Hi Paul,

What kind of sensor are you using? A do-it-yourself or commercial? Either way I would expect the output to be a small varying analog voltage that will have to be amplified and converted via ADC to something the uC can use.

Then you can start collecting samples, implementing either a peak detector or zero cross detector to determine cycle time, and from that the frequency.

Or with some more external signal processing it should be possible after amplifying to do some pulse shaping so you end up with a digital signal you can feed an external interrupt with. Then it's a simple matter of using a timer to measure the time between interrupts.

Sounds like an interesting project.

Dennis