Transducers

Introduction

The term transducer describes a device for translating a signal from one form into another. An oscilloscope is one type of transducer; it transforms electrical signals into a useful visual or photographic pattern. A loudspeaker transforms electrical energy into sound, while a microphone performs the reverse process. A phonograph pickup is also a familiar type of transducer; it accurately translates the needle motion into an electrical form for amplification. At the risk of overgenerality, the transducers may be boldly divided into those which directly generate a voltage and those which depend upon the variation of some electric circuit parameter such as resistance, inductance, or capacitance.

Many electronic measuring and reproducing systems employ both transducers and electric circuits. First an input transducer provides an electrical signal representing the physical quantity observed. This signal is then amplified, operated upon by electric circuits, and transmitted to the desired point. Finally an output transducer transforms the electrical signal into the most convenient form for the particular operation. Although it is often possible to obtain a similar result without the intervening electrical system, the electrical link usually provides the advantages of flexible transmission, amplification, and speed.

Input Transducers

Input transducers convert a nonelectrical quantity into an electric signal. They can be subdivided into several classes. Some of the classes are introduced below.

Mechanical Transducers

Input transducers in this class respond to movements, changes in pressure, or other mechanical changes. A simple kind of motion detector is a variable resistor. Any movement of the movable contact along the "slidewire" changes the resistance in the circuit, and this change becomes the electrical signal.

A mechanical transducer could also produce changes in inductance or in capacitance. For example, the capacitance of a capacitor depends, among other things, on the distance between the two plates. Some record-player pickup arms use variable inductances. In both inductance and capacitance transducers, a moderately high-frequency "carrier" current is varied by the signal because both inductances and capacitances have their greatest effect at higher frequencies. The variations in "carrier" current then can be amplified.

Certain crystalline materials, quartz for example, exhibit the piezoelectric effect. If a voltage is impressed across a wafer of the crystal, the crystal changes its shape. If an alternating current is used, the crystal vibrates, vibrating most strongly when the frequency corresponds to the natural period of the crystal. This effect is used most commonly in regulating the frequencies in communications circuits. The reaction is reversible, however, making it possible to use such crystals as transducers.

If the crystal is compressed or vibrated, it will generate an alternating voltage. This signal alternates with a frequency corresponding to the vibration of the crystal, so the device can be used to detect small motions. Probably the most spectacular of the mechanical transducers are the strain gauges. These devices depend upon the fact that the resistance of a wire changes a tiny amount when the wire is stretched. The wire (or a flat strip of a conducting metal) is arranged so that the pulling force is exerted on it, and at the same time, the wire is part of a Wheatstone bridge. Changes in resistance can be amplified and processed.

Temperature Transducers

The thermocouple, the resistance thermometer, and the thermistor are examples of temperature transducers. The thermocouple produces a DC voltage directly, while the other two instruments give variations in resistance. In either case, it is not difficult to fit the transducer into an electrical system.

Radiation Transducers

The radiation transducers respond to radiation, such as light. The photovoltaic cell is an example of semiconductor transducer device. The quanta of light can displace electrons within the crystal of this semiconductor device. With a proper arrangement of semiconducting and conducting materials, the continued displacement of electrons becomes a small current which can be measured. Other devices sensitive to light include photodiodes and phototransistors.

Output Transducers

Output transducers convert an electric signal into a nonelectrical quantity. Some of the output transducers are introduced below.

Of the many types of output transducers, the galvanometer was the earliest used. The galvanometer is an electromechanical transducer that converts electric current into a rotary deflection. If a coil of wire is suspended between the poles of a magnet, any current in the wire tends to deflect the coil transversely across the magnetic field. A typical galvanometer consists of a coil mounted on pivots surrounded by the magnet. The torque or rotating force in the meter is proportional to the current through the coil. An attached pointer moves across a scale, so current can be read directly in amperes, milliamperes, or microamperes. The same meter can be used to measure voltage if an unvarying resistor is connected in series with the coil. Current through the resistor (and the coil) depends directly on the voltage. If the resistor is built into the instrument, the meter scale can be graduated in volts. A battery (or other constant voltage source) and a meter can be used to measure resistance, once again because I = V/R.

Solenoids

A solenoid is an output transducer that uses electromagnetism to convert electrical energy into mechanical motion. The movement of the solenoid may be used to close a set of electrical contacts, cause the movement of a mechanical device, or both at the same time.

Loudspeakers

The use of loudspeakers as output transducers is advantageous for example in a complex instrumental arrangement. The operator's eyes might be too busy to watch a meter. His/her ears, then, could detect changes in volume or pitch from the loudspeakers. In some instruments loudspeakers are used chiefly for demonstration purposes, while in others the loudspeakers gives a warning when some misuse of the instrument or some other disaster is imminent.