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Dead-Accurate 6-Digit GPS-Locked Clock, Pt 1

Looking for a digital clock that's always dead accurate? This one derives its time signals from the GPS (Global Positioning Satellite) system, so it never needs setting or adjusting. It features big, bright 58mm-high digits for the hours and minutes, plus smaller digits to indicate the seconds.

By Jim Rowe

In the March 2009 issue, we featured a GPS-controlled analog clock that’s proving very popular. Strictly speaking though, this wasn’t a GPS clock but a “GPS-corrected” clock.

Click for larger image
The main board uses 58mm-high 7-segment displays for the hours and minutes readouts plus smaller 13mm-high digits to indicate the seconds. The GPS time signals are derived either from a small add-on module to be described next month or from the GPS-Based Frequency Reference (see text).

Basically, an external module carrying a PIC processor and an EM-408 GPS module was used to replace the clock’s own crystal oscillator drive. The PIC processor provides the timing signals for the clock and the GPS module is then used to re-synchronise the clock once every 44 hours.

By contrast, this digital clock is permanently locked to the GPS time signals and always displays the correct time. It can display UTC time (Universal Time Co-ordinated), local standard time or local daylight saving time, all at the touch of a button.

The digital clock display described here can derive its GPS time signals from the GPS-Based Frequency Reference described in the March-May 2007 issues of SILICON CHIP. However, you don’t have to go to the expense of building the GPS-Based Frequency Reference. Instead, you can use the above-mentioned EM-408 GPS module on a small PC board which can be housed in the same case as the display board to form a self-contained clock. This will be described in Pt.2 next month.

GPS Frequency Reference

The GPS-Based Frequency Reference described in the March-May 2007 issues already displays UTC time on its small LCD readout. In order to get your local time, you have to mentally add (or subtract) the appropriate offset for your particular time zone and also add another hour if your state or region is currently observing daylight saving.

As it turned out, many readers were more interested in the timekeeping aspects of the GPS-Based Frequency Reference, rather than its very accurate frequency outputs. They also wanted a much larger display that could be read at a distance. And they wanted the display to automatically show both local standard time and local daylight saving time.

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