May 2005
Beginner to intermediate
398 pages
12h 12m
English
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Crystals
Finally in our component tour, we come to crystals . Just as their name suggests, they are small blocks of quartz (silicon dioxide). Quartz crystal is a type of material known as a piezo-electric. This is a substance that generates a voltage when it is stressed (compressed, stretched, twisted). This effect is utilized in microphones. The sound vibrates the piezo-electric material, and it produces a small AC voltage that is directly proportional to the original sound that created it. This voltage is then amplified for broadcast, recording, or processing.
The opposite effect is also true for piezo-electric materials. Apply a voltage and the piezo-electric material will contort or vibrate. Some speakers use piezo-electric materials to produce sound. (However, most use other techniques, such as electro-magnetics.) Most loud buzzers are based on piezo-electrics.
Now, the neat thing about quartz is that for a block of a given size, it will vibrate at a given (and fixed) frequency. As such, it can be used as an oscillator to generate a sine wave, which in turn can be used to generate timing signals for microprocessors and other digital circuits. Just about every computer system will have a crystal (or two) somewhere on its circuit board, generating the timing that ultimately drives the whole machine. That crystal is simply a small block of quartz, plated at either end with wires attached and encased in a metal can.
The schematic symbol for a crystal is shown in Figure 4-54, and ...
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