Tri-state output with one input

Discussion in 'The Projects Forum' started by gordinho, May 11, 2011.

  1. gordinho

    Thread Starter New Member

    May 11, 2011
    I need to design a basic traffic light, which operates from one input. Basically, we want to be able to switch between 3 LEDs, with only one input signal. We can control the voltage of this input (lets say 0-5V). This voltage source could also be the power source, but does not necessarily have to be.

    Do tri-state switches exist? For example, a switch that can direct current 3 ways, dependent upon the amount of voltage it receives? If so, it seems like we could use this.

    Is there some way to do this with transistors? I am reading up on transistors, and how they can be used as switches, but not really fully understanding it.

    Another idea I had was to simply use LEDs with varying operating ranges all on the same circuit, so that when the voltage changed out of one's range it would shut off and the next would turn on.

    I would use something like a tri-state switch, or the varying LED idea, but I haven't been able to find anything like that online so far.

    Any ideas? It seems like a simple problem, but I'm relatively new to circuit design.
  2. SgtWookie


    Jul 17, 2007
    You could accomplish that using a "window comparator" circuit.

    Have a look at Rob Paisleys' page on comparator circuits:

    A window comparator circuit is about halfway down the page.

    You would need to add two more comparators to that circuit; one to detect if the input voltage was below the window in the middle, and another to detect if the input voltage was above the window in the middle.