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One-Pulse-Per Second Driver For Quartz Clocks

This simple add-on module for the GPS-Based Frequency Reference is designed to drive the escapement coil of a low-cost quartz clock movement. It uses the 1Hz GPS pulses available at the rear of the Frequency Reference so that the clock can display local time with GPS-based accuracy.

By Jim Rowe

Click for larger image
Fig.1: a clock stepper motor uses a multi-pole permanent magnet rotor which rotates inside a circular gap in a soft-iron stator. It's made to step in the same direction by reversing the polarity of the current pulse at each step.

If you built the GPS-Based Frequency Reference described in the March-May 2007 issues, you’ll know that it provides a continuous readout of "Universal Time Coordinated" (UTC) on its LCD. This time is derived directly from the GPS satellite system and is therefore very accurate.

In practice, it’s not all that difficult to mentally convert UTC into local time. In most cases, you simply add or subtract a certain number of hours, depending on the nominal longitude of your local time zone and, of course, your time of year. For example to convert UTC into Eastern Australian Standard Time, you simply add 10 hours, or 11 hours during the summer months when we’re on "Summer Time" (daylight saving). So 05:15:00 UTC becomes 15:15:00 (3:15pm) EAST, or in summer 16:15:00 (4:15pm).

That’s all well and good but most people would find a direct readout of their local time a little more useful. And that’s where this project comes in. It uses the 1pps (one pulse per second) output from the GPS system to drive a quartz wall clock. All you have to do is set the display for local time at the start, after which the clock will be accurately controlled via the GPS seconds pulses.

It turns out to be very easy to interface the GPS Frequency Reference to a standard ‘analog’ quartz clock movement. First, you have to remove the existing circuitry from the clock (usually just a chip and a crystal on a tiny PC board) and bring out the connections to the clock’s escapement coil. That done, the coil can be pulsed instead by the little driver module described here. This driver module is small enough to fit inside the clock (next to the movement) and gets its power from the GPS Frequency Reference, along with the 1Hz (1pps) pulses.

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