Page 399 - Physics Coursebook 2015 (A level)

Chapter 25: Electronics
Electronic devices
Electronic devices are widespread in the modern world. From birth to death, electronic sensors watch over and control many of our environments. The chances of survival of the newborn baby in Figure
25.1 are improved as a sensor inside the incubator measures the temperature of the air. The output of the sensor operates heating and cooling devices which keep the temperature as constant as possible. Other sensors sound alarms if the baby’s breathing or heart rate falls to dangerous levels.
In this chapter, we will explore how some common electronic sensors work. You will use what you have learned about electricity and electric circuits in Chapters 8–12.
Figure 25.1 A newborn baby in an incubator. Sensors inside the incubator monitor and control the temperature, oxygen and humidity levels.
Components of an electronic sensing system
In its basic form, an electronic sensor may be represented as a sensing device, a processor that provides an output voltage, and an output device (Figure 25.2).
Figure 25.2 Block diagram of an electronic sensor.
The sensing device is an electronic component with
a property that changes when a physical quantity such
as temperature or light intensity alters. We have already looked at one such component, the thermistor (Chapter 11), with a resistance that changes when temperature changes. A similar component, called a light-dependent resistor (LDR), has a resistance that changes when the light level alters. This change in resistance causes the processor to produce an output voltage that drives the output device. In this way the change in a physical quantity such as resistance can be used to trigger the output device. When the air in the incubator in Figure 25.1 becomes cold, the change in resistance of the sensor causes the processor
to switch on the output device, a heater, to keep the baby warm. When it becomes too hot, the same sensing device and processor will switch the heater off.
It is important to note that we are not concerned with all the details of how a particular sensing device works or what there is inside the processor. We are only interested
in knowing the output for various inputs and how the sensor can be used in a range of useful applications. Sensors make use of many basic ideas about electric circuits: resistance and current, for example. A processor may contain many transistors, but it is not necessary
to understand how a transistor works to appreciate the function of a processor. Some detail will be given, but if you wish to delve further into how these devices work, there are many useful websites that will help you to understand how physicists have used their knowledge and inspiration to good effect.
The sensing device is sometimes called a transducer. A transducer changes energy from one form into another. A microphone is an obvious example, as it changes sound into electrical energy. However, even the thermistor used as the sensing device in the incubator in Figure 25.1 can be thought of as a transducer; a change in the internal energy of the air alters the electrical energy in the thermistor circuit.
Piezo-electric transducers
Some crystals such as quartz produce an electric field when a force is applied and the shape of the crystal changes. This is known as the piezo-electric effect. A piezo-electric crystal consists of positive and negative ions in a regular arrangement. When the crystal is stressed, a small voltage is produced between the faces of the crystal. The crystal acts as a transducer since energy is changed
sensing device
processor
output device
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