Trying to understand a circuit

Discussion in 'General Electronics Chat' started by Artfldgr, Sep 19, 2010.

  1. Artfldgr

    Thread Starter Active Member

    Aug 15, 2008
    I am working on a project with some LEDs, and trying to use the junk drawer had me playing around with a circuit so that i could get what i needed. well, the playing around worked in simulation, and i am trying to figure out the circuit so that i can set better values of the parts.

    the original idea was to have one led and using a small signal transistor to turn it on and off (since the logic chips dont sink source enough). then i thought i had quite a number of LEDs and i can use two leds in the circuit.

    then i remembered i had a reel of white leds which were designed for cell phone flash (i think), and if you reverse the current on them another led would light up.

    however, here is the catch. they are PLCC 2... so if i want to use them the first thing that came to mind was i would have to make a bridge circuit. and that would need 4 transistors... which isnt worth it since 2 trans and 2 leds, (and two resistors) could do the same.

    i hope that is enough to describe why i am going through the trouble of trying to use these LEDs but not wanting to use so many parts.

    so playing around with a few ideas i eventually came up with this circuit.
    it actually seems to work well in simulation. the main led is white and it gets more juice than the other led which is red and would get less. which is what the circuit does.

    if switch A is on and B is off, then one color lights
    if switch B is on and A is off, then the other color lights
    if BOTH are on, neither of them light, though my intuition says this depends on the values, and variation of wiring that one can still have one light if one tried.

    of course, both being lit is impossible (other than flashing the circuit so fast that they seem to both be on).


    I would like to understand this circuit so that i can tune it to operate optimally with the values of the white and red LEDs in the PLCC2 package.

    personally i am happy i figured out how to avoid an H bridge..
    the classic two led circuit would be 4 resistors, 2 transistors, 2 leds
    this circuit is 4 resistors, 2 transistors, and 1 part with 2 leds inside...

    you can argue whether i am actually saving a part. to me i am since i have over a thousand of the plcc2 leds... and would have to buy the second set if i didnt get it to work.

    I am hoping that if i get pointed towards better theory, i can get to better part selection, and ability to use the circuit with different values.
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    I am quite confused by what I am seeing here.

    What is the ultimate plan?

    Do you want 2 buttons that each turn on an LED by reversing polarity?
  3. Artfldgr

    Thread Starter Active Member

    Aug 15, 2008
    The Leds in the circuit are wired that way and i cant change them
    i want to use both colors of the plcc2 units, but i dont want to make an H bridge for each of them so that the current can go either way. its a 6 volt positive only circuit, one LED is white, the other is red.

    the switches are only to activate the test circuit, the circuit for which this will be in will be digital 74hc chips. i had to get how this works in isolation though.

    it works on workbench, but my parts selection as far as what was needed was a hunches and a bit of trial and error, not worked out to some set of optimum values derived from understanding it. (i went through about a half dozen ideas before i had it work in workbench. will try to wire it up later as a test as i dont have that much time in one block right now).
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    What is PLCC2 ?

    If you are going to use the gate then try this circuit

    Or connect them directly to the A and B terminals of a 74hc circuit
    • 3.PNG
      File size:
      14.1 KB
    Last edited: Sep 19, 2010
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    As for your circuit, you can easily remove those 30 Ohm resistors in the base.
    And select resistors to get LED in around 10mA..20mA.
    Artfldgr likes this.