Rpm counter for root2cnc

The counter makes use of an optical sensor (transmitter + photodiode) and of a small piece of reflective tape attached to the spindle's fan. A circuit with an opamp interfaces the photodiode to an arduino mini measuring the resulting signal period and providing the rounds per minute to a 0.96'' OLED display (available for about two euros on aliexpress). The box is connected to the splindle magnetically by two small magnets in each arm.
There are two versions of the support with different hole s