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All materials are made up of tiny particles called atoms. Atoms are made up of even smaller particles called protons, neutrons and electrons. The protons found in the atom have a positive charge and the electrons have a negative charge. These charges balance each other, giving the atom an overall neutral charge.

Electric currents

Electricity can be manifested as a flow of electrons or charges. The flow of electrons or charges is known as electric current.

An electrical circuit will have loosely held electrons all along its path. When electrical energy is provided in the circuit, an electric field is set up, causing these electrons in the circuit to flow all at once like water flowing in a pipe or hose.

Electrical circuits

Artificial sensors are based on electrical circuits. Electrical circuits are made up of specific electrical components, a power source and connecting wires, and they can switch or change an electric current. The flow of electric charges in a circuit is controlled by the electrical conductivity of the material used, the components and design of the circuit. A circuit can be designed to allow different amounts of electric charges to flow in different parts of the circuit, so parts of the circuit can have different but interacting jobs.

Electronics is the use of small components such as semiconductor devices in electrical circuits to control the flow of electric charges or perform a function. This is done by increasing or decreasing the current or by stopping the flow altogether. Most electrical devices use electronics – from a simple switch that turns on a light when it gets dark, to a complex circuit that has many jobs to do, such as that found inside washing machines or robots.

Conductivity

When electric charges flow through something, we call this electrical conduction. The substance that the electric charges are flowing through is called a conductor.

Different materials have different electrical conductivities. This is a measure of how easy it is for electric current to pass through the material. The reverse of conductivity is resistivity – how hard it is for an electric charge to move through a material.

Some materials, such as metals, have loosely held electrons in their atomic structure, which allow electric charges to flow easily and are therefore very useful as wires connecting the various components in a circuit. Copper metal is an example of a good conductor and is often used as connecting wires.

Substances that don’t allow the flow of electric charges are called insulators. For example, rubber, plastic and air are poor conductors and are therefore useful as insulators to block the flow of electric charges.

Other materials with conductive properties that are between good conductors and insulators, such as silicon, are called semiconductors. Their electrical conductivity can be altered by the type of atoms used for doping them with impurities. Semiconductor components such as diodes and transistors can change their ability to conduct charges depending on certain conditions such as voltage. This makes semiconductors useful as sensors and switches that respond to the changes in physical conditions.

For example, a heat sensor called a thermistor changes its ability to allow the flow of electric charges through it in response to temperature. By placing a thermistor in an electrical circuit, the current can be switched on or off in another part of the circuit, such as turning off a heater if the air gets too hot. Just as sensors in the human skin send impulses to the brain where the information is analysed and we feel hot or cold, in machines, electronics are used to analyse the physical conditions being sensed through changes in electric current.

How do sensors work?

Sensors react to changing physical conditions by altering their electrical properties. Thus, most artificial sensors rely on electronic systems to capture, analyse and relay information about the environment. These electronic systems rely on the same principles as electrical circuits to work, so the ability to control the flow of electrical energy is very important.

Put simply, a sensor converts stimuli such as heat, light, sound and motion into electrical signals. These signals are passed through an interface that converts them into a binary code and passes this on to a computer to be processed.

Many sensors act as a switch, controlling the flow of electric charges through the circuit. Switches are an important part of electronics as they change the state of the circuit. Components of sensors such as integrated circuits (chips), transistors and diodes all contain semiconducting material and are included in the sensor circuits so that they act as switches. For example, a transistor works by using a small electrical current in one part of the circuit to switch on a large electrical current in another part of the circuit.

Active and passive sensors

Most sensors use radiation such as light or laser, infraredradio waves or other waves such as ultrasonic waves to detect objects and changes in their environment. They can do so by having an energy source inside them that enables them to emit the radiation towards their target object. This radiation is reflected back by the object and detected by the sensor – this is called an active sensor, for example, in the use of radar.

Passive sensors don’t send out their own radiation or waves – they detect radiation that is emitted by their target objects, such as heat or thermal infrared radiation, or they detect radiation from some external source such as the Sun that is reflected off the objects. An example is a thermistor for measuring temperature electronically.

The value in using sensors is that they are non-intrusive and able to sense at a distance. Both active and passive sensors can be mounted on satellites orbiting the Earth to capture information about our environment. The radiation or waves or other physical phenomena detected by the sensors are converted into electrical signals and processed by computers.

MARVIN the robot is equipped with active sensors such as infrared sensors, ultrasound sensors and laser sensors. The counter sensor that determines the distance MARVIN travels is a passive sensor.

Nature of science

Science ideas are subject to change. Ideas about electrical conduction have changed, which led to the development of electronics.

Useful links

Learn about active and passive sensors from NASA.

    Published 2 September 2010