to end up building 8-10 of the circuits that are described in the second link, and then I will need to build 6-8 bandpass filters , plus a high-pass filter and a low-pass filter and then feed my audio input into that whole mess.

    Does anyone know of a circuit design that would give me multiple band bandpass filters all in one project, that I could use as a basis to feed into the LED circuits?
    Last edited by a moderator: May 2, 2017
  5. Nov 27, 2006 #4


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    One alternative is to use Digital Signal Processing (DSP) and a fast microcontroller (uC) to process the audio and calculate the running filter output values. Have you explored the DSP approach at all?
  6. Nov 27, 2006 #5
    No, I haven't. I'm not sure if such a circuit would be beyond my capabilities or not....I guess it would depend on how much I had to start with....

    How expensive or complex would it be to build such a circuit? Would I still be able to use the same basic LED VU meter design for the outputs?
  7. Nov 27, 2006 #6


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    If you haven't programmed a uC before, and haven't had an introduction to DSP yet, then it probably is not a good match for your current project. If you were going to use the DSP option, then you would probably buy a medium-end uC evaluation board (like for a PIC18F4455 or Atmel AT91SAM7S64 or similar), and build an audio ADC circuit and an interface circuit to your LED drive board from the uC evaluation board. Then you'd program the DSP algorithm (mostly in assembly with a C wrapper) and test it out using the appropriate debugger.
  8. Nov 27, 2006 #7
    Yeah, probably true. I am more of a programmer than I am a circuit designer, but this might be a more abitious project than I would want to take on as a first time uC program.

    I found this Velleman kit that I think is what I was looking for:

    However, from what I understand, one of the ICs is no longer available.

    Still trying to figure out if there is a way around that, or to at least use this as a building block...
  9. Nov 27, 2006 #8
    I just found this page...

    It seems too simple, would it actually work?

    It looks like all I would need to do would be to add some additional LEDs coming off of the LM3914 or LM3915

    Is there something I'm missing?
  10. Apr 25, 2007 #9
    Have you made any progress on this project?

    I've been beating my head against the wall, trying to find a practical solution to the same issues you faced. Please let me know. I'm very interested!

    Thanks a bunch.
  11. Apr 25, 2007 #10


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    Homework Helper

    The key element to the Velleman circuit is the Filter Sections A and B which pick out the frequency bands for the display.
    A DSP solution would implement these as a FFT in software.

    With some minor modifications, you should be able to substitute the LM3914 for the obsolete IC
    Or you could use the first circuit Berkeman posted as a substitute.
  12. Apr 26, 2007 #11
    No DSPs...


    Thanks for the reply. I am looking for a clean, simple elegant solution to the filter design for the different LED bars. The LM3914 is not scarce... I just bought a dozen off ebay for about $30. Using a DSP is not an option for me... and I know absolutely nothing about them. I'm not into saving pennies on transistors either. I'm just looking for suggestions on a simple filter design, maybe a modular design using standard/basic components that I can replicate and adjust the component values to filter the different frequency bands.

    Does anyone have a simple solution that does NOT involve a DSP??? I know there is one, because the old stereos had this before DSPs were availlable.

    Many thanks
  13. Apr 26, 2007 #12


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    Old stereos used high pass and low pass fillters.
    Using this approach a 3 channel display can be done with only a few passive components.

    For more channels you need active bandpass filters like the ones in the Velleman circuit.
    All 10 filters are identical except for the component values.
    They do not use DSP.
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