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Stillspin Strobe Tachometer · Revised 15 September 2026
Before anything else: the flashing
A flashing light between roughly 5 and 30 flashes a second can trigger a seizure in people with photosensitive epilepsy, including people who have never had one and do not know they are susceptible. Do not use the app around anyone who has not agreed to it, and stop immediately if anyone feels unwell. A strobed machine only looks stopped; never reach towards anything you are measuring.
How do I measure something with the strobe?
Put a piece of tape on one blade or spoke if you can, so the pattern is no longer symmetric. Press Flash, take the sweep to the top of the range and come down. Above the true rate you will see several evenly spaced ghosts of the mark; the first rate on the way down where a single image stands still is the answer. Press Lock, and the app lists every rate that freeze could also mean, with the one it ranks first marked Most likely. The first time, a card at the top of the strobe screen walks you through it.
Can I type the speed instead of sweeping?
Yes. Tap the large reading at the top of the strobe screen, type the speed in RPM or Hz and tap Set flash. The flash rate is worked out for the marks per turn you have chosen. If the emitter cannot flash that fast, the sheet tells you where it tops out rather than quietly using a different rate.
Why does my fan read three times what I expected?
A three-bladed fan looks identical three times a turn, so it freezes at three times the shaft rate. Set marks per turn to 3 with the buttons beside MARKS, or tape one blade and leave it at 1. The app never guesses that number, because it cannot see how many blades your fan has.
The screen went black and the tabs disappeared
That is the screen strobe. When the emitter is Screen and Flash is running, the whole display becomes the lamp so no lit controls wash out the freeze. Tap anywhere, or press Stop at the bottom, to bring everything back. With the torch as the emitter the screen stays as it is.
How do I set a turntable's speed?
On the strobe screen, choose 33⅓, 45 or 78. The flash is held at exactly that speed; start Refine by drift and stop it when a marker on the platter has crept round by a known number of steps, and the error comes out in percent and in cents. With the unlock, More › Turntable speed counts one small paper marker through the camera against the iPhone's own clock and keeps reading while you turn the adjuster, with a centre-zero meter and a settled indicator. Aim the box at the marker, lock the exposure, and rest the phone where it will not move.
Can it measure wow and flutter, or rumble?
No, and both turntable screens say so. Wow and flutter is defined against a test record — a 3150 Hz tone under IEC 60386 and DIN 45507, or 3000 Hz under NAB and JIS — and is read from how that tone's frequency wavers; a platter-speed excursion is a different quantity. A phone sitting on the platter also turns with the record, so it cannot see the once-per-revolution wow caused by an off-centre spindle hole, which is cured by re-centring the record. Rumble under DIN 45539 sits sixty to eighty decibels down in the 2 to 100 Hz band and needs a rumble test record and a calibrated chain; iPhone microphone response below about 50 Hz is unpublished and varies by model.
How do I record the freeze?
Lock a freeze, then tap the video icon at the top right. Recording lasts eight seconds; if the flash is off, it is switched on for the clip and off afterwards, and you can cancel at any time. With the torch, the screen shows what the rear camera sees; with the screen as the lamp it records with the front camera and shows the flash instead. Whether a phone camera reproduces the freeze your eye sees depends on its shutter and has not been checked on every model.
How do I get a tighter number than the freeze gives me?
Lock the flash as close as you can, then open Refine by drift. Time how long the pattern takes to creep round by one whole step and enter how many steps it moved. The app turns that long interval into a rate far more precise than the freeze itself, and shows the uncertainty it computed from what you actually timed. More marks make it faster: twelve bars reach a given precision twelve times sooner than one piece of tape.
What is the emitter check?
The Check button in the Emitter panel flashes at a ladder of rates and times what this iPhone managed, then replaces the assumed range with a measured one. Until you run it, the app labels the range as not measured. For the torch it times the moment the app asked the LED to switch, not the light itself, because an iPhone cannot watch its own torch.
I refused camera or microphone access
The screen that needs it shows an Open Settings button. Turn the permission on there for Stillspin, then come back to the app.
Why do two methods disagree?
Almost always by a whole-number factor, and that factor is a symmetry setting rather than a fault. The Compare screen names the factor and says which setting to change.
What does the purchase include, and what stays free?
The strobe is free and complete: the sweep, typed rates, the lock, the candidate ladder, the procedure, refine-by-drift, the turntable strobe check, the emitter check, the freeze clip, saved measurements and targets, and the guide. One in-app purchase, paid once, adds the optical instrument, the acoustic instrument, Compare and the camera turntable screen. Deleting the app or moving to a new iPhone on that Apple Account costs nothing: Restore purchase, under More, reopens the routes. Refunds are handled by Apple. Saved measurements stay readable whichever method took them.
A reading looks wrong, or something broke
Send the details to alex@bukhalov.com, with Stillspin in the subject line: the iPhone and iOS version, the app version, what was spinning, and the reading you got against the one you expected.