Normally, non-contact tachometers are optically coupled, using photoreceptors to measure light flashing off of a shaft or a tool ... But we think this is a great tip. [Matthias] is no stranger ...

Tachometers are used to measure the velocity of a rotating object. Tachometers operate on the principle that a driven motor (i.e., a motor operated as a generator) produces a voltage that is ...