Does your drill go as fast as the manufacturer says it will? Well, you’d need a tachometer to figure that out. They’re not that expensive to buy, but as [Elite Worm] shows, they’re not that expensive ...

The SRPM1 optical laser tachometer is a stand-alone, battery-operated device that senses the passage of reflective markings on a rotating fan or shaft in order to determine the target's rotational ...

If you’re into anything even vaguely mechanical on the broad hacking spectrum, you’ve come into contact with things that spin. Sometimes, it’s important to know precisely how fast they are spinning!