MadSci Network: Engineering |
At the heart of a digital watch or timer is a quartz crystal that vibrates at a specific frequency, much like a tuning fork. As you know, the frequency, (rate of vibration) of a tuning fork depends on the material it's made of and the size and shape of the legs of the fork. In the case of digital watches or timers, the quartz crystal is cut to very accurate size (often by laser) to make sure it vibrates at a very particular frequency. This frequency is typically, 32,768 vibrations per second or hertz (abbreviated Hz), with an accuracy of plus or minus 1 Hz for cheap watches to perhaps 0.001 Hz for precision timers. These accuracies correspond to accuracies of 3 seconds per day to 1 second per month. The quartz is not actually cut to the shape of a tuning fork but into a slab. Nonetheless, this slab of quartz has a particular frequency at which it naturally resonates (much as a solid block of metal suspended by a thin wire will ring at a particular pitch when struck). As the crystal vibrates, it shrinks and expands slightly. The quartz crystal has an interesting property in that mechanical forces create a voltage called piezoelectricity. This means that if you compress the crystal slightly, a small voltage is created. Conversely, if you try to pull the crystal apart, the opposite voltage is created. The vibrations act to compress and expand the crystal, causing an oscillating voltage to be created across the face of the crystal. Conversely, if you apply a voltage to the crystal, it will shrink or expand slightly. An aside: piezoelectricity is also what makes the "needle" work on crystal pickups for phonographs, or how the "crystal headphones" used in the early days of radio work. This piezoelectric voltage is picked up by electrodes affixed to the crystal, and amplified by electronic circuitry and converted into 32768 pulses per second. A digital circuit known as a divider has the neat property that it will produce one output pulse for every TWO input pulses. A chain of dividers is used to count the pulses: the first gets 32768 pulses per second, and produces 16384 per second. The second divider takes this signal and produces 8192 pulses per second, feeding the third, which produces 4096, and so, and so on, until the 15th divider takes the two pulses per second it's given and produces one pulse per second at its output. Note that 32768 = 2**15 = 2*2*...*2 (fifteen times) This one pulse per second is used to trigger the display electronics to change the state of the display to the next higher count. (When you "set" the time, you are initializing a count in the display). Quartz watches with analog hands either use the 1 pulse per second (or some higher multiple) to trigger a tiny electric stepping motor that drives the hands, or to supply a tiny synchronous AC motor whose motion is locked to the input frequency. One further refinement in understanding: left to its own devices, the vibrations of the quartz crystal would quickly die out. In practice, the electrodes on the crystal serve to make it part of an electronic circuit known as an oscillator. The highly resonant nature of the crystal serves to lock the electronic oscillator's frequency at the crystal's resonant frequency. The oscillator circuit in turn gives the crystal a bit of energy every cycle to keep the vibration going (it gets just enough of a energy kick every cycle to offset energy losses due to internal friction within the crystal and vibrations transmitted to surrounding air and packaging). If you want to find out more about how these circuits work, you might try your school library to find books on simple electronics projects. You might also try looking at "cookbooks" of simple electronics circuits. Radio Shack used to sell these, as well as the necessary parts (or part kits) to experiment with building your own digital timers. Another great reference that will discuss quarz crystal oscillators is the "Radio Amateur's Handbook". Your school library should have a copy. Try experimenting with electronics or even amateur radio yourself! More than one grown-up electrical engineer cut their teeth this way. It's fun, and there's a lot more to electronics than the simple cogs and gears of a computer that lets you see Web pages :-) Steve Czarnecki
Try the links in the MadSci Library for more information on Engineering.