Science & Technology in Action

2nd Edition

Sensors, Motes and Environmental Monitoring

Higher Education Authority

This lesson outlines the structure and function of the potential divider and shows how it can be modifi ed to act as an environmental sensing element. The operation of a wireless network of low-cost monitoring devices is described.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

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. 

Physical sensors
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.

True or False?

  1. A temperature sensor produces heat.
  2. An LDR is a resistor whose resistance depends on how brightly it is illuminated.
  3. Remote monitoring is not necessarily wireless.
  4. A microphone measures sound.
  5. A voltage divider splits a voltage in the same ratio as the resistors of which it is composed.
  6. A bimetallic strip conducts electricity only when it is cold.
  7. Air expands when heated but metals do not.
  8. An ad hoc network is a network that is connected to the Internet.
  9. A mote contains a single-chip computer.
  10. A transducer is the same thing as a sensor.
  11. Motes were invented by Newton.
  12. Motes are used in the Botanic Gardens to ensure that the pressure in the glass houses remains constant.