Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [3]
Due to their accuracy and stability Quartz crystals are used in the communications field where high frequency stability is required, the generation of clock signals in computers, and the reference signals in most clocks and watches.
LOUDSPEAKERS AND MICROPHONES
If a cone is pointed at a sound source it will tend to vibrate at the frequency of the sound. These vibrations are used to generate movement of a coil mounted at the centre of the cone. The coil moves within a magnet so a small voltage proportional to the sound is generated and it becomes a microphone. If voltage is fed to the coil then it will be translated into a movement of the coil and therefore the cone, thus producing sound. It then becomes a loudspeaker.
Microphones can also be constructed using capacitance effect. In some cases in line amplifiers or even small transmitters are incorporated into the microphone body.
TEMPERATURE MEASURING DEVICES
Thermocouples:
The previous diagram shows a simple thermocouple circuit with junctions at T1 and T2. The thermocouple will generate an output voltage proportional to T2 - T1, (For NiCr/NiAl approx 4mV per 100°C). The main disadvantage with this system is the need to reference the junction at T1. This is sometimes achieved by inserting a second thermocouple in series and placing the end in melting ice (0°C), or using a thermistor to generate a voltage equivalent to the ambient temperature (T1). Where many thermocouples are terminated one reference may serve all thermocouples if they are mounted in a sealed junction box, the box is then considered to be isothermal.
Thermocouples are used at all temperatures from ambient to above 600°C although platinum resistance thermometers give better results under 100°C.
Thermistors:
These devices vary their resistance with temperature (+ve or -ve response) and are generally used in control applications. It is necessary to calibrate systems using thermistors as they are non-linear. A simple circuit is shown above. A supply voltage Vs feeds the voltage divider formed by resistor R and the thermistor Rt. The voltage across the thermistor will then vary with temperature.
OTHER DETECTORS
Light Dependant Resistor (LDR):
These devices are housed in a moulding or case with a clear end window to allow light to fall on the cadmium sulphide resistor. These devices have negative response to light, ie resistance reduces as the light falling on the device increases. LDRs are generally used as sensors for remote outside lighting.
Strain Gauges:
Strain gauges are a thin element of a conductor constructed as shown above mounted on a thin flexible film, which can be bonded to a sample. They are only effective in one plane so they are generally mounted in groups. As the sample moves the strain gauge is stretched which reduces the cross sectional area of the conductor, increasing the resistance.
AC Theory
The UK mains supply is specified as 240V rms at 50Hz. The waveform generated by a rotating magnetic field can be considered to be sinusoidal.
The drawing below, (see fig 1), shows a sinusoidal voltage waveform. The average value of the waveform = Vpk/Ö2 (pk = peak and pp or pkpk the peak to peak value) and for current waveform Ipk/Ö2.
These values are known as the rms values, which equate to the dc equivalent, ie Power = V * I.
The voltage and current waveforms shown below in fig 2 are displaced by the phase angle 90° (pure resistive load). The phase shift of the voltage and current waveforms is dependant on frequency as is the impedance Z. The impedance of a circuit is its AC resistance in ohms.
An inductive load will produce a lagging current waveform whilst a capacitive load will produce a leading current waveform. The reactance X of the load can be represented as a vector drawn at 90° to the resistance vector (pure capacitance or inductance only). The value