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timer circuit stumper

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Discussion in 'General Electronics Discussion' started by [email protected], Jan 21, 2013.

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  1. Jan 21, 2013 #1
    Matt@OPS

    [email protected]

    3
    0
    Dec 12, 2012
    It's been a long time since I've been in the world of dc component based circuits. My stay in the land of plcs and programmables has left me dumb to the basics of dc circuits. I would appreciate any help offered on this circuit.
    The circuit is supposed to light the led when the switch is closed.
    If the switch is closed for more than 1.2 seconds the led will go out.
    The led to stay lit for .5 seconds after the switch is opened.

    This should allow a pulse of 2 hertz or greater to keep the led lit but prevent it from staying lit if the circuit is stuck high.

    Thanks for any help,
    Matt
     

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  2. Jan 21, 2013 #2
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    Jan 21, 2010
    I think you'd be better with a monostable based on a 555.

    Your circuit isn't going to provide any exact timings and will not switch the LED on and off sharply.

    In addition, there may be significant current demands from your signal source, and unless you prevent it from sinking current, it will discharge C1 more quickly than you might desire.
     
  3. Jan 22, 2013 #3
    KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,272
    Nov 28, 2011
    That's a strange circuit! It won't do anything like you want.

    Is this supposed to be some kind of watchdog circuit? While the switch is opening and closing at a rate of 2 Hz the LED stays ON, but when the switch gets stuck either closed or open, the LED turns OFF? That's the impression I get from your description.

    The circuit will need a power source, as well as a switched voltage for the signal that it's monitoring.

    If you write up a complete definition of the behaviour you want, including a general description of how you want to use it, I will design a circuit for you.
     
  4. Jan 22, 2013 #4
    Matt@OPS

    [email protected]

    3
    0
    Dec 12, 2012
    Thanks,

    I got it working although a bit crude I admit.

    thanks for the advice.
     
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