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Power on/off bi-color led circuit?

  1. Dec 5, 2012 #1
    In a robotics project and need an indicator to show the operator when the safety circuit is open or closed. Got it done in software but need a physical idiot light. Thinking a bright red/green bi-color led. Have 24 vdc constant supply and a pin that sinks to ground (100mA max) just while the door is closed. Can only use the channel on the I/O connector (the pin) as the signal for the open/closed condition. No additional switches or contacts and KISS would be greatly appreciated.
  2. jcsd
  3. Dec 6, 2012 #2


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    Welcome to PhysicsForums!

    I don't know if I'm completely understanding the problem, but couldn't you just apply 24V to the LED cathode (choose one colour, unless you want both), put that in series with a suitable ballast resistor, and put the resistor in series with the I/O pin? The LED would turn on whenever the pin goes low.

    If you want one colour on during one condition, and the other colour on during the other condition, this would only be possible if your bicolour LED has separate anode and cathode for both LEDs (they often have one tied together so the package has three pins instead of four)
  4. Dec 7, 2012 #3
    Yes, thats a simple and direct led circuit. I was trying to get fancy and have a single circuit be green while pulled down by the I/O and red while "open". Remembered just enough to get lost in a reversed bias transistor circuit.
  5. Dec 7, 2012 #4


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    Hmm. What if the operator looking at this safety circuit is red-green colorblind?


    For jellybean applications, bi-color LEDs are fine. But for a safety application, you should consider separating the 2 LEDs spatially, to accommodate such potential operators.

    There's a reason that traffic lights still use 3 separate lights for red-yellow-green... :smile:
  6. Dec 8, 2012 #5


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    Here is a circuit to use two LEDs either as individual devices or in a package where each of the LED cathodes are joined together.

    A high input (in this case, +5 Volts) turns ON the left LED and turns off the one on the right. A LOW input does the opposite.

    http://dl.dropbox.com/u/4222062/LEDs.PNG [Broken]

    Bicolor LEDs sometimes give different apparent brightnesses for the same current and they also give different voltage drops for the same current, so the 220 ohm resistors may need to be adjusted.
    In this circuit, both LEDs get about 10 mA.

    EDIT: diagram modified for reduced current drain on input signal.
    Last edited by a moderator: May 6, 2017
  7. Dec 8, 2012 #6


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    Good thinking! :smile:

    If he really desired a single indicator, and safety was not involved, say, just some indication of STANDBY/ACTIVE status, then perhaps a blinking red and a steady green would serve the purpose.
  8. Dec 8, 2012 #7


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    In principle it can be done using 3 resistors, but is wasteful of current. Instead, the arrangement below is almost as simple and is more economical with current. A 2 pin bi-colour red-green LED connected between the X-X points will have 18mA in one direction or the other. The 4.7 volt zener diode can be a low power zener, 400 mW will do, otherwise 1 W.

    The assumption I used is that your I/O pin can sink current (approx 45mA in this case), but does not source current (i.e., on HIGH it's an open circuit).

    Bear in mind the caution given by Berkeman regarding a composite R-G indicator being inherently unsafe. (pun intended). :wink:

    Attached Files:

    Last edited: Dec 9, 2012
  9. Dec 10, 2012 #8
    If this is about safety (outside of a purely experimental setting) you better check the codes and regulations applicable. Probably there are a lot of them.
    You will find that there are standard circuits designed to comply with the codes. Just find out and use them.
    DonĀ“t try to cut costs or to reinvent the wheel. You even might be liable for damages and injuries.
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