Digital watch
In five years watches stopped ticking. Wheels, springs and the escapement gave way to a sliver of quartz ringing in the ultrasound and a chip that counts its vibrations.
- 32,768 Hzthe quartz frequency, exactly 2¹⁵
- ± 15 sa month, better than the best mechanical watches
- ≈ 7 yearson one coin-sized battery
A watch that doesn't tick
On Christmas Day 1969 Seiko released the Astron, the first quartz wristwatch. It still had hands, and it cost as much as a small car. Three years later came the Pulsar with red LED digits, and in 1973 the first watches with a liquid crystal display.
The drawing shows a generic digital watch of the 1980s. The digits show your time. Press "Light".
A tuning fork the size of a grain of rice
A tuning fork cut from quartz crystal is sealed in a tiny metal can on the board. Put a voltage on quartz and it bends slightly; bend it and it produces a voltage. The chip nudges it with pulses, and the fork rings 32,768 times a second.
That is beyond hearing, and the number isn't chosen by chance: 32,768 = 2¹⁵. Halve it fifteen times and exactly one pulse a second is left.
A drop of black resin
The chip itself hides under a black drop of epoxy: the silicon die is bonded straight to the board. Inside are the quartz oscillator, the frequency dividers, the counters for seconds, minutes and days, and the display driver. Next to it sits a small trimmer capacitor, which the factory turned to shift the frequency slightly until the watch kept time.
Digits that barely use any power
Liquid crystal sits between two sheets of glass, with polarising films above and below. With no voltage the molecules are twisted and let light through. With a voltage they line up, and the segment goes dark.
The display gives off no light, it only blocks it, so it uses microwatts. The Pulsar's LEDs needed thousands of times more, and its digits lit up only at the press of a button.
Wires made of rubber
The display connects to the board without any soldering. Two rubber strips are made of hundreds of layers: conducting, insulating, conducting again. Squeezed between the glass and the board, each strip links every contact on the display to the pad below it and never shorts its neighbours.
Seven years from a coin
A 3-volt lithium CR2016 holds about 90 milliamp-hours. A simple watch draws about one and a half microamps on average, and the light and the alarm use up the rest.
90 mAh ÷ 1.5 µA ≈ 60,000 h ≈ 7 yearsA disc that beeps
A brass plate with a ceramic piezo disc is glued to the inside of the case back. The chip feeds it an alternating voltage of about 4 kilohertz, the plate flexes to and fro, and the case back acts as a resonator. That is the familiar beep-beep on the hour.
Eight layers
Bezel, display, zebras, the board with its battery, the case with its buttons, and the back with the buzzer. Everything is held by four screws and pressure. Not a single part rubs against another.
Fifteen halvings
Each cell on the right is a flip-flop. It changes state on every second pulse arriving from the left, so it divides the frequency by two. Fifteen such cells in a row turn 32,768 vibrations into one a second.
In real time the eye can only see the last few cells blink.
A thousand times slower
We have stretched time 1,024 times. Now you can see the switching ripple along: the first cell blinks, the second half as often, the third half as often again. The row of lamps is a binary number that simply counts the quartz's vibrations.
One vibration a second
Slowed down 32,768 times, the quartz swings once a second and you can see every vibration. The next second at the output is now more than nine hours away. Move the slider under the drawing yourself.
The quartz crisis
Electronics got cheap fast. The first gold-cased Pulsar cost $2,100; by the end of the 1970s digital watches sold for less than twenty. The Swiss workshops that had made the world's best watches for centuries couldn't adapt in time: between 1970 and the mid-1980s jobs in the industry fell from about 90,000 to about 30,000.
In 1983 Casio released the G-Shock, a watch that survives a ten-metre drop. In 1989 came the F-91W: simple, with a seven-year battery. It is still made today, and it may well be the most widely produced watch in history.
The Swiss answered that same year, 1983, with the plastic quartz Swatch, made of 51 parts instead of the usual hundred or so. Mechanical watches didn't disappear; they became jewellery. Accurate time now costs less than the strap.