What are you trying to do with the outcome of this sensor?
Explain to us why you cannot buy an already-engineered speed-monitoring switch and use that instead. There is one type that uses a trigger-wheel (a sprocket often works) and a couple of sensors, there is another type that measures back-EMF at an induction motor. If there is anything remotely safety-related to what you are doing with the outcome of this switch (e.g. using it to hold a guard-locked door closed) you had better be really careful about this.
