One LED on then second by switch

Discussion in 'The Projects Forum' started by SDon, Dec 26, 2014.

  1. SDon

    Thread Starter New Member

    Dec 26, 2014
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    Hi, I'm new to this so forgive me if this question is very basic.

    I'm trying to figure out how set up two LEDs with a switch between them so one LED will be on and the second can be turned on by a switch. I tried the most obvious setup (power - resistor - LED - split one way to ground other to switch - LED - ground) but the second wouldn't light. I assume the electricity took the path of least resistance and just went straight to ground before getting to the switch. How do I get around this?
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Got a schematic?
     
  3. MikeML

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    Oct 2, 2009
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    Not clear from your post:

    When the switch is thrown, do you want the first LED to go off as the second LED goes on?

    Or do you want the first LED to go on when power is first connected to the circuit (i.e always on), and have the second LED control only the second LED?
     
  4. SDon

    Thread Starter New Member

    Dec 26, 2014
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    Hi Mike. The second option - first LED always on and the second goes on when I push the switch. I wanted to play around with the difference between connecting components in series vs parallel. I wanted to see the different between one light then close a switch and add a second in series, then I was going to set it up to do the same in parallel. I got as far as adding a second in series, but it didn't work.
     
  5. SDon

    Thread Starter New Member

    Dec 26, 2014
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    Blocco, I'm trying to find some software to let me do a quick schematic. I'm heading out for post-Christmas shopping but will try to find something this evening.
     
  6. blocco a spirale

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    Jun 18, 2008
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    Have you tried a pen and paper or MS Paint?
     
  7. MikeML

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    Oct 2, 2009
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    Does this show you what you are looking for. I'm using LTSpice both for drawing and simulation. Note the voltage across both the red and green LEDs...

    256.gif
     
  8. MikeML

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    Oct 2, 2009
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    Here is a series version. Downside is that the current through the Red Led increases when the current through the Green Led is shunted by the switch. As a practical matter, the apparent change of brightness to the Red Led with be barely perceptible to the human eye... 256s.gif
     
  9. tjohnson

    Active Member

    Dec 23, 2014
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    Sorry for my wrong answer.:oops: I deleted it.
     
    Last edited: Dec 26, 2014
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Is there a restriction for the switch to be SPST? With a SPDT, you can switch either a second diode or second resistor so the current will remain the same in both cases... Something like this..
    swseries.png 1LED.png 2LEDs.png
    The graph on the left is with one LED switched out. It is drawing 0 mA (D'oh!). The graph on the right is with both LEDs illuminated, and the green trace is overwritten by the blue trace. In all cases, the LEDs are drawing about 29 mA
     
  11. SDon

    Thread Starter New Member

    Dec 26, 2014
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    Hey Blocco, you know, it didn't even dawn on me to hand sketch it. :confused:

    And thanks to everyone else for their help. Much appreciated.
     
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