here a basic video explaining how to make a tachometer for ARDUINO.
can be helpfull on CNC or classic machines to mesure rotation speed.

https://www.youtube.com/watch?v=Em_RfpFjTvY



much more details for OPB704

https://www.youtube.com/watch?v=jXFg7EnsQS0



using a I2C 16x2LCD screen

https://www.youtube.com/watch?v=Wp_aVpa-lfc