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GPS Synchronisation For Clocks With Sweep Hands

The GPS Synchronised Clock described in the March 2009 issue only worked with crystal clocks that ticked once a second. The more upmarket clocks have silent sweep hands, which are much more acceptable in a quiet lounge room situation. Now, with just a few simple modifications, you can upgrade to one of these.

by Geoff Graham

There has been a lot of interest in the GPS Synchronised Clock published in the March 2009 issue of SILICON CHIP. It introduced a completely new way of driving the humble analog wall clock and turned it into an amazingly accurate time-keeper.

However, it was only capable of driving clocks that stepped once per second and that stepping mechanism can be very annoying to some people – especially in the dead of night and even more especially when sleep eludes them! Tick. . . tick. . . tick. . . tick. . .

The four modifications can clearly be seen in this under-board photo. Make sure you use insulated wire (or a length of insulation spaghetti slid over a wire) for the link (D) as it crosses over another track underneath the microcontroller. Similarly, ensure that the leads for the two Schottky diodes do not come even close to the tracks underneath, just to be safe!
Click for larger image
Fig.1: the waveform used to drive the coil of a sweep hands clock. The clock pulses alternate with a positive pulse, an idle period and a negative pulse. This is repeated at 16 pulses per second to drive the clock's hands around the dial.
Click for larger image
Fig.2: this is how the clock's coil is driven in the modified circuit. The micro can take pin 17 high for a positive pulse on the coil, low for a negative pulse, or open circuit (represented as a centre-off position) for the idle period between pulses.

They crave the silent, continuous sweep hand on the old-style electric clocks.

The good news is that some of the more expensive crystal clocks, such as those from Seiko and Citizen, now have a sweep second hand that continuously and silently glides around the dial.

You do not have to part with a lot of cash to get this new silent treatment. K-mart sell a reasonably cheap clock with a continuous sweep second hand while replacement movements with a continuous sweep hand are available on the internet for $10 to $15 (Google “clock movement continuous sweep”). Note that some suppliers use the term “sweep second hand” when referring to the old stepping movement, so look for the words “continuous sweep” or “silent”.

We had already had emails from readers who wanted to upgrade these improved crystal clocks to GPS accuracy and so we thought it worth revisiting the GPS Synchronised Clock design to see if it could be modified to drive the new sweep hand movements. The answer was yes, although with an important caveat.

A standard crystal clock movement uses a coil with a soft iron core and a small bar magnet (the rotor) positioned in the magnetic field. An alternating current flows through the coil which causes an alternating magnetic field and the rotor rotates to follow this field. It is this rotation that, via gears, drives the clock’s hands.

Fig.1 shows the driving waveform for a clock with continuous sweep hands. It consists of a positive pulse, an idle period, a negative pulse and another idle period. This repeats eight times a second. The rotor in the clock’s movement has a certain amount of momentum which keeps it spinning while driven by this pulse train, so it never stops. This is different to the stepping clock movement where the voltage pulse on the coil pulls the rotor around and then stops it dead – once every second – thereby creating that tick sound.

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