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How to catch a high signal?

Discussion in 'Electronic Basics' started by Irina, May 25, 2007.

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  1. Irina

    Irina Guest

    Hello all,

    I have got a problem with a digital circuit. I got a signal from a
    comperator, which changes from 0 to 1 and back. Now I want to catch
    the first high signal and hold it until infinity. I try to use a RS-FF
    with R = 0 and S = 0,1,0,1,... (my input-signal) , but I got, of
    course, an undefind state until the first high comes. How can I get a
    zero in the first moment and not an unknown state?

    Thanks a lot!

  2. Bob Eld

    Bob Eld Guest

    Use a "D" flop. Connect the "D" input high (5V) and connect your signal to
    the clock input. Take the output on the "Q." When the first positive edge
    of your signal appears, the "D" flop will change state to high out where it
    will remain. The flop may be reset by using the reset input and should also
    reset on power up.
  3. Blake

    Blake Guest

    It seems to me that an RS latch is just want you want for this application.

    If I understand you correctly, by saying that R=0 and S=0 is an undefined
    state, you are implying active-low logic. If that's the case, you want R = 1
    for your circuit to operate. Your circuit can't latch if it's perpetually

    But you do want a one-time reset when you set the latch into action. The
    obvious solition is to use a momentary reset switch to reset it. Another
    option is a high impedance pull-up resistor and a large value capacitor on
    the reset pin. Maybe 10K and 1 uF. That way, on ititial power-up, the reset
    pin will be held low for roughly one time-constant. You get a one-time reset
    pulse, and then the latch is ready to catch the first (active low ?) pulse
    on the set pin.
  4. John Fields

    John Fields Guest

    View in Courier:

    NOR Y--+
    +--B | Vcc
    | | |
    | A--+ [0.1µF]
    +--Y NOR |
    | B-------+
    | |
    +-->OUT [100K]
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