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Driving an LED through RC circuit(RC time constant)

  1. Jul 25, 2008 #1
    Hi, i am trying to design a circuit, where i am trying to light up an LED long enough based on my pulse signal.
    Here is what i am trying to do.

    a 10us pulse(every second) --->74HCT04 buffer---> (RC circuit)----> LED.

    since this is a 10us pulse, i want to make sure to light up the led for long enough time, so that one can see it. So i am trying to use RC time constant to add delay so that the LED remains ON for a longer time. I am not able to figure out the exact circuit for that and how i should design the RC circuit and how and where to place it with/before/after the LED.
    Also i am not able to figure out how to calculate R and C.

    It would be great, if someone can help me out.
    Any kind of input is appreciated.
  2. jcsd
  3. Jul 25, 2008 #2


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    Staff: Mentor

    Traditionally, you'd buffer the pulse, drive through a diode into a cap (with parallel bleed resistor to give the RC time constant), and follow that with a Schmidtt trigger gate to drive the LED.

    Quiz Question -- why does the second gate need to have a Schmidtt trigger input?
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