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4-input controlled relay

  1. Apr 18, 2013 #1
    I am trying to implement the following device:
    It takes a 4-bit input string, and a control input. If the control input is 1, the output is a 4-bit string. If the control input is 0, the output is 0000. What sort of device might make this possible? Obviously, brute force logic with AND gates works, but is there an easier method perhaps?


  2. jcsd
  3. Apr 18, 2013 #2
    Go with a Quadruple 2-Line to 1-Line Data Selector.

    Connect your input string to the B inputs, and your "default" (0000) to the A inputs. This gives you the option of setting what you want your default to be if it might change.

    If you want it as a fixed circuit and the 0000 has no chance of changing, you can just use 1 AND gate per bit. You can get quad AND gates in one chip pretty easily, so "brute force" logic isn't really that difficult.

    Just wire your control signal to one input on each gate, and your input string to the other inputs. When the control signal is high:

    A & 1 = A

    So the bit is passed through. When the control signal is low:

    A & 0 = 0

    and the output is forced to be 0, no matter what the input string.
  4. Apr 18, 2013 #3


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    If you're looking for an implementation of an actual 4-input relay (i.e. a relay that switches when any one of four inputs goes high), you can just use blocking diodes: connect each input to the anode of a diode and connect all four of the diodes together and to the relay. This'll prevent shorting out of the other inputs. Pro-tip: put a flyback-diode across the relay to reduce back EMF (Google for flyback-diode to see the interpretation).

    If you're handy with microprocessors / microcontrollers, a single low pin-count micro can now replace what used to be a small network of glue logic (albeit, at a little higher cost / complexity).
  5. Apr 19, 2013 #4


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    The output being the same 4-bit string as the input, I presume?

    If the input changes while control = 1, you want the output to immediately reflect that change?
  6. Apr 19, 2013 #5
    Yes I apologize for leaving that out. And yes, if control = 1, output must respond to changes in input.

  7. Apr 19, 2013 #6


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    Staff: Mentor

    If the logic inputs are clean and of settable levels, and the input of the following stage is consistent and understood, you may be able to use resistor AND gates. Don't use this for a mission-critical stage, or something you are being paid to design for an employer. You'd need to have a good reason for using this dodge.


    Tailor input logic HIGH voltage to be towards the minimum usable. Make input 1 a data bit and input 2 the control. Choose R1 and R2 so that when either input is LOW, the junction voltage is below that recognized by the following gate as HIGH. The noise margin won't be as wide as otherwise.

    This idea may not be feasible for every logic family.

    Attached Files:

  8. Apr 19, 2013 #7


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    Good idea using two inputs (A & B). I was going to suggest the CD4016 Quad Bilateral Switch
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