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Transistor LED flasher

Discussion in 'LEDs and Optoelectronics' started by jcmpix, Feb 23, 2011.

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  1. jcmpix

    jcmpix

    10
    0
    Jan 26, 2011
    Hi, I built the circuit on this page:
    http://www.josepino.com/circuits/transistor_led_flasher
    but couldn't get it to work. The LED just stays on and does not blink. The only thing I did differently was use a 9V battery.

    Also, I don't understand the circuit very well. The way I see it, the capacitor charges through the PNP emitter, collector and NPN base. I don't see a discharge path though.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'd try 3 to 6 volts like they suggest and see if it makes a difference.

    I have placed it in a simulator and it doesn't seem to work. There's also some worryingly poor design -- such as nothing limiting LED current.
     
  3. jcmpix

    jcmpix

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    Jan 26, 2011
    I added a 1k resistor in series with the LED. Removing it makes no difference though.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I did the same on the simulator :)
     
  5. jcurrie

    jcurrie

    128
    1
    Feb 22, 2011
    reverse the connections on the npn did it it kinda works led dosn't go all the way out but is flashes on three volts don't work at all in his confurgration.
    jim
     
    Last edited: Feb 23, 2011
  6. jcmpix

    jcmpix

    10
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    Jan 26, 2011
    Meaning switch the collector and emitter connections on the NPN? Looks strange, wouldn't the NPN emitter and PNP base currents conflict?
     
  7. jcmpix

    jcmpix

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    Jan 26, 2011
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Actually, the ones shown on that page seem far easier to explain.

    You're essentially charging up a capacitor until the voltage on the capacitor is sufficient to turn on the transistors. The additional current through the 22R resistor lifts that end o f the capacitor, turning the transistors on harder. The capacitor charges in the reverse direction.

    As the capacitor nears filly charged, the current available to the base of the first transistor reduces, turning off the second transistor. The 22R resistor has less current flowing through it, which lowers the voltage on that end of the resistor. This reduces further the voltage on the base of the first transistor, turning both off some more. The process continues until both transistors are hard off.

    Then the capacitor slowly starts to charge through the 100K resistor again...

    (Or at least I think that's what is happening).

    If I am right it is pretty obvious why replacing the 22R resistor with a LED is never going to work.
     
  9. jcmpix

    jcmpix

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    0
    Jan 26, 2011
    Yes, but look at the cap orientation. Shouldn't the positive be on the left for it to charge through the 330k?
    Using conventional current flow, it should go from the + end of the battery, through the 330k, then to the + end of the 10uF right?

    Edit: Same circuit on talkingelectronics, but with the cap reversed, I guess the other one is in error?
    http://talkingelectronics.com/projects/200TrCcts/200TrCcts.html#5
     
    Last edited: Feb 25, 2011
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
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    Jan 21, 2010
    The capacitor charges up in the reverse direction by only about 0.6V, when the transistors turn on, the capacitor gets discharged, then charged (the correct way) by potentially more than 0.6V.

    If you use a polarised capacitor, it is the right way around.
     
  11. jcmpix

    jcmpix

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    Jan 26, 2011
    I think I'm starting to "get it" now. Thanks! Charging the capacitor "the right way" (with the + end on the right) is done when the transistors are conducting, with conventional current through the PNP collector, then the cap, then the base-emitter of the NPN. When the transistors are off the cap charges "the wrong way", from the 330k to the left end and down through the 22R.

    By the way, do you use conventional flow or electron flow where you live? Sometimes I encounter people/websites/books that use electron flow.
     
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