Delay circuit

Discussion in 'General Electronics Chat' started by umphrey, Mar 8, 2013.

  1. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    Hello all,

    I was wondering if you guys could help me out with something. I have a circuit carrying a logic signal, +5V or 0V, and I need to add a component such that when the voltage is transitioning from low to high it has very short prop delay (< 200 nanoseconds or so), but when it is transitioning from high to low it gets delayed, maybe by 5 microseconds or so. Any thoughts?
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Assuming you are attempting to make the transitions more abrupt, look into Schmitt triggers. Namely, a Schmitt trigger buffer, or two inverters....

    Edit: Sorry, I just reread the post... So a Schmitt trigger would work on making abrupt transitions. Would charging a discharging/capacitor work in your application?
     
  3. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    Well it's state dependent so hysteresis isn't really an issue, the signal is coming out of an AND gate so it's either high or low, but I need one transition to take longer than the other one.

    Yes, but, only if I can make it so charge up time from 0V - 5V is =/= charge down time from 5V - 0V
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
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    Are you looking to delay the transition, which would essentially ramp up to a voltage, or are you just wanting the transition delayed from where it would normally be?
     
  5. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    It's just a digital signal, so if it's transitioning from high to low, it needs to get a few microseconds of prop delay, but if it's transitioning from low to high, delay needs to be minimal.
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
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    If that's the case, you could input a buffer, buffer charges a cpacitor, and teh capacitor goes to the input of a Schmitt triggered buffer, with the trip-up voltage set to close to Vdd and trip-down voltage to slightly less, given the impulse response of a capacitor, the transition point to a low output is shortly after the transition, using the step response, the capacitor will take some time to get up to the trip-up voltage of the Schmitt trigger.
     
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  7. tshuck

    Well-Known Member

    Oct 18, 2012
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    675
    I am curious, why would you want to do this?
     
  8. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    If I get too much current, I have a transducer that sends a signal to break the circuit in 2 different places. But they need to open/close in a different order.
     
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