three 1's detector circuit

Discussion in 'The Projects Forum' started by abuarab88, Feb 19, 2009.

  1. abuarab88

    Thread Starter Member

    Nov 6, 2008
    19
    0
    hello everyone
    i am attempting to design alogic circuit that give me 1 when i have three 1's adjacent and zero otherwise
    for example:
    01110101 input must give 1
    01011011 input must give 0
    01011100 input must give 1
    as you see guys that the input number has 8-bits
    please help me in designing the circuit theoritically and practically

    thank you all
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    Oh my...isn't THIS a fun one!

    If you have to do it with combinational logic, it will be a nightmare.

    HOWEVER....if this is a clocked application...that is, if every bit lasts for a precise length of time, you can use a simple integrator and threshold detector.

    Is this a timed circuit or not?

    eric
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Okay....it can be done combinationally without TOO much nightmare. :)


    You need EIGHT three-input nand gates, and a single EIGHT input Nor gate. (YOU can build this with a few 2 input NOR's)


    Eric
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Are you ready?

    Build an eight line bus, with each line representing a bit. Number each bus 1 through eight.

    Connect 1st 3 input NAND gate (inputs) to bus 1 2 3
    Connect 2nd ....................................to bus 2 3 4
    Connect 3rd .....................................to bus 3 4 5
    Connect 4th......................................to bus 4 5 6
    Connect 5th......................................to bus 5 6 7
    Connect 6th......................................to bus 6 7 8
    Connect 7th......................................to bus 7 8 1
    Connect 8th......................................to bus 8 1 2


    Now, NOR all 8 outputs.....and there you have it!

    Your bill is in the mail. :)

    eric
     
  5. autorelease

    Active Member

    Jan 26, 2009
    39
    0
    If you don't have to detect wrap-around (bits 7-8-1 and bits 8-1-2), then you could use 6 NAND gates. Then you could use two 74HC10's or 4023's.

    The 4078 is an 8-input NOR gate.

    This only works if you need to output high if there are at least three consecutive ones. If you need to detect the presence of exactly three consecutive ones, this won't work because of the NOR gate, and things get much more complicated...
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Good point on the wraparound

    I'm not sure it would be MUCH more complicated if we wanted to detect precisely three 1's. I think if you used EXCLUSIVE NOR gate, it would be taken care of. I'll have to think about this. :)

    eric
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Take another look at the original problem statement.

    Any time there are three adjacent '1's with a zero on either side, the output should be true, else false.
    Eric, your original solution won't work as is, because your three-input NANDS ignore the bits on either side of three adjacent bits.

    The wrap-around question is a valid point too; but we need to hear from our OP on that question.
    For example, would:
    11101110
    be considered true or false?
     
    Last edited: Feb 19, 2009
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Also it would ibe of nterest to have the OP to confirm whether the 8 bit pattern 00111100 would be true or false?

    hgmjr
     
  9. abuarab88

    Thread Starter Member

    Nov 6, 2008
    19
    0
    yes it is considered true
    and also this is considered true

    and i will be gratefull if you supply me with the name and numbers of components i need to use and better a draw of the combination
    cause i am too begginner

    thank you all
     
  10. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    This being the case, the approach outlined by KL7AJ should be a reasonable one.

    hgmjr
     
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