5

(b), d2 is increased to 10 units C = 50 = 5 units. w 2 10

A.C. Supply from an oscillator.

Sensor . capacitor

Sensor . capacitor

Resonant frequency of this = 1 circuit is approx = 2rcCC

Resonant frequency of this = 1 circuit is approx = 2rcCC

Changes in C caused by pressure changes at the sensor thus cause the resonant frequency of the circuit to change. The frequency of the AC supply does not change.

Fig. 8.24 An RLC circuit and the principle of the variable-capacitance sensor introduced into a circuit of the form shown in Fig. 8.24(c) it has an effect on the frequency at which the voltage across the capacitor and the voltage across the inductance (coil) cancel each other out, electrically. The frequency at which this happens is known as the resonant frequency of the circuit. When the circuit is resonating, the voltage across the resistor R is in phase with the a.c. supply. Because the resonant frequency will change as the pressure at the sensor causes the capacitance value C to change, the terminal voltage V at the phase detector circuit, is related to the resonant frequency and manifold pressure.

For electronics purposes, light is often considered to consist of discrete elements called photons. Photons possess energy and in certain materials and under suitable conditions photon energy can be transferred to and from electrons to change their electronic behaviour. One way in which the change in behaviour of electrons affects a material is to change its conductivity and this is made use of in photodiodes, light sensitive sensors, etc. Another way that it is made use of is to produce a voltage and this is the basis of a fuel cell.

Figure 8.25 shows the basic principle of an optocoupler. This is a device which forms the basis of sensors that are used for position sensing, as in

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