50-100 LED Vu Meter How?

Discussion in 'General Electronics Chat' started by SavedByTheLight, Oct 6, 2011.

  1. SavedByTheLight

    Thread Starter New Member

    Oct 6, 2011
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    I need tons of LEDs for a massive VU meter. How do I get say 50 or 100 LEDs working together? I have looked all over for a diagram and have found none. Accuracy it not an issue but wow factor is.

    I have been looking at:
    The LM3916
    The LM3915 is the logarithmic version of the LM3914 chip
    The NTE1508
    The NTE1549
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    For that amount of leds I would use a micocontroller and multiplexing. A simple 10 bit ADC will be enough and say 15+6 pins will be needed to control 90 LEDs.
     
  3. SavedByTheLight

    Thread Starter New Member

    Oct 6, 2011
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    Can you cascade say 5-10 of any of the ones above? How?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If you're thinking of cascading the LM3916 - please don't. :) You might cascade an LM3915 with an LM3916, but it would be better to stick with LM3915's. Avoid using the NTE models unless you like to pay considerably more for an NTE labeled LM3915.

    You can cascade LM3915's to take care of a large dynamic range. Each LM3915 covers 30dB. If you cascaded 3 of them, you'd have a 90dB range. Cascading more than 3 is not recommended. Look in National Semiconductor's datasheet for the LM3915, starting on page 10 for how to cascade them.

    You will need a couple of very fast opamps to amplify the audio signal by 30dB. An MC33078 RRIO 15MHz dual opamp would be a good choice.

    You didn't say what your power supply will be.

    You need to provide more information about your LEDs; what is their typical Vf @ current rating. If they are colored LEDs, then indicate Vf @ current by color.

    You will need to use driver transistors for the LM3915 outputs. Even though the LM3915 can drive one or two LEDs from it's outputs, you really don't want that for this application; there would be too much power dissipation in the ICs themselves.

    It would also help if you could post a sketch or drawing on how you were considering placing the LEDs, and at which sound level they should activate. If you are cascading 3 LM3915's, that will give you 90dB in increments of 3dB. (every 3dB is a half-power point; 6dB is twice as loud as 3dB, but only half as loud as 9dB)
     
  5. SavedByTheLight

    Thread Starter New Member

    Oct 6, 2011
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    I am just starting this and I don't need anything exotic on the LEDs. I would like to stack them all in a line. Think VU meter on stereo LEDs but tons of them. I will settle for 30 if I must but I would like more. Why do they limit this at three?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Perhaps you should start with breadboarding just one LED per output; that way it won't take a lot of time, and you can have something working fairly quickly.

    While the LM394x series are pretty nifty ICs, you need to be careful to not ask too much of them; if you're operating it in bar mode with lots of LED current and the voltage supply is a bit high, you can easily burn them up due to excessive power dissipation. Using transistor drivers is really quite easy, and it takes most of the load off the IC.

    You can use multiple LEDs per output if you'd like.
    It has to do with the resolution of the IC vs voltage limitations.

    You do realize that every 3db is a half-power point, right? The signal doubles or halves?
     
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