How would u trigger 555 monostable on both edges?

Discussion in 'General Electronics Chat' started by Wendy, Apr 25, 2010.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I've been thinking about it, still haven't come up with an answer I like.

    One 555 monostable, being feed with a symmetrical square wave. How would you trigger the monostable on both edges?

    I think diodes might be the answer, to suppress the positive edge.
     
  2. SgtWookie

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    Jul 17, 2007
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    You could feed a square wave through a differentiator circuit, then through a precision rectifier circuit to invert the negative pulses. You would wind up with a stream of positive pulses.
     
  3. KMoffett

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    I've used a relay with a capacitor and an SPDT switch was an edge detector before.

    A bit clugy, untested, and would only work on slow square waves, but....;)


    Ken
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    If you exclusive OR a square wave with a delayed version of itself you get a frequency doubler that is a string of short positive pulses. You can delay a square wave by using a single pole RC filter or you can use a high speed clock and a flip-flop to produce a delayed version of a square wave.
     
  5. KMoffett

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    Or maybe replace the relay with an AC input optocoupler (back-to-back LEDs) with a transistor output like an H11AA1: http://www.vishay.com/docs/83608/83608.pdf Would need to add another resistor.

    Another thought: Bill are you thinking of pulsating DC or AC square wave?

    Ken
     
  6. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    A simple square wave, with an inverted version of the same waveform available if needed. I still have diode in the back of my mind as the answer, to allow a quick rush of current one way (negative edge) and a slow charge the other (positive edge).

    What has me thinking there might be a problem is a positive pulse is mixed with a negative they could cancel out.

    I'm going to draw something out to model it.

    *********************************

    This might work, I'll have to try it out.

    [​IMG]

    If someone has a different idea I would be interested.
     
    Last edited: Apr 25, 2010
  7. KMoffett

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    Bill,

    Do you think this will fly?

    Ken
     
  8. Papabravo

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    Don't know if they still make it but the 8T20 double edge triggered one-shot would fill the bill. We used them a lot to make data separators for FM and MFM encoded disk drives.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A XOR gate with an RC will also work.

    [​IMG]

    The RC will delay the input and give a puls at the output when the two inputs differ.

    Bertus
     
  10. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Run the trigger train through a full wave bridge rectifier! I had to do precisely this trick a while back so that i could use T1 timing as a clock source.


    Eric
     
  11. Ron H

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    Apr 14, 2005
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    XOR will give a short positive pulse. You can follow it with another XOR configured as an inverter.
    Alternately, you could use a single XNOR (still with the RC delay).
     
  12. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I think it would. I know why D3 is there, I wonder if I could eliminate it without messing anything up.

    A thought similar to that occurred to me, but I couldn't see how to implement it.
     
  13. Wendy

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    That would work too. I'm thinking of a real application, the time is a little critical.

    When I first thought of a circuit like the ones show I went into brain freeze. I think I've been staring at this screen too long.
     
  14. Ron H

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    This is pretty simple. The alternate pulse widths are not identical, but it shouldn't matter.
    One of the pulses goes below GND, but the current is pretty low, and this could be fixed with another part or two if it is a problem.
     
    crutschow, inwo, m121212 and 2 others like this.
  15. KMoffett

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    D3 just provides a rapid discharge across R6.

    I still don't know if your input is a monopolar square wave, or a bipolar one. ???

    Ken
     
  16. Wendy

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    Think single power supply, and a digital train from that.

    I really like Ron's approach, I would have never thought of using a common base transistor for the positive edge, and the whole arrangement replaces the signal conditioner.
     
  17. Ron H

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    The common base transistor handles the negative edge.
     
  18. Wendy

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  19. inwo

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    Thank you, it works for me....................
     
  20. inwo

    Well-Known Member

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    Ron's circuit works perfectly on the bench, with minor changes.
    It will not double when triggered in actual circuit using hall sensor.
    Rising edge is slow.
    Looking for a simple way to square up rising edge.
    If I need to add a schmitt trigger gate, maybe I should use another solution.
    I'm all about simple. clockdoubler.gif
     
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