Building bi colour LED selecter...

Discussion in 'General Electronics Chat' started by RRITESH KAKKAR, Feb 6, 2013.

  1. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    Hi,

    I am working on LED display 8x28. so, both anode are connected together to 74164 and i want to connect the cathode of both LED to PortB via selector by bit RD7...
    BC547 will be used via portB for LED.
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    Do you literally mean a 74164? Because if so, you'll be trying to source current from its outputs to drive the LED anodes, and a TTL chip has very limited ability to do that. You'd be better off using an HC series chip. With any matrix-scanned display, you have to consider the amount of current that will flow, both instantaneously and as an average. Can you make the LEDs bright enough?
     
  3. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    I am using 74HC164.
    here is the single color display
    http://www.youtube.com/watch?v=pvVnOHjuoKM

    I have no data sheet of led but i think it takes 20mA app, pls explain more...
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    The video shows that you've solved that problem, if it ever was a problem. Your LEDs seem bright enough, at least for use indoors.

    To use the bicolor LEDs, all you need to do is build the chain of 74HC164's twice, once for the "red screen" and once for the "green screen". Then every time your transistor turns on a particular row of LEDs, you drive the red and green outputs separately. It would mean that the transistors would each be driving 56 LEDs instead of 28, but maybe each of the red/green pairs would only have one color on at once.
     
  5. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    The video was for single REd LED...
    currently I am working on BI color two separated LED!!
    But i have already connected the anode lead together of LED, and i think i can switch single Color LED by simply connecting two transistor with base connected with 3.3K Resistance to uC pin's and the emitter to gnd removing GND from emitter will turn of all single Led and glow other, i hope you are getting me!!
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Sorry, I thought you had (effectively) an array of 8 rows by 28 columns becoming 8x56 when you put in the extra LEDs. What I believe you're saying is that it's now 16 rows by 28 columns. I should have picked this up when you said "both anode are connected together to 74164".

    However, I can see a problem here. You're going to be running each of your LEDs on a 1/16 duty cycle, because you have to drive each of your 8 red rows and each of 8 green rows in sequence. That means that to get an average current of 5mA in each LED, the instantaneous current must be 5*16, or 80mA. Obviously your 74HC164's can never do this, so your current will be less. I'm sure you'll see the LEDs do something, but I expect the output to be quite dim.
     
  7. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    NO, i am only using 8 Rows not 16....
    the two led will not glow at same time for example....
    Scrolling with RED LED then next time with yellow.
    so, i have to change the rows connected to PORT B with code for that i need circuit!
     
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  8. RRITESH KAKKAR

    Thread Starter Senior Member

    Jun 29, 2010
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    still waiting for help............
     
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