The Quartz Wall Clock


The familiar quartz wall clock is a modern marvel worth investigating. Available for under $10, it provides timekeeping accuracy that once was only available from a large, expensive pendulum clock (itself a marvel, too, but not a cheap one). I had a quartz clock that failed to work properly, so I took the opportunity to disassemble the clock and see how it worked. (This is the only clock of this kind in my experience to fail to work. It is also the only USA-made clock.) My clock had a General Time Corp. GT2000 movement, which is probably similar to all such movements. It is powered by a single AA alkaline cell that should be replaced about annually. The clock is made mostly of plastic, showing the great utility of this material for making inexpensively pieces of complex shapes and functions. It can be disassembled with the aid of a small screwdriver, which is employed only to release the snap fastenings that are used exclusively. Tweezers are useful for handling the small parts. There are no threaded fasteners in the clock. Note how the use of plastics makes inexpensive manufacture possible, but gives excellent serviceability and an attractive appearance. Several different plastics are used, each adapted to its function. All the parts of the clock are shown in the photograph.

Start by detaching the clear plastic front. Then carefully pry off the hands, after which the paper dial will come out easily. Turn the clock over, and release the movement by prying aside one of the spring catches, and lifting out the movement. The clock movement is about 55 x 55 mm square, and 13 mm high. The case is in two pieces, which again can be separated by prying the spring clips, two on one side and one on the other. The white plastic gear trains then fall out easily. The clock motor and circuit board are in one module, that comes out easily. Two metal contact strips, a short + one and a longer - one, are some of the rare metal parts, and fall out easily. The circuit board makes contact by pressing against the ends of these strips. The top of the movement case contains the setting wheel and its pinion; these can be left assembled.

The circuit board is held by spring fingers on a plastic support that is connected at one end of the motor coil. Note the small metal cylinder of the quartz crystal, which resonates at 32,768 Hz, or 215 Hz, that can be counted down to 1 Hz by a binary counter. Two small horns have copper-wire leads to the coil soldered to them. The coil is of fine wire, and surrounds a ferromagnetic core of strange shape. At one end is the space for the motor pinion, which has a cylindrical permanent magnet on one end, and a 10-tooth pinion on the other. The cylindrical magnet is magnetized transversely, N on one side and S on the other. When the polarity of the field established by the coil reverses, the pinion makes one-half a revolution, or 30 revolutions per minute. Just how it manages this is not clear from superficial observation, but it is probably similar to the action of a stepping motor. In my clock, it did not, in fact, rotate properly and the second hand just oscillated back and forth without going anywhere. You should be able to see the rotation in a disassembled working clock by putting in a battery, and maybe guiding the loose end of the arbor with tweezers. The chip in the circuit board contains the oscillator, a 215 divider, and a driver for the motor coil. It operates on 1.5V, and is "potted" in an epoxy compound. This module takes the place of the escapement in a mechanical clock, giving us a rotating pinion of very accurate speed. The timekeeping of the quartz clock is not affected at all by friction.

The remainder of the clock consists of the gear trains that link the movements of the hands. The gearing is schematically represented in the diagram. Note that gears 3 and 5 rotate independently, but on the same axis. The second hand is driven by the clock motor alone, connected to the motor by a gear train with a 30:1 reduction. The minute hand is driven jointly by the second hand and the setting wheel. A gear train with 60:1 reduction drives the minute hand from the second hand through the frictional resistance in the minute hand shaft. The minute hand drives the hour hand through a 12:1 reduction. The setting wheel also drives the minute and hour hands by means of the intermediate gear in this train. If you observe the setting wheel while the clock is working, you will see that it is rotating. When you move the setting wheel, the minute and hour hands are moved while the connection to the second hand slips. These trains of gears are exactly the same as in a mechanical clock. In this clock, the plastic gears are easily and accurately made, and are much cheaper than metallic gears.

The seven gears can be identified by numbers. 0 is the motor pinion, 1 is the second hand, 2 is the motor-second spur gear, 3 is the second-minute spur gear, 4 is the minute hand, 5 is the minute-hour spur gear, and 6 is the hour hand. Gears 1,4 and 6 are on the same axis, and their shafts telescope. Gears 3 and 5 rotate independently on the same axis, with gear 5 driven by the setting wheel. Gear 4 is driven both by gears 5 and 3, with a frictional connection. The gears can be removed in the order 1 to 6, and replaced in the same order. Gear 0 has 10 teeth, gear 2 50 and 10 teeth, and gear 1 60 teeth on the large gear. Counting the numbers of teeth on the other gears was inconvenient, but perhaps the reader may do so and verify the ratios.

References

M. Fischetti, Scientific American, March 2004, p. 98f., "Rock Clock." Neither the motor nor the "gears and wheels" are explained, but there is a picture of the quartz oscillator. The watch motor may make "two beats a second," requiring 120:1 reduction to the second hand, but the wall clock uses two seconds per beat. This article was probably not prepared by looking at an actual watch, but by paper research. Incidentally, the "power" mentioned in the article in a watch microprocessor is of no consequence; the job is an exceedingly easy one. Microprocessors are very flexible--that's the advantage. The Apple II processor did much, much more than the processor in any watch.


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Composed by J. B. Calvert
Created 6 February 2004
Last revised 10 February 2004