another version for this circuit pls

Discussion in 'The Projects Forum' started by whitearrow, Jan 23, 2009.

  1. whitearrow

    Thread Starter Member

    Jan 6, 2009
    13
    0
    hi, can anyone help me to design another circuit which has the same outputs as the one in the attachment? the problem is that it works perfectly in simulation but in reality it is not working. the problem is that pin 12 of U2:B is an input which is determined by its own output. pls help!!!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your truth table is disorganized. In two places, you indicate the output true for A*B* C^ (C not). It also has two listings for a true output for A*B^*C^, which contradicts the two false conditions for the same states.

    The circuit has 4 inputs. Perhaps a table based on inputs A, B, C, & D might help. Notice that the U2B gate is a latch function - that is why the output of the OR gate is one of the inputs. The output will be maintained after the top input (A?) has gone low, but the others are maintained.
     
  3. whitearrow

    Thread Starter Member

    Jan 6, 2009
    13
    0
    no i need the two of them. ie A*B*C^ and A*B^*C^ will be a true output. this is becauce a bulb should switch on at point B and switches off at points A and C where the sequence starts from A ie A starts with a true value than B and than C. that is why i have an output serving an input. can you help with this problem

    thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    As Beenthere mentioned, you have four inputs.
    Or, is the topmost input (to U2:A pin 2) always a 1?
    If U2:A pin 2 is always a logic 1, then I'll assume that your inputs are A, B, and C, from the top down on the far left.
    If A=1, B=1, C=0 (leaving U2:A pin 2 high) then the output is 1.

    If B is then toggled low, there's a problem; the output is indeterminate - how long does it take B to toggle low in relationship to the propagation delays in the gates? If the toggle is instantaneous, it might work - but only in simulation. It won't be instantaneous in the real world.

    You need to simplify the circuit, and eliminate the feedback.
     
  5. whitearrow

    Thread Starter Member

    Jan 6, 2009
    13
    0
    how can i syplify the circuit and remains with the same outputs?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    You will have to set up conditions where A*B*C^ is true, and A*B^*C^ is true, and use an OR gate to pass either condition. Otherwise, the states are mutually exclusive.
     
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