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Will this work?

Discussion in 'General Electronics Discussion' started by foTONICS, Mar 13, 2014.

  1. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    So I have this 12V 20A SPST toggle switch that has a built in LED. Now when I first bought this thing I was under the assumption that the LED only turns on when the switch is in it's 'ON' position, but after looking at the datasheet some more I realized that it's always on, regardless of switch position. I wanted to do a little circuitry to achieve this effect.

    The photo named 'before' is obviously how the circuit is before I did anything and the photo named 'after' is my idea.

    The only thing worth mentioning is that I left out component values as I will just pick these based off load requirements, I left out the current limiting resistor on the base of the BJT (forgot), and I don't know if I should use a BJT or a FET.
     

    Attached Files:

  2. kpatz

    kpatz

    247
    52
    Feb 24, 2014
    A SPST switch with an LED typically has 3 terminals: in, out, and ground, and the LED (with a built-in resistor) is connected between out and ground.

    If you wire the two switch terminals incorrectly, supplying 12V to the output terminal instead of the input, the LED will stay on all the time.

    Just reverse the two switch terminal connections in your circuit (1 and 3 in your diagram) and bingo! LED turns off with the switch. Your schematic (1st pic) is correct, but you must have gotten the two terminals mixed up when constructing the actual circuit. Or maybe the LED is connected to 1 instead of 3 in the switch.

    Your circuit (2nd pic) will work but it needs a base resistor. A BJT is fine for the application, though a FET would consume less power since there's no base current. But, you shouldn't need a transistor at all.

    Can you post a link to the datasheet, or give the part # of the switch?
     
    Last edited: Mar 13, 2014
  3. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    thanks a lot!

    here is a link to my switch:
     
  4. kpatz

    kpatz

    247
    52
    Feb 24, 2014
    Got it. :)

    The datasheet doesn't say which pin is 1 and which is 3 on the actual switch, which is probably what led to your confusion when the LED wouldn't turn off. Just swap the connections on the two main pins and you're good to go.
     
  5. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    yes it works perfectly now, and to think I was about to design all this circuitry around something as simple as swapping the leads.
     

    Attached Files:

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