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How can I make the LED light when the motor stops running (aka the output from the op amp (pencil drawn) is low and so the transistor base input is low)?

In this configuration. My LED lights when the motor is running and thus the input at the transistors base is high. How can I change the arrangement of my components and/or connections so that my LED will light when the motor stops running and so the input at the transistors base is low?

enter image description here

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    \$\begingroup\$ Welcome! Please rotate your schematic drawing. \$\endgroup\$
    – winny
    Commented Nov 21, 2023 at 12:54
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    \$\begingroup\$ Exchange the inverting and non-inverting inputs to the right hand comparator / opamp. \$\endgroup\$
    – Russell McMahon
    Commented Nov 21, 2023 at 13:25
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    \$\begingroup\$ Hand drawn diagrams are acceptable BUT they must be correctly oriented, tidy, readable and well labelled. || If you want people to put some effort into helping you then you need to try to help them do so. Apartt from tidiness, label ALL components wity designators - eg Q3. Don't jameverythingintoacorner because your paper is too small - think ahead and/or add some more paper. Help us to help you. || Having -5V rail up and +5V rail down is VERY unusual convention. Having it reversed in some location(s) is diabolical :-). Having it "upside down" on the opamps is especially pointless. \$\endgroup\$
    – Russell McMahon
    Commented Nov 21, 2023 at 13:25
  • \$\begingroup\$ PLEASE improve the diagram ! \$\endgroup\$
    – Russell McMahon
    Commented Nov 22, 2023 at 4:01

2 Answers 2

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This is not an actual answer, but I have to do it this way to provide a schematic, so bear with me.

Your output stage may give you difficulty, especially if you are trying for output near zero, which does seem to be your intent. If the output is nominally zero, slight shifts require the op amps to cycle +/- 1 diode drop in response to noise. Depending on your physical setup, this can lead to the op amp producing a high-frequency oscillation around zero, and the op amp getting hot while injecting noise into any surrounding circuits. Try something like

schematic

simulate this circuit – Schematic created using CircuitLab

However, this will have the side effect of reducing the voltage swing available for low load resistance,so check to see if this is a problem.

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Swapping the inputs to the op amp (or comparator) ringed in red will achieve what you want.

OR

Change the LED driving transistor to a PNP, emitter to +5, collector to ground via LED (with appropriate polarity) and resistor to ground.

In both cases the 0.3K resistor on the 4.7K:0.3k divider may benefit from being changed to a larger value - maybe around 1k. This increases the trasition threshhold voltage.

enter image description here

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