pulse detection circuit...

Discussion in 'General Electronics Chat' started by raffter, Mar 10, 2008.

  1. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    hello,

    what chip functions like:

    input is AC(12V).... chip detects positive nd negative sides of input , and has 2 outputs.... 1st output pin will output positive PULSE from "positive side of AC input" and the 2nd output pin will output positive PULSE when it has detected negative side of AC input....

    or what is this circuit called??

    I hope my explaination is clear enough....

    anyone??


    Ralph
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    My first impulse (sic) is to say a full-wave bridge rectifier, consisting of four diodes.

    However, you indicate a pulse - were you expecting a square wave output?

    A bridge rectifier has two inputs, and two outputs. With 180° out of phase AC inputs connected, one side of the bridge will output the positive pulses, the other side will output the negative pulses - but that's not really what you're looking for - or is it?

    You could use a dual comparator, like an LM311, or most any dual op amp - if the inputs were given suitable reference levels - to do what you're asking.
     
  3. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    Something like this:

    kindly check attached..... output/s need not necessarily reach +12V.. but should be above +3V or so...and outputs need not be a "square wave".. I can just add schmitt outputs... supply for circuit is +12V only...



    Ralph
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
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  5. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    nice link!! :)

    thanks... ok I will go check it out...



    ----------------------------
    EDIT:
    spoke too soon... negative detector circuit/s(on link above) require bipolar supply... I just have single supply(+12V)



    Ralph
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Sounds like you might be able to use the full-wave bridge as recommended by sgtwookie in combination with a comparator.

    You did not mention the frequency of your 12VAC input. This will be a factor to consider.

    hgmjr
     
  7. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    How about: Rectify the AC, detect the peaks (or 0V points), then feed it to a JK flip-flop? Unless you need to indicate the phase of the AC (one specific output goes active for ONLY the positive side of the AC signal).

    --Rich
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached; a level-shifted zero crossing detector. It uses 1/2 of an LM339 comparator, but you could use a dual comparator, or a dual op-amp in open loop mode.

    Note that C1 should be a low-ESR type.

    It's important that R2=R6 and R3=R8, or the comparators will not trip at the right time.
     
  9. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
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    oooppsss.. input freq is not much... 5-250Hz max.. :)
     
  10. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0

    Sgt,

    nice level-shifting ... :)

    I have a discrete(all tr) w/c outputs EXACTLY like in the scope sceenshot... but I could not "mix" the inputs nor the outputs of the "detector" w/o affecting the other...I will give that circuit a try.. thanks!

    btw, how does the circuit function w/o an input(float)??


    and, R3 = R8?? or R3 = R7?? :)




    Ralph
     
  11. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    my simulation...
    R6 and R3 play important roles.. :)



    Ralph
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    If there is no input, both outputs will be at zero volts - if R2 = R3 = R6 = R8.

    Ideally, all of the 100k resistors are equal - or very close.

    R7 is your "finagle factor", or your tolerance from 0v on where to trip the comparators. As drawn, it's about 0.15 volts from the reduced-by-roughly 1/1.7 input, which means that the input signal would have to vary above or below zero by about 0.255v to trip the comparators.

    Rather than using a 39K resistor to limit the voltage within "the rails" (0v-12v) of the comparators, I could've used a 1k resistor and some diodes to limit the voltage swing, which would increase the resolution of the comparator trip network - but I took the easy way out. I'll leave optimization up to you. ;)
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Why are you using 630k to set the comparator trip value? :confused:

    OK, well - you're playing around... have fun ;)
     
  14. raffter

    Thread Starter Active Member

    Feb 28, 2008
    113
    0
    back2back zeners will do the job nicely at clamping the input :)

    and the 620k?? comparator adjustment..at least thats what my sim shows..sort of "spacing" the positive and negative output..

    and your R7... another "trip" adjustment :) same as the OTHER 100k tied to Vcc..
     
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