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I recently built the following circuit: my op amp circuit

In case you can't tell, it has 23 volts on the power rail and VEE is connected to ground. I don't have any feedback on the output. I have a simple voltage divider on the inverting input. The resistors are chosen so that when the input voltage, Vin, is above the threshold set by the divider (about 1 volt), the output should swing up to VCC, 23 volts. However, in practice, it doesn't seem to be doing anything when I switch Vin between 0 and 5. The output remains low. I haven't used op-amps much at all, so I figure I'm doing something wrong. I know of Schmitt Triggers, however those at first seemed more complicated than what I wanted, and I also didn't want the signal to invert. I digress...is this totally wrong, or should I suspect the problem elsewhere?

Here is the op amp: https://www.mouser.com/datasheet/2/348/lm358f-e-1873592.pdf

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  • \$\begingroup\$ Why 21V out? Vcc-2V drop. why not use 5V logic output and scale the inputs accordingly. \$\endgroup\$ Aug 18, 2020 at 20:17

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You have swapped the placement of the two resistors. The way you have it set up, the switching threshold is about 22V, not about 1V.

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  • \$\begingroup\$ Oh geeze, that's a pretty big mistake. Let me try fixing that. Regardless of that. Is there anything wrong with my hypothesis of how it should work? \$\endgroup\$ Aug 18, 2020 at 19:49
  • \$\begingroup\$ No, you have the right idea. The LM358 is pretty old and not the greatest op amp but I think it should work for what you are trying to do. \$\endgroup\$ Aug 18, 2020 at 19:50
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    \$\begingroup\$ One limitation of the LM358 is that it would not work for the value of 23V for the trigger point because that would exceed it's input common mode range. 1V is ok. \$\endgroup\$ Aug 18, 2020 at 20:14
  • \$\begingroup\$ Hmm, are you sure @KevinWhite? The data sheet seems to say 36V (page 2). Nevertheless, that seems to have solved the problem. Thank you guys for the insight and pointing out my silly error! \$\endgroup\$ Aug 19, 2020 at 0:02
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    \$\begingroup\$ @thisissparzo - That is the absolute maximum that won't blow it up, doesn't mean it works. The relevant parameter is on pages 3 and 4 with a note "VICM=VEE to (VCC-1.5V)". In your case, the maximum is 21.5V (23v supply rail). \$\endgroup\$ Aug 19, 2020 at 0:35

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