Need some help with triggering an LED circuit

Discussion in 'General Electronics Chat' started by outsider787, Nov 11, 2011.

  1. outsider787

    Thread Starter New Member

    Nov 11, 2011
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    I have an input signal which is either 3V or 0V.
    From a battery I need to drive an LED with this signal.

    I need the LED to be on when the signal is at 0V and I need the LED to be off when the signal is at 3V.

    Anyone have any idea how I can wire this up?

    Thanks,
    adrian
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    A one transistor invertor should do for a red LED @ 20 mA. One k Ω base R, 50 +- Ω for LED current limiting. 2N3906 or similar.
     
  3. outsider787

    Thread Starter New Member

    Nov 11, 2011
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    Perfect.
    Exactly what I've been looking for.
    http://www.kpsec.freeuk.com/trancirc.htm#inverter

    Thank you!
     
  4. Audioguru

    AAC Fanatic!

    Dec 20, 2007
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    Is the transistor used to short the LED to turn it off when the 3V is present?
     
  5. outsider787

    Thread Starter New Member

    Nov 11, 2011
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    From what I understand, yes, the transistor is used to short out the LED (through a resistor) when the 3V is present.

    why do you ask?
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Ouch: Shorting the LED would be quite wastefull of battery life.' should have mentioned that 2N3906 is PNP. As they say, apicture is worth a 1000 words.
     
  7. sheldons

    Well-Known Member

    Oct 26, 2011
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    Heres some led drivers(for info ...)
     
  8. Audioguru

    AAC Fanatic!

    Dec 20, 2007
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    BUT with a 3.5V blue or white LED their current is only 6mA.
     
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