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I don't understand this circuit

  1. Nov 2, 2009 #1
    I have a kit for designing circuits and this was one of the projects in the book:
    That was the best scan I could make... On the right side, right next to the NPN transistor there is an LED and to the right of that there is a 6V power source, positive side is up, negative is down. If there's anything else unclear about the picture, ask me.

    So the circuit does exactly what the book says it would: the light turns on for a few seconds and then turns off. But looking at the connections made, the current has a direct path through the LED without even going through the transistors, so it seems like it should stay on at all times. I think the basic idea of the rest of it is that when the capacitor is charged it provides the base current necessary to allow the collector-emitter current to flow. Whether it flows or doesn't flow, I still see one direct path for the current to take through the LED. Why does it still turn off after a few seconds?
  2. jcsd
  3. Nov 2, 2009 #2
    The text states "You should have no difficulty in figuring out how this circuit works". Which leads me to think you may not have learned from the previous projects or skipped them completely. Remember, when a transistor is in the "on" state it will conduct current and so the voltage across the emitter collector will be low.
  4. Nov 2, 2009 #3
    I've done all of the previous projects. Transistors are the only things that confuse me and I understand them on basic level now, which should be enough to understand this circuit. But I still see a path through the LED that doesn't go through the transistors and shouldn't be affected by any state they are in.
  5. Nov 2, 2009 #4
    The transistor stages are set up in such a way that they act as NOT gates. If voltage in the base of one transistor is low then the voltage on the collector side will go up, and if base is high, then the collector voltage will go down. There are two such stages in the schematic, so you have a NOT and a NOT that cancel each other.
  6. Nov 2, 2009 #5
    But what is the voltage across the emitter collector (of Q2) when it is "on", and what voltage is needed to turn the led on?
    http://www.ledsmagazine.com/features/4/8/1/MarlFig1 [Broken]

    Maybe a simpler way for you to think of this is that when Q2 is "on" it shorts out the led (current flows through R3 and Q2). When Q2 is "off" it is like an open circuit (current flows through R3 and the led).
    Last edited by a moderator: May 4, 2017
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