How does a digital watch work?
No gears, no hands, no ticking. A sliver of quartz, a row of counters and a sandwich of liquid crystal that twists light keep the time on one coin cell for seven years. Take a digital watch apart in 3D and count the seconds yourself.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A chip, a crystal, a cell and a piece of glass. Inside the resin case is a small module: a circuit board with one chip under a blob of black epoxy, a quartz crystal in a tiny can, and a 3 volt coin cell. The display glass has no wires. Two rubber zebra strips squeezed between board and glass carry every signal, and a piezo disc on the steel back makes the beep.
- One tick a second. The quartz crystal is cut like a tuning fork and rings 32,768 times a second. That is 2 multiplied by itself 15 times, so a chain of fifteen flip-flops, each halving the beat, ends at exactly one pulse a second. Faster beats along the chain feed the beeper and the display.
- A row of counters. The chip does not know the time. It counts. Seconds run 0 to 59, roll over to 0 and pass a carry to the minutes; minutes carry to hours, hours to the date. The date counter checks the month for its limit, and a watch that stores the year gives February 29 days in a leap year.
- Seven bars make every digit. Each digit is kept as four bits. A decoder turns them into seven on or off signals, one for each bar of the figure of eight. A watch face can have 80 bars, so they share wires: three back electrodes take turns, and 30 chip pins do the work of 81.
- A sandwich that twists light. A segment is liquid crystal between two polarisers set at right angles, over a reflector. With no voltage, the molecules form a quarter-turn twist that guides room light through both polarisers and back out: clear. About 3 volts stands the molecules up, the twist is lost, the light is blocked: dark. It makes no light of its own.
- Four jobs on a millionth of an amp. The Mode button steps the chip from clock to alarm to stopwatch to setting, and the other buttons change meaning in each. An alarm is a comparator and a 4,096 Hz tone into the piezo disc. The whole watch averages about 1.5 microamps, so one coin cell lasts seven years or more. A smartwatch uses over ten thousand times as much.
| Term | Meaning |
|---|---|
| Quartz crystal | A tiny tuning fork of quartz that the chip keeps ringing at a very steady 32,768 swings a second. |
| Divider | A chain of flip-flops that each halve the frequency; fifteen of them turn 32,768 Hz into one pulse a second. |
| Counter | A circuit that adds one for every pulse, rolls over at its limit and sends a carry to the next counter. |
| BCD | Binary-coded decimal: each decimal digit stored on its own as four bits, from 0000 to 1001. |
| Seven-segment display | A digit drawn with seven bars, named a to g. Different sets of bars make 0 to 9. |
| Liquid crystal | A liquid whose rod-shaped molecules share a direction and can be turned by a small voltage. |
| Twisted nematic | The LCD type where the molecules make a quarter-turn twist that guides light between two crossed polarisers. |
| Multiplexing | Driving many segments with few wires by letting two to four back electrodes take turns, many times a second. |
| State machine | A circuit that is always in one of a few modes and steps between them when a button is pressed. |
From a crystal that melted twice in 1888 to a ten-dollar watch that keeps time for seven years on one coin cell.
- 1888 · A substance with two melting points (Friedrich Reinitzer, Prague (then Austria-Hungary))
- 1968 · RCA shows the liquid crystal display (RCA, New York, USA)
- 1970 · A watch with no moving parts (Hamilton Watch Company and Electro/Data, New York, USA)
- 1970 · The twisted nematic cell (Martin Schadt and Wolfgang Helfrich; James Fergason, Basel, Switzerland and Kent, Ohio, USA)
- 1972 · The Pulsar goes on sale for $2,100 (Hamilton Watch Company, Lancaster, Pennsylvania, USA)
- 1973 · Six digits that never switch off (Seiko (Suwa Seikosha), Suwa, Japan)
- 1974 · A watch that knows the months (Casio, Tokyo, Japan)
- 1976 · The $10 digital watch (Texas Instruments, Dallas, USA)
The full story, with 32 moments, charts, people and 37 sources: glassbox.how/e/digitalwatchclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
digitalwatchclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/digitalwatchclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
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Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/dwatch.js |
What the chapters share: the time and date counters, the seven-segment table and watch face, the liquid-crystal and battery sums, and a generic LCD wristwatch in 3D |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |