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Homework Help: Can u make if A>=B then output=1 Else output=0 using these logic gates?

  1. Mar 24, 2008 #1
    [SOLVED] can u make "if A>=B then output=1 Else output=0" using these logic gates?

    1. The problem statement, all variables and given/known data

    this is not quite a homework problem. I am designing a device and I need to use A>=B comparator.

    The challenge is I need to do this using only NAND, AND, NOR, OR and ExclusiveOR gates. Because these are all we have at the electronics laboratory.

    This might look like a computer science problem at first. But I have to make this using actual gates, cables and LEDs. (Hence, this is electronics :))

    2. Relevant equations

    no equations, just boolean logic

    3. The attempt at a solution

    tried using some different arrangements but can't really find one that works
    Last edited: Mar 24, 2008
  2. jcsd
  3. Mar 24, 2008 #2

    I have just got the equal to part completed. now I need the "greater than" part.

    equal to is

    EOR ==> NOT ==> (output)

    (inputs A and B are going to the EOR gate)
  4. Mar 24, 2008 #3
    The truth table of your gate is [C is the output]:

    Code (Text):

    | A | B | C |
    | 0 | 0 | 1 |
    | 1 | 0 | 1 |
    | 0 | 1 | 0 |
    | 1 | 1 | 1 |
    This logic can be given by:

    C = \overline{A}~\overline{B} + A\overline{B} + AB[/tex]

    C = A + \overline{A}~\overline{B}

    So, you get,

    A (OR) (!A (AND) !B)


    http://img89.imageshack.us/img89/6882/pflogicxbt7.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  5. Mar 24, 2008 #4


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    How many bits are A and B? If they are small enough it wouldn't be that hard to write out the K-Map and find the circuit by brute force, would it?
  6. Mar 24, 2008 #5
    from his second post, i'm guessing both inputs are 1-bit values only.
  7. Mar 24, 2008 #6
    thanks, that helps a lot
    (the inputs are one bit each, either high (1) or low(0) )
    Last edited: Mar 24, 2008
  8. Mar 24, 2008 #7
    hey rohanprabhu, what program did u use to make that drawing?
  9. Mar 24, 2008 #8
    it's made by hand.. in photoshop.. nothing automated. I used the gate images from wikipedia.
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