Control LED brightness with sound

Discussion in 'The Projects Forum' started by Glen M, Mar 15, 2016.

  1. Glen M

    Thread Starter New Member

    Jan 12, 2016
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    I've been trying to research this for the past fews days but I can't seem to find much of an answer. A lot of people out there seem to have used a tip31 transistor circuit to switch a bank of LEDs but I'm not sure this is the best way to do it. I've also looked at VU meters in the terms of an LM3914/5/6 but these (as they are designed to do) switch a line of LEDs depending on sound whereas I want all the LEDs on but their brightness to increase as the sound level increases.

    Perhaps I'm looking for to simple an answer to too complicated a question. Is there any way of using the outputs of the likes of an LM3915 to control the brightness as I describe?
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you want a group of LEDs to vary brightness all together, they you connect all of them to a common power source and use a control signal (some aspect of a music audio signal) to vary the output of the power source. If you want to be able to see the variations, then having the LEDs respond to the audio waveform directly will not work. You eye cannot perceive something changing 1,000 times per second. So the standard approach is to filter the audio to select the frequencies you want the LEDs to respond to, then envelope-detect or amplitude detect the audio waveform go get a signal that varies with the loudness of the notes, not the frequencies in the notes. This signal drives the power source.

    A TIP31 is an excellent lower-level power transistor. What is the total current of the LED array?

    ak
     
  3. Glen M

    Thread Starter New Member

    Jan 12, 2016
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    The LEDs will be blue 3mm flat top 20ma 3.4volt. The array has 24 LEDs in it.
     
  4. AnalogKid

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    First thing you'll need is a 90 Vdc power supply.

    ak
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Yes a 90 V source is one method. Six rows of 4 LEDs powered by a common source and 15 Volts would work also. Something like the attached but 6 rows wide and using the music as an input instead of a constant current. Or four rows of 6 from 24 V?
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    Okay, thrown together quickly and not tested, something like this?
    I'm sure better people here could improve on the design.
     
  7. AnalogKid

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    Aug 1, 2013
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    Actually, the point I was making was that if you withhold important details, you get responses that are technically correct but completely useless.

    Discrete current mirrors do not work nearly as well as predicted. Separate from that, 687 needs a bit more work.

    ak
     
  8. hp1729

    Well-Known Member

    Nov 23, 2015
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    An early attempt at a simulator for me ... a modified version of 687. The basic idea seems to work. I'm learning to use Multisim Blue. No particular reason for choosing this simulator.
     
  9. Glen M

    Thread Starter New Member

    Jan 12, 2016
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    My power source is 12v but my intention will be 6 rows of 4no LEDs each. The LEDs won't get to their full brightness at this but it'll be enough.

    I really appreciate the schematics above but being not that familaire with such things could someone break it down into a more simple parts list?
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The datasheet for the LED probably has a chart of forward voltage vs. normalized intensity. What does it say for Vf = 2.75 V or 80%? Note that the power circuit will eat up some headroom.

    ak
     
  11. #12

    Expert

    Nov 30, 2010
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    Not work. 4 x 3.4V = 13.6 volts.
     
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Yes, the design needs to be modified to fit your specific needs. 15 V is no problem. What is your audio source like? The input needs to be modified to fit your needs also. Those things are up to you (the OP).
     
  13. Glen M

    Thread Starter New Member

    Jan 12, 2016
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    I've run several of these led already in groups of 4 from the 12v supply and they are fine. The LEDs are rated 3-3.4v I was merely presenting the highest voltage so 3x4 = 12v.

    I would of liked to use a small microphone as the audio source to completely separate out the LEDs from the audio equipment but I'm not sure if this will be possible. If necessary I can take a signal from the line out of the cd player or the speaker output of the amplifier. I assume the RCA line out would suffice?

    Long story short they are going to be installed in my car to operate as a part of my boot build for car shows etc. Engine off they run on the battery at 12v, engine running the voltage jumps to 14.4v but I can fit a voltage regulator to correct this. I would of thought 99% of the time they will be running just off the battery. They will only be used at most 2-3 times a years and only for a brief periods, as I say its a show item only.
     
  14. hp1729

    Well-Known Member

    Nov 23, 2015
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    LEDs are pretty forgiving on a little extra voltage. My LED flashlight runs four white LEDs (rated at 3.5 V) straight from three AA batteries (4.5 V). I figure I get about 200 hours of life out of them before they die. You may get less. cCar batteries don't have much internal resistance my flashlight depends on. A low dropout regulator may be wise.

    Input from a mike might need a preamp. From a speaker output ??? I don't know. It will either work or it won't. I don't think either side would be damaged by trying. A line output would work better.
     
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