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A GPS-Synchronised Clock

Tired of resetting the time on your wall clock? This GPS circuit will convert a standard battery wall clock into a precision timekeeper that never needs to be corrected. It will even automatically adjust for daylight saving time.

By Geoff Graham

Battery-powered quartz crystal clocks are very common and they keep good time, with a typical accuracy of two seconds per day. However, that couple of seconds can add up. After a month it could be a minute out and after a few months, you are up on a chair again to reset it to the correct time.

Wouldn’t it be nice if you never, ever had to do that again? 

This design replaces the electronics in a standard quartz wall clock with a controller that synchronises itself via the GPS (Global Positioning Satellite) system. It uses a relatively inexpensive (US$70) GPS module to get the precise time and a microcontroller to control the hands of the clock. It will run for about a year on two alkaline AA batteries and over that period will keep the time accurate to within a few seconds.

Even better, it understands the rules of daylight saving (DST) and will automatically adjust by skipping forward an hour at the legislated time and date when daylight saving starts. When daylight saving stops, the clock will stop for exactly an hour at 3.00AM, – and start again when the indicated time is correct.

It is also easy to use. All you need do is set the hands of the clock (including the seconds hand) to the 12 o’clock position and then insert the battery. The controller will use the GPS to get the current time and then step the clock hands at double speed around the dial until it has reached the correct time. It will then drop back into normal timekeeping mode with the time derived from a crystal oscillator.

To conserve the battery, the GPS module is only used to synchronise the clock every 44 hours and following synchronisation, the clock will either skip seconds or double-step to reach the correct time. After synchronisation the microcontroller is also able to calculate the inherent inaccuracy of its crystal oscillator and will compensate by occasionally skipping or double-stepping a second. This process can also compensate for aging of the crystal and will keep the clock accurate between synchronisations.

The controller also monitors the battery voltage and when it has dropped below 2V, the microcontroller will stop the clock at exactly 12 o’clock. You then replace the battery and it will repeat the start-up process by stepping to the correct time. 

In short, you never have to set the time.

Click for larger image
It looks just like a bog-standard battery-powered wall clock and in fact, it started out life as such. But it's only when you turn it over . . .
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You can see it has something that's not bog standard - a GPS module, microprocessor and driver. And no, we havenâ�t forgotten to put the clock movement battery in!

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