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