Delayed Power Shutoff Timer

Discussion in 'General Electronics Chat' started by PhilBrink, Dec 10, 2014.

  1. PhilBrink

    Thread Starter New Member

    Oct 12, 2011
    7
    1
    I am attempting to create a circuit which will power itself off seconds after a button is pushed. But, I am not successfully keeping the power on after the button is released. I am very new to using transistors so I am guessing the problem is how I am using it.

    The completed project will use about 150 mA including this portion and will be powered by two lithium batteries.

    I apologize for the poor quality drawing and the colored lines through it. This office scanner has some issues and I am away from my computer.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    Yes, it's a transistor problem. You can't drive an emitter follower from its own output, as you are trying to do with the 4401, since the emitter voltage is always about 0.7V less than the base voltage. One option is to add a PNP transistor as shown in the simulation below.

    555 Auto off.gif
     
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  3. PhilBrink

    Thread Starter New Member

    Oct 12, 2011
    7
    1
    Thank you. So much to learn.

    I will have to rummage around and determine if i even own a PNP transistor. May have to run to the store.
     
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  4. crutschow

    Expert

    Mar 14, 2008
    12,977
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    You could also use a logic-level type P-MOSFET in place of the PNP. Another alternate is to use six CD4050 CMOS non-inverting buffer gates in parallel (there are six in one package), in place of both transistors for minimum number of added components.
     
  5. PhilBrink

    Thread Starter New Member

    Oct 12, 2011
    7
    1
    I had a different thought this morning and quickly tried it out. Instead of stopping the positive, I have it stopping the negative. For the most part it works, but it stays on indefinitely. I traced the problem to the low voltage coming from the output at 0.55v being enough to trigger the transistor.

    Is there a way to drop the low voltage to 0V but still have enough to trigger the transistor when the output is high?
     
  6. PhilBrink

    Thread Starter New Member

    Oct 12, 2011
    7
    1
    I wanted to say thanks, the pnp transistor option works very well. I don't pretend to understand it, but it works.
     
  7. sdowney717

    Member

    Jul 18, 2012
    385
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