Inside How Apple devices work, one part at a time

Apple Watch

Digital Crown

The crown on the side of Apple Watch is not connected to anything you can see. Turning it moves a striped shaft past a light and a sensor, and the watch counts the stripes.

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The stripe under the read headCrown, shaft and drum: 12 light and 12 dark stripesRead head: an LED and a photodiode side by sideTurn the crown and the drum turns with it. The LED lights the stripe in front of the read head, and the photodiode sees the reflection change at every stripe edge: 24 edges, so 24 counts, per turn, one every 15°. The watch knows only the counts, and scrolls its list one row every 3 counts.

Counting light

The crown sits on a short shaft. A pattern of light and dark stripes runs around that shaft, and a small light shines on it. A sensor beside the light sees the reflection change each time a stripe edge passes.

Each change is one count. In optical encoders like this one, two readings taken slightly out of step tell which way the shaft is turning, so the same stripes can scroll a list up or down.

Why it feels like clicks

Since Apple Watch Series 4, the crown has haptic feedback: the Taptic Engine taps as you scroll, so a smooth wheel feels like it has detents.

All explainers