Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Many domestic devices contain sensors—devices that detect some change and usually trigger some result. When you open the door of a refrigerator a lamp lights automatically. In this case the sensor is a simple switch that is located on the hinge side of the door. When the water in a jug kettle comes to the boil the kettle switches off automatically. When a temperature sensor located in the handle is heated by the steam it switches off the electricity. In this lesson we will examine the basic structure and function of electronic sensors and how they can be used for remote monitoring.
The operational component of the switch in a jug kettle and of the thermostats used in many electric heaters and cookers is abimetallic strip. Metals expand when they are heated but not all by the same amount. A bimetallic strip is composed of two different metals bonded together along their length. When it is heated it bends and when it cools it returns to its original shape. When the bimetallic strip moves it can physically push a switch, changing its state.
Many sensors in common use produce some physical movement in response to an environmental change in position (door alarm), temperature (thermostat), pressure (washing machine) orhumidity (tumble dryer). Increasingly however, environmental sensors are electronic; they provide a range of outputs (not just on/off) and are more easily linked to data-monitoring and recording systems.
The potential divider
The operational principle of many electronic sensors is thepotential divider (voltage divider). Essentially this consists of two resistors in series; the electric potential (voltage) applied across the pair is halved if the two resistors are exactly equal. Unequal resistors will split the voltage unequally.
The voltage across any of the resistors is a fraction of the voltage supplied to the pair. For example, if the resistance of one of the resistors is a tenth of the total resistance then the voltage across it will be a tenth of the total voltage. This can be expressed mathematically as follows: V2/V = R2/(R1+R2) or V2 = (V x R2)/(R1+R2) where V is the total voltage.