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120 LED indication device

Discussion in 'LEDs and Optoelectronics' started by paddy, Dec 5, 2012.

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  1. paddy


    Sep 11, 2012

    I have to make an indication device which has 120 LED's.

    I have connected that LED's to one port of micro controller via 74HCT574 and ULN2004.

    i.e Micro controller port -> 74HCT574-> ULN2004 -> LED's

    For 120 LED's I have used multiple 74HCT574 IC'c and ULN2004 IC's.

    I am sending common data on all 74HCT574 and enabling particular 74HCT574 by giving clock to it.

    Is this circuit okay?

    Please suggest.

  2. GreenGiant


    Feb 9, 2012
    without a picture of the schematic or anything I can't say for sure.

    It sounds ok, but do you have resistors for the LEDs? You need something to limit the current through them.

    You said that all of the LEDs are connected to one pin of the micro? with the information given it sounds like all are going to come on and off at the same time, and the circuit is a little over complicated for just doing that, you wouldnt need the ULN2004's unless you are using high voltage (6V+) LEDs. Just a thought
  3. BobK


    Jan 5, 2010
    I presume the 74HCT574s are used as shift registers by connecting each Q out ot the next D in.

    But a 74HCT575 would be better, as it has fewer pins, is already set up as a shift register and has a seperate output latch so that you can switch all 120 outputs at the same time, vs. getting flickering as you shift the data in.

  4. paddy


    Sep 11, 2012
    Please have a look on attached circuit.

    I have to check 120 values with some defined limit values and depending on result i want to turn on/off the LED's.

    1. I am comparing first 8 values
    2. Set/clear corresponding pin according to result
    3. Giving clock to 74HCT574 latch
    4.Again comparing next 8 values and so on.

    So the output of the latch will drive the LED's through ULN2004.

    Attached Files:

  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The circuit looks o.k., but can be simplified: The ULN2004 has internal base resistors for each driver. Therefore you don't need the external resistors between HC574 and the ULN.
    The resistors for the LEDs are required as shown.

    I suggest you put a resistor from /OE to GND. That way you can easily put Vcc to /OE for test purposes (e.g. blanking all LEDs). You may even want to connect all /OE pins of the registers together and feed this signal to one spare pin of the µC. Thus you can make any pattern blink by just disabling all register outputs via the µC.
    Or you could dim the LEDs by applying a PWM pattern to /OE.

    Or just turn them all off to save energy.

    Last edited: Dec 6, 2012
  6. paddy


    Sep 11, 2012
    Okay.I will check out with this.

    Thanks a lot for your kind support.

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