pulse on threshold crossing - the most convenient method?

Discussion in 'General Electronics Chat' started by pgo1, Jan 28, 2017.

  1. pgo1

    Thread Starter Active Member

    Nov 7, 2012
    I am trying to find a compact, elegant way of deriving a rising edge every time a threshold voltage is crossed. This is different from a comparator, as in that instance when you are below the threshold the output is one state, and when you are above it the output is the opposite state, so you get a rising and falling edge depending on which "direction" you cross the threshold from. I want a rising edge every time the threshold is crossed.

    one solution i am playing with is to use an open collector dual comparator with the same threshold but wire the output so that one output is inverted with respect to the other, then AC couple the rising edge, diode clip the falling edge and mix the signals back together. I can't find a way of conveniently mixing the signals back together without using an op-amp, whilst maintaing the signal level enough to drive a 5V chip, and also having a high enough RC constant that the signal can recover in time for the next (i am expecting crossings around every 10uS max). The whole thing starts to feel pretty lumpy (i attached a picture of one - poor - solution. yes, i know, i'm not a great analogue designer...)

    I seem to often need to use this kind of circuit and i never seem to come up with a solution that seems elegant. I have also tried AC coupling the comparator outputs and full-wave rectifying them which also seems "lumpy". is there a better way?
  2. shortbus


    Sep 30, 2009
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
  4. DickCappels


    Aug 21, 2008
    A comparator followed by a one-shot should do it.
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    A one shot that triggers on both the positive and negative going edges?


    OK, I forgot I had one :)
    555 Edge Triggered Monostable2.gif
  6. crutschow


    Mar 14, 2008
    Run the output of a zero-crossing comparator through an XOR edge detector.

    Last edited: Jan 28, 2017