On, off LED circuit

Discussion in 'The Projects Forum' started by heywood, Dec 17, 2008.

  1. heywood

    Thread Starter New Member

    Dec 17, 2008
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    Hi, I'm trying to do something pretty basic, power a Green and a Red LED based on the state of a switch, on/closed or off/open. When the switch is closed, I'm also powering a relay coil. This is what I have come up with. I'm curious if there is an easier way to do this or with fewer components.

    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Here's how I would do it...

    [​IMG]
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Here is another choice for you.
     
    Last edited: Dec 18, 2008
  4. Wendy

    Moderator

    Mar 24, 2008
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    [​IMG]

    Depending on the resistance of the coil, there is a good chance both LEDs will light with this scheme.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    I would say one has a 99.9% chance of success if the circuit is for low voltage(12V or less) relays. Most of them have coil resistance less than a few hundred ohms.

    Furthermore, one could also use high intensity LEDs that take 1mA or less so the circuit will most definitely works.

    However, I do agree it is a different story for special or high voltage relays that has high coil resistance.
     
  6. heywood

    Thread Starter New Member

    Dec 17, 2008
    2
    0
    Thank you very much for the suggestions. The input to open and close the switch is actually a ground signal (floating=open) coming from an ECU so unfortunately I can't do too many clever things with switch.

    I do think that the right side transistor can be deleted. I don't see why I can't power the relay + green LED directly from the signal line.

    I need to study more the suggestion to chain the LEDs together. At first glance it would seem that the relay would always be powered but I guess I need to look at the math more closely. It's certainly a very simple solution.
     
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