Total current draw when multiplexing LEDs

Discussion in 'General Electronics Chat' started by illusive, Sep 10, 2015.

  1. illusive

    Thread Starter Member

    Jul 9, 2015
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    Hello everyone,
    I'm trying to figure out the total current draw in a project i that i will be building in order to design the power supply. The project consists of a graphic equalizer MSGEQ7 which takes the audio signal, splits it into 7 frequency bands and multiplexes the 7 signals into one output. The MSGEQ7 is connected to a microcontroller which controls the signals and powers up the correct leds (example: as the 400Hz signal is present the controller sends the signal to power up only the green leds in my case).
    This is from where i got the idea:
    https://ubiyubix.wordpress.com/2015/06/27/7-band-audio-spectrum-analyzer-w-msgeq7-lm3915-attiny2313/
    I'll just remove the LM3915 and connect different color LEDs to the different frequencies directly with miner changes. I will connect 6 leds for each frequency band, lets say 15mA per led that will give me 90mA per band and the total current when all leds are on will be 630mA. But the problem is that the bands are multiplexed and this confuses me. Does this means that the total current at any time will be about 90mA? Or am i missing something?

    Thanks!
     
  2. JJM101

    New Member

    Jul 27, 2014
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    I looked over the schematic from the project you referenced. So if you aren't going to use the LM3915, how do you intend to control 6 x 7 = 42 LEDs?

    I'm a little confused on what you want to do compared to the project you referenced...

    The MSGEQ7 cannot drive anything itself, it is only intended as a reference as the datasheet says it can only output 1mA. It takes the 7 frequency bands, and outputs a voltage 0-5 for each band. You select the frequency band utilizing the strobe and reset pins.

    The LM3915 can read the analog voltage and turn on the correct LEDs for a SINGLE bar graph. In the project referenced, for a SINGLE frequency band, the LM3915 interprets the MSGEQ7 analog signal and displays the amplitude for that frequency band on ONE of the bar graphs. THEN, the microcontroller switches the MSGEQ7 to output the next frequency band, while simultaneously grounding all of the LEDs to the next bar graph. The 'multiplexing' is done by the microcontroller, in selecting each group of LEDs contained in the 7 different bar graphs.

    So to answer your question, your total current draw will be the max draw for one bar graph for each band, and in your case, 6 LEDs.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    You're missing something and you're not giving us enough information to give you an answer.

    By definition, when LEDs are multiplexed, they're not all on at the same time... When multiplexing, the LED drive current should be increased so the average power dissipation is close to the direct drive power; otherwise the multiplexed LEDs will appear dimmer.
    • Are you connecting the LEDs for each band in series or parallel?
    • What is the multiplexing frequency?
    • Are the LEDs all the same color?
     
  4. illusive

    Thread Starter Member

    Jul 9, 2015
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    This is my basic concept. It is just to show my idea and it's far from final.
    circuit.jpg
    I drew just 3 bands with only 1 led in every group. If i understand correctly how MSGEQ7 works then my idea is that at the output pin 3 the chip puts out an analog voltage from 0 to 5 depending on the current band frequency "strength" and the microcontroller activates the respective led group (transistor in my case).

    I think you can understand what i'm trying to do, maybe i have just based my project on the wrong article.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Ask your questions when your idea is closer to final.

    I understand your circuit is preliminary, but what you have drawn now is a combo LED/transistor tester that produces dead LEDs and transistors.
     
  6. illusive

    Thread Starter Member

    Jul 9, 2015
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    My idea is final, the circuit is basic.. so to speak... I was trying to say that i i know that a bunch or resistors are missing and so on.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, since it is multiplexed only one is on at a time, although you may want to drive them higher than their rated current so the will appear bright.
    Bear in mind you will need to keep track somewhere which band is being outputted.
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    That didn't come across in your post...

    The chip you mentioned outputs a voltage proportional to the peak for a frequency band. You haven't given sufficient information on how you're going to connect the LEDs or how you're going to drive them. In your mind, your idea may be final; but I'm not a mind reader.
     
    Last edited: Sep 10, 2015
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    He said a microcontroller. Therefore, it implies using his on-board ADC, converting the voltage of each band to a register value, then he said he will drive LEDs from his microcontroller.

     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    If you understand his requirements so well, why aren't you giving him a solution???
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I'm getting on a plane right now and no drawing tools available.
     
  12. illusive

    Thread Starter Member

    Jul 9, 2015
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    Well yes, i mentioned the microcontroller and also posted the link to the article from which project i got the idea. I'm also planning to use the original C code for the Attiny2313.

    I'm sorry if i wasn't clear enough.
     
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    That really narrows down the solution space. There are probably only a few million ways to connect LEDs and drive them from a microcontroller.

    But just wait a bit. GoperT knows exactly what you want and will be drawing you a schematic.
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If you plan to use the original code in C, you are going to have to use the LM3915 as well as all the other circuitry of the author in your first link.
     
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