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Homework Help: Logic gates (and and nand)

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Using only AND and NAND gates, design a circuit in which 4 switches must all be turned on before a switch in another circuit turns off.

    i think half the reason i dont understand may be the wording of his question, i know the tables but i am not sure how the logic gates themself work. i am researching now, would really appreciate the help!

    2. Relevant equations
    the charts can be found on this site: http://www.kpsec.freeuk.com/gates.htm#summary

    3. The attempt at a solution
    Hello all, just after abit of guidance with a physics question i made for myself.
    I have this really good idea that i thought would work but it didn't and my university professor told me i should learn more about how these logic gates work!

    sorry this is my first post i editted to fix things
    Last edited: Jun 10, 2009
  2. jcsd
  3. Jun 12, 2009 #2


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    Science Advisor
    Gold Member

    Logic gates simply act out the appropriate boolean algebraic operator. You already have everything you need to know from the link you provided to layout the circuit. The next step is to figure out the logic table. A simple, but usually inefficient, way to implement a logic table is to use a gate(s) to create each possible high state and then OR all the outputs from each of these high states. For example: we want a high output when x=1 and y = 0 or when x = 1 and y = 1. Thus, for the first instance we would AND x and NOT(y). The second case we AND x and y. Then to get the final output we would OR the output from these two AND gates. A more astute observer would note that the y value is irrelevant in my example, you could simplify the output to just being x which shows how this method can be inefficient but it is the basic starting point for deriving a simple logic circuit.
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