BillO:
justjed:
typically, in tach applications, I see hall sensors used.Well, I'm not sure where you have looked,
Well, e.g., under the hood of my decrepit Volvo (R.I.P.).
Optical is cheap, has huge bandwidth, and is easy to implement. You might want to consider it.
A hall-effect sensor is ~$1 (cheaper in various places, but not where I can also find other stuff I want to put on the same order), and the magnets are cheap as well. I'm sure optical can work too, but magnetic will be, I think, more robust in this application.