Making an led vu meter - LM3915

Discussion in 'The Projects Forum' started by biscuitownz, Aug 9, 2013.

  1. biscuitownz

    Thread Starter New Member

    Aug 9, 2013
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    Hello everyone, I'm new here and wanted to ask for some help with an upcoming project of mine. I want to make a vu meter that looks like this (http://www.youtube.com/watch?v=_UDw6NOazt0).

    However, all of the diagrams I've found seem to have people saying they don't work or this and that should be changed. So I want to ask people who have dealt with this IC first hand so I know what to expect and so on.

    I would also like it to be ran in dot mode and not the bar mode. I know bar mode can be turned up by connecting pins 9 and 3.

    Here's the specs of the IC (http://www.ti.com/lit/ds/symlink/lm3915.pdf)

    Thanks
     
  2. Mussawar

    Member

    Oct 17, 2011
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    LM3915 is a log base ic. It means that it process the input signal in log scale. You have the datasheet, fine. Try to understand the basic working of this chip. Major task is to manipulate the hi and low ref at pins 7,8. You may try variables at these pins to adjust the ref. Leave pin 9 open to work in dot mode. In general, you can try 390 Ohm resister bw pin 7,8 and a 2.7k resister to ground from pin 8. Also use a variable at input. It should work fine.
     
  3. biscuitownz

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    Aug 9, 2013
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    I wired it up according to the diagram given by manufacturer. However, I used 1k ohm resistor between pin 7/8 and then tried a bunch of resistors from 1k-10k on pin 8 to ground. All of them seem to give me the same result of the leds not fully turning off when they move in dot mode. They just seem to flicker but they don't turn off completely. I'm not sure what to do now.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    1k between 7 and 8 is fine but you also need a resistor between pin 8 and ground. You also need to connect pin 6 to pin 7.

    You cannot just assume there are errors in datasheets. connect them correctly first. If you get the datasheets directly from the manufacturer's website (as you did), you will see they are generally correct. Also, look for the revision date at the bottom of the datasheet. The one you posted was revised in MARCH 2013. What was the date on your link that pointed out the error?
     
  5. biscuitownz

    Thread Starter New Member

    Aug 9, 2013
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    Yes, 6/7 are bridged and there is a resistor between pin 8 and ground. Right now, I'm using 1k between 8 and ground. I will post a video showing it soon.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    What is and exactly how is your audio signal connected to the circuit?
     
  7. biscuitownz

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    Aug 9, 2013
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    I have it connected directly to my amp. I've tried plugging it into the preamp but it gave the same results. I also tried rectifying the signal but it was still doing the same thing. I'm uploading the video right now.

    EDIT: Link here: http://www.youtube.com/watch?v=eH9qgYzbi8A&feature=youtu.be
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    However surprising it may be, your chip is working perfectly. The signal you are putting in is vibrating up and down at 1000 times per second (from zero to peak voltage). The chip is displaying the current (instantaneous) input voltage constantly.

    If you want more of a single dot, you will have to add circuitry that will trap the signal from the music at discrete times (five times per second). You also need a peak detector circuit (see the DATASHEET). The peak detector will make sure you measure the highest value in the 1/5th second interval.

    You might want to invest in a $7 breadboard to let you prototype some circuits.
    Cheers
     
    Last edited: Aug 9, 2013
  9. biscuitownz

    Thread Starter New Member

    Aug 9, 2013
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    Well, that would seem to explain a lot since they do flicker.
    What would be the simplest peak meter to make? I've looked around and most of them seem to use the LF351 which seems to be discontinued, unless I get it on ebay. Are there any alternative components that would act the same way as a peak detector?

    I have a proto breadboard....It just doesn't seem to like the LM3915 as it doesn't work unless I press down on it.

    I've found this diagram, http://www.aaroncake.net/circuits/vumeter.gif

    Is the circuit next to the input a peak detector?
     
    Last edited: Aug 9, 2013
  10. GopherT

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    Nov 23, 2012
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    Why do you look for a second source instead of the top of page 12 in the TI DATASHEET you linked above.

    There are some "hints" in the DATASHEET on op amp selection if you go that route. See page 11.
     
    Last edited: Aug 9, 2013
  11. biscuitownz

    Thread Starter New Member

    Aug 9, 2013
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    Sorry, I got carried away. Anyway, I decided to go with the "Half-Wave Peak Detector" which was the one on top of page 12. Although, where would I connect the other wire of the audio input to? Would it just connect to ground?
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You don't need an op amp that fast for an audio level meter, I use an LM358. I just finished building one using the LM3914 IC and you can steal the peak detector off it. U2A, U2B and associated components peak detect the input signal. I didn't check the peak detector on the data sheet but the one I have shown is a proven performer I have used many times.
     
  13. biscuitownz

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    Aug 9, 2013
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    Thanks for all the help guys, I got it to work in dot mode after adding the peak detector. :D
     
  14. GopherT

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    Which peak detector circuit did you use?
     
  15. biscuitownz

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    Aug 9, 2013
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    The one on top of page 12 of the pdf
     
  16. bountyhunter

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    Sep 7, 2009
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    That's a really low performance circuit. It has no active current drive to charge the storage cap so it's ability to catch the peak voltage gets worse the higher the voltage signal goes. Fast transients are just going to wave as they go by.

    The accuracy is very poor, there is no feedback. It's a sort of voltage follower from the input to the emitter of Q1, then down a diode drop across D1... so it's sort of "up a VBE and down a VBE so it cancels out" except the diode drop of a 1N914 is probably 200 mV higher than the VBE of a 2N3906. This circuit is for flashing lights only, no accuracy.
     
    Last edited: Aug 11, 2013
  17. vmsa

    New Member

    Sep 27, 2010
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    Hi, I know very well the LM3915 operation, and I could not keep from writing something to you.
    I use it from years. For example, see my madness design in http://www.youtube.com/watch?v=fMNRLboMBIg , where I used 3 LM3915 in a novel design.

    Please, excuse my bad English, but I try to help you, explaining a couple of simple facts, without "electronics maths":

    The pin 7 is the voltage reference fixed at some 1,25V above pin 8, and the current drain from it is 1/10 the LEDs current. One example if you connect the pin 8 to ground, and pin 7 with 6, (as page 7 in datasheet) then the IC is a voltmeter with 1.25V FS. If you connect a 1K2 resistor between pin7 and ground (1 mA), the LED current will be around 10 mA (typical for small LEDs).

    In this configuration you need a 1,25V in the pin 5 to obtain the last LED turned on. If you connect it at the output of a power amplifier, is sure that you have more than 1,25V, and you need some type of level control (as a potentiometer).
    Next you need to RECTIFY and smooth the signal. How to do this depend what quality you want. A simple diode and a capacitor can be a simple solution, but you see only the positive semicycle of the signal, and have a threshold of 0,6V.
    The input resistance of 3915 is high and you need to put a resistor to ground, for example 10K, and the capacitor may be over 1uF. Test different values. If the value of the capacitor is small the lights jump quickly, if it is big the lights marks the peak level for a time.

    As this IC have a logarithmic scale, every LED is -3dB below the previous LED, from up to down. Starting from the example, if you connect Pin 7 to 6, and pin 4 and 8 to ground, the maximum voltage is Vref. This correspond to the voltages indicated in the table in FIG 30 of datasheet, with -3dB between voltages (about 0.7 times).

    I hope this information be util for your project. :)

    Sincerely,
    Victor
     
    Last edited: Aug 23, 2013
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