[rhn_mk1]

Is there a fixed-function circuit that can read a digital value out of a sensor and compare it with a threshold?

I'm not dounting it's possibly, but I'd be surprised if it's available without involving a Turing-complete computer.

[JohnFen]

Yes, there are tons of these sorts of circuits. It's something that is often needed, and these sorts of circuits have been used before ultra-cheap microcontrollers were a thing. A turing-complete computer is serious overkill for this sort of application.

I'm not sure if there is a single-chip solution (although I'd be a bit surprised if there isn't), but the fundamental circuit isn't that ambitious. Using a very low-end microcontroller is likely less expensive, though.