Stopwatch and Timer Calibrations - National Institute of Standards ...

Description of Timing Devices
Figure 2. Type I digital stopwatch. Figure 3. Type II mechanical
stopwatch.
2.A.1. Basic Theory of Operation
Every stopwatch is composed of four elements: a power source, a time base,
a counter, and an indicator or display. The design and construction of each
component depends upon the type of stopwatch.
2.A.1.a. Digital (Type I) Stopwatches
The power source of a Type I stopwatch is usually a silver-oxide or alkaline
battery that powers the oscillator and the counting and display circuitry. The
time base is a quartz crystal oscillator that usually has a nominal frequency of
32 768 Hz, the same frequency used by nearly all quartz wristwatches. The
32 768 Hz frequency was originally chosen because it can be converted to a 1
pulse per second signal using a simple 15 stage divide-by-two circuit. It also
has the benefit of consuming less battery power than higher frequency crystals.
Figure 4 shows the inside of a typical digital device, with the printed circuit
board, quartz crystal oscillator, and battery visible. The counter circuit consists
of digital dividers that count the time base oscillations for the period that is
initiated by the start/stop buttons [6, 7]. The display typically has seven or eight
digits.
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