How a quartz watch turns vibrations into seconds

On 25 December 1969, Seiko released the Quartz Astron 35 SQ. Inside a wristwatch, a quartz crystal and an electronic circuit supplied a rhythm that could drive the hands. The challenge had been to make the crystal, electronics and motor small enough, and economical enough with electricity, to work together on a wrist. Seiko's museum documents the release and the engineering behind it.

Look at an ordinary quartz watch today and a question remains: how does a piece of crystal become a sequence of seconds? The battery supplies energy, but it does not contain a clock. The answer comes from keeping a vibration going, turning that rhythm into an electrical signal and counting its cycles.

A crystal that connects motion and electricity

Squeeze quartz in the right direction and electrical charge appears on its surfaces. Apply an electric field and the crystal changes shape slightly. This two-way connection between mechanical strain and electricity is called the piezoelectric effect.

Pierre and Jacques Curie demonstrated the pressure-to-charge effect in 1880 and its reverse in 1881. Decades later, Bell Telephone Laboratories developed quartz clocks; a quartz-clock design was described in 1927. The wristwatch inherited a principle that had already served much larger instruments. NIST's history traces these steps.

The useful crystal is carefully shaped, often into a tiny tuning fork. Like a musical tuning fork, it has a preferred rate of vibration, called its resonant frequency. Its dimensions and the way it is cut help determine that rate. The resemblance concerns repeated mechanical motion; the watch is not listening for a musical note.

A crystal left alone cannot keep vibrating forever. Motion loses energy. A battery-powered circuit sustains it by feeding an electrical signal back to the crystal with timing that reinforces the vibration. The crystal's motion also produces an electrical response, so motion and voltage participate in the same repeating loop. The crystal and sustaining circuit together form an oscillator. Epson explains why the crystal needs a matched circuit.

Think of maintaining a playground swing with small, well-timed pushes. The pushes replace lost energy; the swing's physical properties help set the rhythm. In the watch, electrical feedback supplies the pushes. The analogy stops at the circuit: nobody measures the fork by eye, and the crystal is not swinging under gravity.

Why count to 32,768?

Many modern quartz watches use a frequency of 32,768 hertz, meaning 32,768 complete cycles each second. The electronics produce a repeating signal from the oscillator and count those cycles. NIST describes this common timekeeping arrangement.

That number looks awkward in decimal, but it is wonderfully convenient for circuits that divide by two. Halve 32,768 and you get 16,384; halve again and you get 8,192. After fifteen halvings, the output repeats once per second. The device that performs this operation is a frequency divider.

One basic dividing stage changes between two electrical states on successive input cycles. It needs two input cycles to complete one output cycle, so its output frequency is half its input frequency. Connecting fifteen such stages produces the full division. Nexperia's divider datasheet shows this chain of fifteen stages.

Nothing here makes the crystal physically vibrate more slowly. It continues at its fast rhythm while successive electronic stages produce slower signals. Nor does the number create accuracy by itself: 32,768 cycles amount to one second only when the oscillator runs at the intended rate.

The 1969 Astron initially used 8,192 hertz. Seiko later adopted higher frequencies as electronics needed less power. Today's common 32,768-hertz choice combines a practical crystal rhythm with convenient electronic counting; it is not a frequency used by every quartz timepiece.

Make the count visible

In a typical analogue quartz watch, the divided timing signal controls brief electrical pulses to a small motor. Each pulse creates a magnetic field that moves the motor through a step. Gears carry that motion to the hands. The familiar once-a-second movement is therefore a mechanical display of an electronically counted rhythm. Seiko describes the Astron's motor and its one-pulse-per-second drive.

A digital quartz watch can use the counted time to update numbers on a liquid crystal display, or LCD. It does not need hands to make the rhythm readable. Seiko's 1973 06LC, for example, displayed six digits on an LCD. “Quartz” names the timing technology; “digital” describes how the result is shown. The Seiko Museum explains the development of LCD watches.

A small error, counted faithfully

Suppose an oscillator runs slightly fast. The divider still counts correctly, but finishes each group of cycles too soon. The watch gains time. A slower oscillator makes it lose time. Counting faithfully preserves a systematic rate error rather than cancelling it.

Engineers often express that error in parts per million, or ppm. An illustrative error of +20 ppm means a rate 20 millionths faster than intended. Over a day of 86,400 seconds, that adds 1.728 seconds, about 1.7 seconds. If the error stays constant, twice as much elapsed time produces twice as much gain.

Keeping the rhythm close

Manufacturing differences can shift the frequency. Temperature also changes the crystal's behaviour, and gradual changes with age can alter its rate. Many ordinary tuning-fork crystals run slower when taken sufficiently above or below their temperature of highest frequency. Microchip's application note explains manufacturing tolerance, temperature drift and ageing.

Calibration means comparing the oscillator with a reference and adjusting its timing. Some circuits can trim the frequency by changing an electrical property called capacitance. That lets the circuit correct a small offset, but does not guarantee perfect timing under every condition. Epson explains frequency trimming and its limits.

The little jump of a second hand hides all of this work: energy replenished, quartz flexing, electrical cycles counted and a motor moving. A watch turns that fast physical rhythm into a useful answer to an everyday question: how much time has passed?