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# Adjustable Schmitt Trigger Circuit Help

#### chrisjrollin

May 3, 2015
5
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?

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.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
The most obviously direct way to do this is to use a pair of comparators driving a flipflop.

#### Arouse1973

##### Adam
Dec 18, 2013
5,178
The most obviously direct way to do this is to use a pair of comparators driving a flipflop.

Interested Steve, what will the FF do?
Adam

#### Laplace

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

#### Colin Mitchell

Aug 31, 2014
1,416
Or use a Schmitt Trigger made with transistors

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Interested Steve, what will the FF do?

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:

use a Schmitt Trigger made with transistors

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

#### Colin Mitchell

Aug 31, 2014
1,416
Designing a transistor schmitt trigger circuit for this requirement is not an easy thing to do.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Designing a transistor schmitt trigger circuit for this requirement is not an easy thing to do.

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.

#### cjdelphi

Oct 26, 2011
1,166
the 555 seems like a good candidate. ..

use a potentiometer on pin 5 to adjust the hysteresis. ..?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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

#### cjdelphi

Oct 26, 2011
1,166
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

#### Alec_t

Jul 7, 2015
3,647
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.

#### Arouse1973

##### Adam
Dec 18, 2013
5,178
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

Ok I see what you mean now.
Thanks
Adam

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
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

#### Attachments

• 555SchmittTggrAdj.JPG
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#### Arouse1973

##### Adam
Dec 18, 2013
5,178
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

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
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:

#### Arouse1973

##### Adam
Dec 18, 2013
5,178
Rome wasn't built in a day Chris It doesn't matter how long it takes.
Adam

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
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

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Yet another OP disappearing act? Sometimes I wonder why I bother. Yet I keep doing it!

Chris

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
yes. But others here appreciate your efforts

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