I have a moisture sensor that I want to connect to a Schmitt trigger so when it reaches the upper voltage, the op amp goes high for watering. It shouldn't turn off until it reaches a lower voltage, then not turn back on again until the upper (dry) voltage is hit.

The supply voltage is 5v, but the output shouldn't go higher than 3.3v since it's going to a microcontroller. But I can figure that part out later, I just would like to see the op amp going high and low when it's meant to.

I thought I understood how these work, but getting some breadboards but not getting expected results.

To calculate the op amp config for the Schmitt trigger setup, I used some online calculators.

Firstly I assumed I needed a non-inverting Schmitt trigger since I want to to ON when high voltage is reached and OFF on the low voltage. But I did try both...

I mapped out what I did in circuit lab.

I also added an output buffer setup because I am just trying things to see if it makes a difference.

Any help would be appreciated


Non inverting Schmitt trigger

Here it is in circuit lab:

enter image description here

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1 Answer 1


The supply voltage is 5v

Basically you cannot expect the antique 741 work from a 5 volt power rail.

Reasons not to use a 741 op-amp and, to underpin this: -

enter image description here

Image from the data sheet

Any help would be appreciated

Rail-to-Rail Operational Amplifiers from Mouser

Rail-to-Rail Operational Amplifiers from RS

Op-amps from Farnell <-- you'll need to check some of these out to decide what is rail-to-rail

Instrumentation, OP Amps, Buffer Amps from DigiKey <-- you'll need to check some of these out to decide what is rail-to-rail

  • 1
    \$\begingroup\$ Thank you guys. I would have taken me ages to figure this out, as per the article linked... I thought this was just a go to op amp for simple projects. \$\endgroup\$
    – Dale Woods
    Commented Nov 11, 2023 at 12:32
  • 1
    \$\begingroup\$ @DaleWoods That's the problem with internet tutorials and articles: They are stunningly obsolete and use antiquated components which are often even not for sale anymore. Lost a lot of time with that myself. \$\endgroup\$
    – Fredled
    Commented Nov 11, 2023 at 13:01
  • \$\begingroup\$ I agree that you should bury the old (56 years old) 741 opamps and also bury the (wires allover the place) breadboard. \$\endgroup\$
    – Audioguru
    Commented Nov 11, 2023 at 19:12

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