Connect with us

Adjustable Schmitt Trigger Circuit Help

Discussion in 'General Electronics Discussion' started by chrisjrollin, Jul 18, 2015.

Scroll to continue with content
  1. chrisjrollin

    chrisjrollin

    5
    0
    May 3, 2015
    Hello,

    Glad to post my question here to get some help.

    Can anyone please introduce me the schmitt trigger circuit, which can individually adjust
    UTP(Upper Trigger Point) and LTP(Lower Trigger Point) voltages?
    Schmitt_Trigger_Waveform.png

    I could find several schmitt trigger circuits having adjustable reference voltage(Vref), but the problem was that
    UTP and LTP change together when single Vref changes.

    It would be the great help for me if you let me know the schmitt trigger circuit which can control UTP and LTP
    individually.

    Thank you.

    Thank you.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    The most obviously direct way to do this is to use a pair of comparators driving a flipflop.
     
  3. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    Interested Steve, what will the FF do?
    Adam
     
  4. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The FF should be wired to provide the hysteresis that would normally be accomplished by positive feedback in the single-comparator circuit.
     
    Arouse1973 likes this.
  5. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    Or use a Schmitt Trigger made with transistors
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    The design is pretty much identical to a 555. It has a pair of comparators driving a flipflop. In the case of the 555, the voltage reference to the two comparators is approx 1/3Vcc and 2/3Vcc, however it is fairly obvious that you could set these to whatever value you sensibly required in your own circuit.

    It's a simple, and easy to understand circuit, providing complete isolation of the two levels from each other.

    Hovever @Colin Mitchell's suggestion of:

    Is also viable, and I expect he is right now working to deliver you an appropriate circuit
     
  7. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    Designing a transistor schmitt trigger circuit for this requirement is not an easy thing to do.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    That was my initial thought, but your initial reply suggested to me that it was rather more trivial than I envisaged. Thanks for the clarification.
     
  9. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    the 555 seems like a good candidate. ..

    [​IMG]

    use a potentiometer on pin 5 to adjust the hysteresis. ..?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    The problem with that is that you can't set the threshhold of the two comparators independently as the OP requires.

    Making this from two comparators and a flip flop yourself allows you access to the inputs of the comparators individually
     
  11. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    true....

    what about modifying the voltage before it reaches pin2 / 6? these 555's have been exploited so much i'm sure someones tried lol
     
  12. Alec_t

    Alec_t

    2,904
    784
    Jul 7, 2015
    To modify the voltage while retaining threshold independence would need two buffer amplifier stages in addition to the 555. Simpler just to use the two comparators as Steve suggested in post #2.
     
    (*steve*) likes this.
  13. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    Ok I see what you mean now.
    Thanks
    Adam
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    I didn't immediately reply to this post but using the ubiquitous 555 tempted me right out of the gate. After fiddling around for about 2 hours spicing this I finally got it to work. The values of the resistors can be played with or changed to Pots by chris.

    Chris
     

    Attached Files:

  15. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    It's probably a stupid question but the waveform the OP supplied appears to be showing a +/- swing. Is this correct, or is he using a half supply common?
    Adam
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Fortunately, my circuit should work regardless whether or not his signal swings below zero.

    BTW, it shouldn't have taken me the 2hrs it did but I was looking at this wrong and initially had D2 wired backwards, which didn't provide the isolation between Thres & Trig that I was trying to achieve. So at that point I played with transistor inputs to the 555 but finally saw the light. Went back to square one and reversed that Diode.

    Chris
     
    Last edited: Jul 19, 2015
  17. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    Rome wasn't built in a day Chris :) It doesn't matter how long it takes.
    Adam
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Here are two more versions of my original circuit. Notes are provided within the schematic images. Since these are spice simulations please follow SOP regarding Spice Vs Real World! ;)
    Chris 555SchmittTggrAdj2.JPG 555SchmittTggrAdj3.JPG
     
    Arouse1973 likes this.
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Yet another OP disappearing act? Sometimes I wonder why I bother. Yet I keep doing it!

    Chris
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    yes. But others here appreciate your efforts :)
     
    CDRIVE likes this.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-