Instrumentation circuits 555 timer

Most electronic repairs consist of replacing entire printed circuit boards, as opposed to individual electronic components and wiring, which was required in early instrumentation. Cam timers have disappeared, to be replaced by digital electronics. Analog electronics have been minimized in order to preserve the integrity of the signal by digitizing as close as possible to the detector. 

The instrument bus furnishes power to those instruments most sensitive to disturbance while the relay bus (also called the "control bus") furnishes power to the less sensitive components, such as relays and motors, which not only are less sensitive to disturbance but are themselves actual sources of disturbance. If one of the alternator supplies breaks down or otherwise becomes unavailable, the relay bus would be switched to purchased power. To permit operation with the "rougher" purchased power, the relay circuits include a 2-sec timer and a number of time delay relays so that reactor shutdown does not ensue unless the r.elay bus voltage is off for 2 sec or longer. 

A sensor that is especially interesting and instructive is made from a quartz crystal microbalance, or QCM. This device is based on the piezoelectric characteristics of quartz. When quartz is mechanically deformed, an electrical potential difference develops across its surface. Furthermore, when a voltage is impressed across the faces of a quartz crystal, the crystal deforms., A crystal connected in an appropriate electrical circuit oscillates at a frequency that is characteristic of the mass and shape of the crystal and that is amazingly constant as long as the mass of the crystal is constant. This property of some crystalline materials is called the piezoelectric effect and forms the basis for the QCM. Moreover, the characteristic constant frequency of the quartz crystal is the basis for modern high-precision clocks, time bases, counters, timers, and frequency meters, which in turn have led to many highly accurate and precise analyTical instrumental svsiems. 

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