# How to design a Schmitt Trigger with Variable Hysteresis and Mid-Threshold

#### kasamakasam321

Joined Nov 21, 2021
3
Hey! I want to design a Schmitt Trigger whose Hysteresis Voltage (Difference of 2 Thresholds) and Mid-Threshold (Average of 2 Thresholds) can be independently varied with Potentiometer as Variable Resistor or Variable Potential Divider (or as a Variable Voltage Reference directly at Inverting or Non-Inverting Terminal).

My operation is Single Supply with +24V and Ground Levels. But I would like to know for general case with dual (and unequal in magnitude) power supply.

I prefer making it with a Comparator IC with Open-Drain (can only Pull Down but not Pull Up). But I would love to know equivalent Circuit with OP-Amps.

Besides, I would love to know how to design these kind of circuit topologies myself rather than asking someone that fulfil the requirements (like the requirement of selected independent variable parameters).

Thank You!

#### ericgibbs

Joined Jan 29, 2010
18,268
hi 321,
Welcome to AAC.
E

#### kasamakasam321

Joined Nov 21, 2021
3
Thanks.

I don't have a basic topology with me. Thats exactly what I need: a topology for a Single Supply Schmitt Trigger with comparator whose Hysteresis Voltage and Average Voltage of the two thresholds can be independently varied with two Potentiometers.

#### ericgibbs

Joined Jan 29, 2010
18,268
hi,
You must have a written or mental description of the project that you post.
For example, details of the signal levels, threshold level voltages, range of adjustment etc.

E

Edit:

If you think about it, you are asking for a universal design that would cover every possible application.

#### WBahn

Joined Mar 31, 2012
29,522
Thanks.

I don't have a basic topology with me. Thats exactly what I need: a topology for a Single Supply Schmitt Trigger with comparator whose Hysteresis Voltage and Average Voltage of the two thresholds can be independently varied with two Potentiometers.
In other words, you need someone to do your homework for you.

That's not how this works. YOU need to do the lion's share of the work. Make your best attempt. Explain what you have done and why. If it's off the rails, we'll help you understand what you are overlooking so that you can do it better. That's how you will learn.

If someone just gives you the topology, that's not how you are going to learn to design something.

#### crutschow

Joined Mar 14, 2008
33,369
Hey! To do what you want is not trivial.
You would need to independently add an equal value of plus and minus hysteresis to the mid-threshold voltage, independent of that voltage..
You could possible generate a trip point voltage that can be independently adjusted for both the average voltage and the voltage difference using op amps, with the difference controlled by the comparator output signal.

It's not clear that the complexity of such a circuit is worth the result.

#### MrAl

Joined Jun 17, 2014
10,927
Hey! I want to design a Schmitt Trigger whose Hysteresis Voltage (Difference of 2 Thresholds) and Mid-Threshold (Average of 2 Thresholds) can be independently varied with Potentiometer as Variable Resistor or Variable Potential Divider (or as a Variable Voltage Reference directly at Inverting or Non-Inverting Terminal).

My operation is Single Supply with +24V and Ground Levels. But I would like to know for general case with dual (and unequal in magnitude) power supply.

I prefer making it with a Comparator IC with Open-Drain (can only Pull Down but not Pull Up). But I would love to know equivalent Circuit with OP-Amps.

Besides, I would love to know how to design these kind of circuit topologies myself rather than asking someone that fulfil the requirements (like the requirement of selected independent variable parameters).

Thank You!
Hi,

I'd like to ask you a question first. Have you ever designed a SINGLE input Schmitt Trigger circuit before this? If not you have a long way to go
Try to design a single input circuit first, then go from there.

#### kasamakasam321

Joined Nov 21, 2021
3
H
Hi,

I'd like to ask you a question first. Have you ever designed a SINGLE input Schmitt Trigger circuit before this? If not you have a long way to go
Try to design a single input circuit first, then go from there.
Hi. Yes, I've done it.

#### ericgibbs

Joined Jan 29, 2010
18,268
hi kas,

E

#### MrAl

Joined Jun 17, 2014
10,927
H

Hi. Yes, I've done it.

Hello again,

Ok thanks for the answer. If you have one already why dont you show it we can go from there.
Just to note, this is not an unusual endeavor. In fact, it is very common in power supplies that have to detect under and over voltage. It's nice to be able to adjust the trip points.

There is a much easier way to do this though i am not sure if you can use it or not. You'll have to show your simpler circuit. I am sure then others here can help too.

#### Ian0

Joined Aug 7, 2020
8,966
In fact, it is very common in power supplies that have to detect under and over voltage.
That's a window comparator, not a Schmitt trigger.

#### MrAl

Joined Jun 17, 2014
10,927
That's a window comparator, not a Schmitt trigger.

I did not want to give that information out just yet, let the OP think a bit more (ha ha).
But there is a little more to it than that. I'll get more detailed when the time is right.
Thanks.

#### WBahn

Joined Mar 31, 2012
29,522
H

Hi. Yes, I've done it.

Start with the schematic for a Schmitt trigger circuit that does NOT have adjustable thresholds. What determines what those threshold are.

Now add it some adjustment for the threshold and ask yourself what determine the center of the threshold and what determines the width of the hysteresis.

Now you are in a position to start asking what you need to control in order to achieve what you want, namely to decouple the center and the width and make them independent of the supply voltages.