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Logic Circuit help

Discussion in 'Electronics Homework Help' started by Madz2007, Dec 8, 2016.

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  1. Madz2007

    Madz2007

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    Dec 8, 2016
    Hey guys,

    Before i post what i need help with, i'll leave my skype if anyone would be kind enough to help me 1 on 1 please do :)

    Skype: Live:mahdi.rajabi_2

    Im having some problems figuring how to make this logic circuit.

    This is the problem:

    https://gyazo.com/1e4e58b73e6ec4881f1b8f1c6682cf66

    I think i've figured out the truth table, but maybe wrong.

    logic.png (uploaded by mod)
     
    Last edited by a moderator: Dec 8, 2016
  2. davenn

    davenn Moderator

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    post your result and some one better at digital than me can check it :)
     
  3. Laplace

    Laplace

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    Apr 4, 2010
    The number is odd only and always when the "1" input is HI, A= 1. The number is even when either the "2" or "4" or both inputs are HI and the "1" input is LO. So the green LED is identical to input A. The red LED is HI when either B "2" OR C "4" are HI, AND A "1" is NOT HI. That is how you get 3 logic gates AND, OR, & NOT for your design. This is a trivial logic design that does not require design tools, so the object is not to get the solution but rather to demonstrate ability to use the design tools, to convert a problem statement to a truth table, to convert a truth table to a logic function, to transfer a truth table to a K-map, and to minimize a K-map.
     
  4. Madz2007

    Madz2007

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    Dec 8, 2016
    thank god for you help, please add my skype: Live:mahdi.rajabi_2 I really need more answers about this.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Post the details here or I will close the thread. We aim to answer questions both for you and for anyone who comes along later with a similar problem.

    If you can't play your part you may find that we won't spend a lot of effort helping you.
     
  6. Madz2007

    Madz2007

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    Dec 8, 2016
  7. Laplace

    Laplace

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    Apr 4, 2010
    Looks good to me. I like to use index numbers for the truth table, and mark as much information as possible on the truth table. See attached image. On my K-maps I also put little index numbers in the corners of the boxes. That makes it easier to transfer the contents of the truth table onto the K-map.
    Image-even-odd.png
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Laplace has given you a nice table to work with.

    The 'logic' for ODD is ridiculously simple as can be seen quickly from Laplace's table.

    The logic for EVEN would also be very simple if it weren't for the quirky definition of zero not being EVEN (zero is an even number).
    However, compare EVEN and ODD for all numbers except 0 to find a very simple logic for the red LED. Then add a little bit more logic to account for the special case 0.
     
  9. Madz2007

    Madz2007

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    Dec 8, 2016
    A friend has helped me put this Logic Circuit up, representing that Truth table.

    Circuit:
    [​IMG]


    Kmap:

    [​IMG]

    What is the equation for the table? Can someone help me get the formula
     
    Last edited by a moderator: Dec 9, 2016
  10. davenn

    davenn Moderator

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    @Madz2007

    It would be really helpful if you would post your images in the forum like I have done for you
    it saves people having to load other pages and then flick between pages for their discussion :)


    Dave
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    For the Green LED, have a very close look at the input A...

    For the red LED, look in the same way at A, then cosider A=B=C=0 as a special case to add the required logic.
     
  12. Madz2007

    Madz2007

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    Dec 8, 2016
    Ok, i have no idea how to do that, im so rusty at all this, i just got into breadboad and any sort of wiring 4 days ago... So im picking up a lot of stuff at once, but im trying.

    In all honesty, I understood that you said to go back to the truth table and look at the Inputs, but i dont know how to write them or anything..

    However, could you tell me whether the the circuit is correct?
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

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    O.K., what is the difference between input A and output ODD (green)? If you don't see the answer for the green LED now, I can't help :(
    For the red LED, compare green and red for all numbers 1...7, there's only one logic function that strikes you. only 1!
    What will be the output of this logic function for the case A=B=C=0? Obviously not the one expected. So you'll have to add some logic to make the red output 0 for A=B=C=0.
    Which logic function can enforce a 0 output (by using one control input) regardless of the state of the other input? It is an elementary function.
    What is the logic level on this control input for this case and how can you create this logic level for A=B=C=0 only?

    Sorry, I do not have the tiem to draw a schematic from your wiring diagram. Show us the schematic diagram, that's quite another thing.
    Also don't forget to label the inputs A, B and C and the outputs red and green.
     
  14. Madz2007

    Madz2007

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    Dec 8, 2016

    Here it is:

    [​IMG]
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    I meant something like this.
    We can live with the schematic from post #14, but it is not ideal.
    The chips are 7408?
    Where's A, B and C?
    Which of D1, D2 is the red one, which the green one?

    Whatever the answers, the logic for the green diode is definitely much too complex.
    Go back to post #7, compare A and green bit by bit what do you see?

    Answering only one part of a post will not help you along. Read the full post, follow all hint, answer all questions.
     
  16. Madz2007

    Madz2007

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    Dec 8, 2016
    Apart from what Laplace already said:

    [​IMG]

    And the Green LED and Red LED are complete opposite to each other apart from #0 i dont know :(
     
  17. Madz2007

    Madz2007

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    Dec 8, 2016
    I also got this, this MAY NOT be correct, please tell me if it is or not.

    [​IMG]
     
  18. Harald Kapp

    Harald Kapp Moderator Moderator

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    You finally got it!

    Right again, so red = NOT green (except A=B=C=0). Now let's tackle that last obstacle. Which logic gate will force its output (red) to 0 regardless of the input (NOT green) when the other input (what I called control input) is 0? This is an elementary logic function of two input signals and one output signal.

    Next task: how can you create a control signal that os 0 for the condition A=B=C=0?

    You then have all the necessary building blocks for the solution.

    Finally heed Laplace's comment:
    You'll stil have to perform these parts of the task.
     
  19. Harald Kapp

    Harald Kapp Moderator Moderator

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    Doesn't look like it's doing what you expect. Check for yourself by writing down the truth table.
     
  20. Madz2007

    Madz2007

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    Dec 8, 2016
    I have no idea how to test my circuit to see if it matches the truth table.

    And is it the And gate?
     
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