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
