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Altera_Forum
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16 years ago

Electronics Newbie question concerning LEDs

Hi all,

I am an electronics newbie, and I am trying to design an guitar that has a light up fretboard. To do this I need to control about 120 individual, single color LEDs, and I am having trouble finding a good method. I've been researching for a few days now, and can't find a board with that many I/O pins to control. I was wondering if the Cyclone II would be able to control the LEDs? and if so would I need any other equipment?

Thanks so much in advance!!!!

-Steve

15 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    I guess you could use a Cyclone part to drive 120 LEDs. The current consumption might be quite high. 120 * say 20mA.

    A better way might be to somehow strobe the LEDs so they are not all on at the same time.

    For example, if your LED matrix is 6 rows by 20 you could light just 6 LEDS at a time then move on to the next 6 LEDs.

    If you strobe fast enough the LEDs should look like they are all on simultaneously!

    You would then only need 20 + 6 Output pins but would probably need to add external buffers to beef up the drive.

    Hope this is of some help
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Thanks for the quick responses guys! I originally was planning to turn on half of them at a time to learn scales but if that isn't feasible I might just do a max of 6 at a time (one for each string). I actually considered the strobing, but it seems to be difficult to program and I'm not sure what external buffers do. Would I need those for the cyclone too?

    Thanks again for the advice!
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Just use 8 or 16 channel serial input LED drivers (or just serial latches). You can do it all with 4 pins and one string of drivers.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    i see i thought it may be limited by the number of fingers you could play with. sounds like a cool project.

    you should take a look at PWM (pulse width modulation). this is a method of "strobing". you could also use external drivers as mentioned, or transistor buffers in between all of the LEDs.

    with PWM, if you used a 4 bit counter you could have 16 groups of LEDs which would be on 1/16th of the time (or off of course). staggering the phase of each LED group would mean you'd have a maximum of 200/16 LEDs on at a time. the programming is still pretty simple.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Hi,

    As was already said, multiplexing the 160 LEDs into a matrix of say 16 rows of 10 LED's each would be the standard way of doing this. What is not so clear yet is that if you cycle through the columns fast enough, it looks as if all 160 LEDs are on at the same time. You need to go through the matrix about 60 times per second in order for the eye not to see the flickering. You would then drive the 16 LEDS in each column 1/(60*10) = +-1.6ms. You would display col 1 for 1.6ms, then col 2 for 1.6ms, then col 3 for 1.6ms etc until you reach col 10 and then you repeat again from col 1. To the eye it will look as if all the LEDS are on at the same time (at least the one you want to be on).

    You do not need an FPGA to do this and a microcontroller with at least 10+16 =26 I/O's can do it (You can use fancier tricks with shift retgisters and latches to further reduce the I/O count). You will need to drive more current through each LED than usual since it is only on for 1/10th of the time. Although it is not purely linear, I would suggest an overcurrent of at least 3 times your normal continuous current. You could probably drive the 16 LEDs directly form the microcontroller (or use 74AC244 or similar buffers to increase current drive). The 10 column you would probably need to drive with transistors. The easiest is to use NPN transistors to ground.

    See the attached drawing.

    Regards,

    Niki
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    The feasability of a multiplexed drive scheme depends on the intended brightness. If you apply 1:10 multiplexing, as suggested, you typically aren't allowed to increase the current higher than a factor of 5, which results in half the intensity achievable in DC operation (not considering current dependant efficiency, which further reduces the effective intensity by a small amount).

    But there are considerable differences in pulse current rating between LED product families, so you should always check the datasheet.

    It also depends on the current level, if you are able to drive the "rows" by standard logic devices or even a microcontroller IO port. In an application targetting to high brightness, this most líkely won't work.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    when staring at a fretboard i don't imagine you'll want it to be bright at all, in fact the dimmer light would be a welcomed feature.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Again, thanks for the input everyone! I do like Niki's idea of strobe matrixing, but I'm confused on what parts I would need. Are there any suggestions of microcontrollers and other parts that I would need? I would like to use my laptop to control the lights that go on