I am designing a Schmitt trigger with a variable resistor. I used this online calculator to help discover my values.

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I have placed it all into a circuit that appears as such and used LTSpice to model it.

enter image description here

My goal is to have it go high at 2.3V and low at 1.97V, but everytime I run it, it does no behave as I would believe.

Any help would be nice, thanks!

EDIT 1: I got rid of the op-amp and the resistor like was suggested. The circuit starts high and goes low when my resistor is at 2.235k and when my voltage is at 2.2V, which is not how I intended it.

[![enter image description here][3]][3]

  • 1
    \$\begingroup\$ Don't say what it doesn't do, say what it does do. \$\endgroup\$
    – Andy aka
    Oct 30, 2016 at 17:46
  • \$\begingroup\$ Just a note: the first opamp (U2) and R7 seem redundant to me. You might eliminate them. Connect the output of your resistor divider directly to the (-)input of U3. Also, voltage supply B2 is redundant. You can connect the top of R3 to B1(+). \$\endgroup\$
    – Rich S
    Oct 30, 2016 at 17:56
  • \$\begingroup\$ I made the changes and added the description. Thanks. \$\endgroup\$
    – Alex
    Oct 30, 2016 at 18:05

1 Answer 1


Here is my simulation, which works:


simulate this circuit – Schematic created using CircuitLab


The upper-trip-point (VUTP) is 2.3 volts, what you wanted. But the lower-trip point (VLTP) is about 2.15 volts. You will need to adjust the resistor values. Do you need more help with that?

And the lesson here is, don't always trust online calculators!

  • \$\begingroup\$ This is exactly what I was looking for. I am going to need to adjust the values, but this is a big help. Thanks! \$\endgroup\$
    – Alex
    Oct 30, 2016 at 18:34
  • \$\begingroup\$ If you could, could you point me in the direction of some good resources. My Hysteresis/Schmitt Trigger calculations are a bit off it seems. Thank! \$\endgroup\$
    – Alex
    Oct 30, 2016 at 20:32

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