Use a cheap 8 pin microcontroller...
Well, I was thinking of ATTiny85 (8 pin DIP) since I have a tube of them.
But the point was the general discrete logic of how to implement this.
Optical, hall effect, reluctance pickups, whatever. It's all about the same.
Do you need/want to process both signal edges?
The circuit I described above (using the 555 as a Schmitt trigger inverter) and discharging the cap on activity would work fine.
Still, I kind of dislike the idea of a cap charging slowly and discharging instantly. Nothing discharges "instantly".
Ok, in this case you could add a resistor to mellow the discharge.
That would also be helpful in making it so that a single event wouldn't trigger the output.
Whether this is useful/desirable is another question.
If it's one pulse per revolution, you probably don't want that.
Technically a capacitor is not a good state storage element unless the changes are gradual.
A digital counter is much better.
A rotation event would reset the counter.
The counter stops when it reaches a value (probably a single high bit).
Your output signal is that high bit complimented.