Need help with active low BJT LED

Discussion in 'General Electronics Chat' started by Sonoma_Dog, Sep 16, 2009.

  1. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    ok I need this LED Active low, means when the Vin is low the LED turns on.

    I have 2 circuit in mind, wanted to know which one works (or works better).
    I don't have time to figure out the resistor values yet.

    please point me to the right direction.

    Thanks a lot in advance!
     
  2. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    see attachment please :)
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Try using a PNP transistor instead, like this:

    [​IMG]

    The signal input to R1 must come within 0.5v of V+ in order to turn Q1 off.

    When the input to R1 is pulled low, 20mA will flow through R1, which is the maximum that many microcontrollers can sink. It's OK to increase R1, but it should allow at least 1/10 of the desired collector current.

    R2 was selected to provide 20mA current through an LED rated for Vf=2V@20mA.
     
  4. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    All right, let me see if there is any PNP phototransistor sensor.
    Thanks!
     
  5. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    but, if i must work with NPN transistor, will my circuit work?
     
  6. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    after a bit of research, looks like PNP phototransistor does not exist. That also means I must work with a NPN transistor. : (
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If you have an old 2N404 around, you can use it as a phototransistor by grinding off the top of the TO-5 can. It may not pass enough current to let the LED do more than slightly glow.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, you threw a curve ball. You didn't start off saying that you were using an NPN phototransistor.

    You might be able to simply use the NPN phototransistor to sink current from the LED, but you wouldn't be able to adjust the light threshold.

    Or you could try it like this:

    [​IMG]

    Similar to the first circuit I posted, except the NPN phototransistor Q2 is turning on the PNP transistor.

    R3 provides some adjustment for the light threshold.
    R4 limits current in case R3 is accidentally turn to 0 Ohms, which would burn up the phototransistor.
     
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