Tachometer using NVE ADT001 noncontact magnetic rotation sensor. Arduino pin 2 to Sensor Sin output; pin 3 to Cos; pin 13 indicates direction; pins 7 to Rx on COM-11441 7-segment display for RPM; pin 5 is RPM variability PWM output.
See https://youtu.be/OcEzG93sloY for a demonstration. See https://www.nve.com/webstore/catalog/product_info.php?products_id=646 for sensor info.