Measuring rotation speed with a strobe
Flash a light at exactly the rate something spins and it appears to stand still, because every flash catches it in the same position. Sweep the flash rate until the pattern freezes and you have measured the speed without touching anything — the oldest non-contact tachometer there is, and one a phone screen can do.
Freeze means match
Put a mark on the rotating part, or use an existing feature like a fan blade or a printed strobe disc. Increase the flash rate slowly. When the mark stops drifting and hangs motionless, the flash rate equals the rotation rate. If the mark creeps forward the flashes are slightly slow; if it creeps backwards they are slightly fast, and that drift direction tells you which way to nudge.
The harmonic trap
A mark also stands still at half the flash rate, a third, a quarter. Always sweep up from below and take the first freeze you find, because that is the true speed — every later one is a harmonic. A quick check: at the true speed a single mark appears once; at a subharmonic you often see two or more evenly spaced ghosts.
Turntables and fans
Vinyl is the classic case, since the three standard speeds — 33⅓, 45 and 78 rpm — are exactly why strobe discs exist. Set the flash to the nominal speed and watch the dots on the platter edge: drifting clockwise means the deck is running fast, anticlockwise means slow. Ceiling and computer fans work the same way, and here the strobe doubles as a fault check, because a bearing that is starting to fail shows up as a speed that will not hold still.
Frequently asked questions
Does this work in daylight?
Poorly. The strobe has to be the dominant light source, so a dim room gives a much cleaner freeze.
Is a screen strobe dangerous?
Flashing light can trigger seizures in people with photosensitive epilepsy. Any strobe tool should warn you before it starts, and you should not point it at someone who has not been asked.
Why does my turntable freeze at 33.3 but sound fast?
Check that you did not land on a harmonic, and check the mark count on the disc — a disc printed for 50 Hz mains behaves differently under a 60 Hz reference.
Sixteen tools, one app. TOOLBOX turns the sensors already inside your phone into a level, an inclinometer, a screen ruler, a caliper, a compass, a decibel meter and a strobe tachometer. It runs in the browser, with no account and nothing to install.
Open TOOLBOX