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Making a bicolor LED blink red/green

Discussion in 'LEDs and Optoelectronics' started by Eddie Klein, Jan 24, 2015.

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  1. Eddie Klein

    Eddie Klein

    Jan 4, 2015
    I'm attempting my first electronics project. I have some two lead biclored red and green LEDs. I want them to blink from red to green using a 3v coin cell. With having only two leads I assumed by simply connecting + to + and - to - that I would have a blinking circuit. WRONG...What I have is constant red or constant green depending upon which lead is connected to + or-. I'm sure there is an easy straight forward way to make them blink but I don't know how to do it. Can anyone help please. Thanks
  2. davenn

    davenn Moderator

    Sep 5, 2009
    That's correct, to light the opposite colour the voltage needs to be reversed.
    So to have it blinking back and forwards between colours you would need a
    circuit to reverse the voltage

    here's a circuit I found, I'm sure you could have found it too ( google is a wonderful thing ;) )


  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Eddie and welcome to Electronics Point :)

    Here's another option that will work (barely) from a single 3V battery and will run for a reasonable length of time.


    The blink rate is determined by the product of RT and CT. You can vary either one to change the blink rate.

    U1 is a CD40106B hex inverter with Schmitt trigger inputs. This may be available from your local electronic components store, otherwise you can get it from Digi-Key ( or Mouser ( It's available in a DIP package (the type with pins that go through the board) or an SMT (surface-mount technology) package that mounts flat onto the board. The DIP package is easier to prototype with, but larger.

    CD is a 100 nF or 0.1 µF ceramic capacitor that must be connected

    RL affects the brightness of the LEDs, although at 3V that circuit won't deliver very much brightness - try it and see whether it's suitable or not. Reducing RL increases the brightness; you can reduce it all the way to 0Ω (i.e. a short circuit) without harming anything, if the supply voltage is only 3V. That will give you the brightest indication you can get from that design (unless you find a more efficient LED).

    You can build the circuit on a breadboard, a piece of stripboard, or using "skeleton wiring".
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