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CONFUSED -wire problem

  1. Apr 16, 2012 #1
    Ok; so basically, I am creating the game of Nim through gate chips (AND/OR/NOT), switches, and yea all the basics.

    I have pretty much completed designing and physically building the game but I have been having issues with one portion of the design -not the logic of the design.

    I have a regular wire coming out the output of a D flip-flop into an AND gate. I also have a resistor-LED coming off the output to ground to see the output of the flip-flop.

    When I physically touch the wire or get really close to the wire, the led lights up accordingly:

    not touching but close = led flickers
    touch the wire = led on
    squeeze wire = led brighter

    SOOO I have no idea why this is happening or how to fix this. (I can touch all other wires in the circuit and nothing happens.)

    OH and I had the same problem using banana clip wires to switches but that problem was solved when I connected the switches using the regular wires.
  2. jcsd
  3. Apr 16, 2012 #2


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    Staff: Mentor

    Welcome to the PF.

    If you leave CMOS logic gate inputs floating, you can see this type of behavior. The CMOS inputs are very high impedance when left unconnected, so static electricity and nearby parasitically-coupled 60Hz noise can make them switch their outputs. Always have something connected to logic gate inputs (tie off any unused inputs appropriately).
  4. Apr 16, 2012 #3
    This circuit will determine who starts the game (computer or human) and will toggle the current turn after the clk is pressed (push button switch).

    heres how I wired the diagram:

    I think I have all the inputs connected? but the circuit reacts in the same way...

    Attached Files:

  5. Apr 16, 2012 #4


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    Staff: Mentor

    And when the switches are open, what is connected to those inputs.... ?
  6. Apr 16, 2012 #5
    Sounds like you just need a pullup resistor.
  7. Apr 17, 2012 #6
    thanx guys! I got the wire fixed some of my gates were bad apparently.

    butta Berkeman

    Ok so I have this basic switch which looks like


    When the switch is ON, current is allowed through the switch while when the switch is OFF, no current is allowed through the switch.

    My problem is where when the switch is OFF. Seemingly, if you connect a switch CMOS logic gate, the input of the switch is floating rather than '0'. Is there a way to make the switch give an output of '0'?

    nvm! I figured it out! I put a large value resistor connected to ground with the input from the switch which gave me a 0 and 1 output to the logic gates!

    Last edited: Apr 17, 2012
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