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ScrollAnatomy of an input

Mechanical
Study / 01

Drag to orbit · drag the tire to roll · tap the tire to click · focus the model, then Shift + wheel to scroll

Preparing exhibit…

Inside the everyday

A small motion.
A precise signal.

Explore the mechanism that turns a fingertip movement into a digital command.

Inspect the assembly

Reference scale · millimeters

Optical wheel architecture.
Original, reference-scale reconstruction.

Engineering notes

An original, reference-scale reconstruction of a real optical-encoder scroll-wheel architecture, with a spring-detented wheel and a depressible axle. It is not a measured replica of a specific retail mouse or a manufacturer's CAD model.

Scroll: the slotted code wheel interrupts two spatially offset optical channels, and their phase order determines the direction. 24 slots a quarter slot pitch apart give 96 quadrature edges per revolution, decoded here as 24 scroll steps.

Wheel click: the left bearing is a pivot; the right shoe moves down in its guides, tilting the axle by up to 1.5°. A non-rotating engager depresses the middle switch. Rolling and pressing are independent degrees of freedom.

Pointer movement: the separate underside navigation sensor images the illuminated surface through a lens. It is not the wheel encoder, and this viewer does not simulate image correlation or cursor motion.

Materials use procedural shading, micro-surface variation, studio reflections and real-time shadows. They are authored approximations, not measured material scans.

The optical paths are enlarged and colored for explanation. The detent count and overall dimensions are modeling choices. Spring deformation and switch travel are kinematic illustrations, not finite-element or electrical-contact simulations, and switch contact details are simplified. The signals are ideal and omit noise, switch bounce and firmware filtering.

Drag empty space to orbit and right-drag to pan; drag the tire to roll it and tap it to click. Focus the model to capture input: the mouse wheel zooms, Shift + wheel turns the tire, Arrow Up and Arrow Down step a detent, Space runs or pauses the drive, hold M, L or R to press a button, 1–4 choose the assembly display, S saves a PNG and Escape releases the capture.

Reference: US 5,912,661 A, a Z-encoder scroll-wheel mechanism.

Operate the mechanism

From touch to scroll.

The tire, axle and code wheel turn together. Two optical channels resolve direction; pressing the axle actuates a separate switch.

Rate

Hold to press. A press is a separate button event; it never changes the scroll count.

Wheel angle

Scroll steps

Net edges

Direction
Illegal transitions
Left click
Wheel click
Right click

Stops the drive, returns the wheel to zero, clears the counters, trace and held buttons, and restores the cutaway overview.

Run and Pause in the transport drive the wheel at the rate below. A step stops the drive and turns exactly one detent.

Live quadrature

A B

0010110100

24 windows · 24 detents · 96 edges per revolution. Four valid transitions produce one scroll step.
Ideal model signals read from the decoder state: no noise, switch bounce or firmware filtering.

At the computer

Decoded wheel movement scrolls this document. A press is a separate button event.

.museum-body--immersive { overflow: auto !important; } .museum-main--fill, .museum-runtime { height: auto !important; min-height: 100vh; overflow: visible !important; } .museum-runtime > .exhibit-root, .museum-runtime > :not(#mw-about):not(noscript):not(script) { display: none !important; } .museum-runtime #mw-about { display: block !important; position: static; width: min(100%, 800px); margin: 24px auto; padding: 24px; box-sizing: border-box; } .museum-runtime [data-about-close], .museum-runtime #mw-about > h2 { display: none !important; }

c. 1998 · Computing

Scroll Wheel & Optical Encoder

Three mechanisms share one axle: a tire your finger turns, a cam that makes the clicks, and a slotted disk that tells the computer which way.

A scroll wheel feels like a single motion, but three mechanisms share its axle. A rubber tire gives your finger something to turn. A formed spring riding a 24-lobe cam makes the clicks you feel and holds the wheel at rest between them. And a slotted disk turning between optical sensors tells the computer not just that the wheel moved, but which way.

Direction is the interesting part, and it cannot come from one sensor. Each sensor alone produces exactly the same pulse train whichever way the wheel turns. So a second sensor sits a quarter of a slot pitch away, and the pair walks a two-bit code — 00, 10, 11, 01 — in one order forwards and the reverse backwards. This exhibit decodes it the way the real circuit does: by counting edges as they are crossed, four to a detent, so jitter inside one window nets to zero instead of scrolling the page.

Pressing the wheel is a separate degree of freedom. The guided right end of the axle tilts onto its own middle switch, so a wheel click arrives as a button event and never changes the scroll count; the left and right buttons work the same way through their own plungers and contact springs.

The assembly is a reference-scale reconstruction of a documented optical wheel architecture, exported as 187 geometries and 371 named parts and shipped with the exhibit. The colored beams are drawn so you can see which path is blocked; real infrared light is invisible, and the signals are ideal, without noise, switch bounce or firmware filtering. No specific commercial product is claimed and no manufacturer CAD was used.

Specifications

Encoder: 24 windows, two channels a quarter pitch apart
Resolution: 96 edges a revolution; four edges make one scroll step
Detent: A formed spring on a 24-lobe cam holds each resting position
Wheel click: The axle tilts up to 1.5° onto a separate middle switch
Scope: Reference-scale reconstruction; not manufacturer CAD

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