two transistor on off switch--what is going on?

Discussion in 'The Projects Forum' started by chimera, Apr 19, 2012.

  1. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Hello. So, its been sometime since I messed around with transistor circuits. I figured it'd refresh my knowledge. This is what I intended ( I have attached a schematic):

    I have wired up two transistors so that an led is always on when a push button is open. I have accomplished that with PNP transistor in a simple pass configuration. Now, the second transistor, an NPN, should only light up another led when the push botton is pressed and should remain lit till the time the button is pressed.

    I wired it up, but the thing is that, when i press the button, the pnp led remain on but dim and the npn led lights up. When I let the button go, the pnp leds becomes bright again and the npn led goes out (as intended).

    Where am I going wrong with this basic set up? I really am amazed how easy it is to forget basic transistors circuits (for me atleast)

    For image purposes, please see attached schematic. THANKS!!!!!!


    EDIT: Everyone was such a great help that I ended up fixing it on my own! Thanks for all the efforts guys :rolleyes: (sarcasm). I have attached the fixed schematic of the problem. Feel free to use it for future references
     
    Last edited: Apr 19, 2012
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The problem is that conduction through Q3 base prevents the voltage across the switch ever being more than about 0.6V, so the other transistor is not switching. You can probably get around that by adding one more resistor, or maybe a diode? Think about it.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Geeeeeez! It was only yesterday that you posted this. People do have lives outside of AAC.:rolleyes:
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    Simpler and avoids PNPs. It's basically a flip-flop.
     
    chimera likes this.
  5. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Thanks for the idea. I need a pnp in a pass element configuration. But yeah, the npn arrangement in deisgn is basic flip flop switch and is a very simple yet efficient design.

    What deisgn software are u using?

    Yeah i tweaked the values a little bit!
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Tina Classic.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Simulation has suckered more than one novice.
     
  8. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Ok..thanks for insight
     
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