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Stillspin Strobe Tachometer · Revised 28 October 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?
If it is safe, mark one blade or spoke so you can check how many marks pass each turn. Press Flash, sweep to the top of the available range and come down. A single image that appears still is a candidate, not proof of the true speed. Press Hold to keep that flash setting. The app shows a bounded set of candidate rates under stated marker and harmonic assumptions; other speeds may also fit. Check the marker count and use another method or an independent reference before relying on a result. The first-time card explains the free checks.
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 can look alike at three points in each turn, so the strobe may show a candidate 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 to set the flash target. That target is not a measured platter speed or a calibration of the emitted light. Open Check by timing the drift and time a marker moving through the steps you count; the app estimates the difference in percent and cents from your timing and marker count. 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 a strobe video?
Hold the flash setting, 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. Camera reproduction of the apparent freeze has not been verified on a real iPhone; rolling shutter and panel timing may make the clip look different from what you see.
How do I get a tighter number than the freeze gives me?
Hold the flash near the apparent freeze, then open Check by timing the drift. Time how long the pattern takes to move through the steps you count. The app estimates the speed difference from that timing and shows uncertainty based on the timing and mark count you entered; this does not establish calibrated accuracy.
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?
A whole-number ratio can come from marker or blade counts, pulse counts, or harmonics. Compare can show the ratio, but it cannot identify its cause. Check both setups and repeat at the same speed; use an independent reference if the result matters.
What does the purchase include, and what stays free?
The strobe is free and complete: the sweep, typed rates, the held flash setting, the candidate ladder, the checking guidance, drift timing, the turntable strobe check, the emitter check, the strobe video, 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.