Newbie question about LED circuits

Discussion in 'General Electronics Chat' started by Arosfa, Aug 6, 2014.

  1. Arosfa

    Thread Starter New Member

    Aug 6, 2014
    Hello everyone, I'm hoping someone can help me!

    I'm trying to design a simple circuit with two LED's that depending on what DC voltage is applied to the input, one or the other LED will light up.

    In more detail:

    1. If I apply a 12VDC supply, a green LED will be ON and a red LED will be OFF
    2. If I apply a 24VDC supply, a green LED will be OFF and a red LED will be ON

    Can this be easily achieved?

    Thank you..
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    It's a bit tricky, as I assume you don't want to use another power supply but run off the input "supply." Also, you don't say what you want if say 11 volts or 19 volts gets applied, so I'm going to assume the 12V and 24 volts are relatively fixed, say from using either one or two batteries in series.

    Getting a "snap on" light will take more parts, but for just a simple indicator it's doable.

    Let me toss the attachment out as a starting point:

    Sorry I cannot get it to view on this computer. Mods?


    Zener Z1 keeps LED1 off until you get greater than 7 volts in, then it comes on. R1 sets the nominal 12V current to about 10mA.

    At around 19V Z2 starts to turn LED2 on. R2 sets the nominal current at 24V to also around 10mA.

    When LED2 is on the current flows into Q1, which turns it on and shorts out LED1. However, current is still flowing thru R1 so the net current draw is about 50 mA.
    • LED.bmp
      File size:
      57.2 KB
    Last edited by a moderator: Aug 6, 2014
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  3. AlphaDesign888

    New Member

    Jul 27, 2014
    Window comparator.
    Arosfa likes this.
  4. Arosfa

    Thread Starter New Member

    Aug 6, 2014
    Thanks ErnieM. That should do it.

    Thanks Alpha, hadn't thought of that (actually, I didn't know it existed - my learning curve is a steep one!)