I am using an LM324n quad-core op-amp. I have connected it between +24 V and GND. One of its op AMPS is USED as a buffer. When I pass the buffer a negative voltage, I get it inverted at the output although I expected it to be zero. Is this weird response somehow LOGICAL or expected?

Here is the datesheet of lm324n


Unfortunately I had some issues with Windows lately and I have no other schematic but for this handwritten one.

MOTOR VELOCITY input can become both positive and negative and that's where the issue is!Maybe I can help you. I have worked with Op-Amops years ago.
Why do you want to use an op-amp?
That device, the LM324 can be used either in a DC aplication or as a n audio amp.
In either case, the NORMAL method is to provide a split rail, + 12 volt and - 12 volt. If you want a bufgfer, you put a reitor from the output back to the invert input. ykou tie another risistor from the other input to grnd. You will get a voltage out of only a few milivolts.

Are you familiar witrh op-amops? The first thing to lern is to use a split ril supply.
Trying to bnias the thing from a SINGEL suppley can drive your ctrazy.

Why did you want to use a LM324?
I have stron opions about op amps, but even so, I might be of some help.
For gbeneral use, the 4558 dual is a good coice. IMO.
But you can make the LM324 work, but it does have a quirk.