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Syllabus reference (October 2002 version)
4.Some devices use output transducers to make connections between the device and the environment	Students learn to: explain the need for a relay when a large current is used in a device describe the role of the electromagnet, pivot, switch contacts and insulator in a relay describe the structure of light-emitting diodes (LEDs) in terms of p-type and n-type semiconductors explain why voltmeters, ammeters, CROs and other electronic meters are considered output transducers	Students: process information to explain the way in which a relay works using a circuit diagram solve problems and analyse information using circuit diagrams involving LEDs and relays analyse information to assess situations where an LED would be preferable to an ordinary light source

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edit: 21 Aug 08]

Voltmeters and ammeters convert electric currents (electrical energy) to mechanical energy (of a needle moving over a scale) or images of numbers (light) on the display of an electronic digital multimeter. The amount of movement of the needle or numbers in the display correspond to the aspect of electrical energy being measured (voltage, current, resistance, watts).
The inputs to Cathode Ray Oscilloscopes (CROs) transfer electrical energy from the circuit being monitored to control plates inside the picture tube of the CRO. The control plates deflect a very finely focused electron beam traveling from the electron gun to the front of the tube in a way that corresponds to the voltage in the circuit being monitored. The electron beam hits a screen coated with tiny particles (phosphors) that emit light when hit by electrons. The greater the deflection of the beam from the centre of the screen the higher the voltage being measured. The screen also has a visible grid printed on it. The grid is calibrated to show the voltage of the circuit being monitored. Because the electron beam sweeps across the screen, changes in the voltage against time can be observed as a visible trace on the screen.

A diagram and more information is available at DOCTRONICS Educational Publications by WD Phillips, UK.

Additional background information listing a number of output transducers:

loudspeakers (convert electrical energy into sound)
light emitting diodes or LEDs made of semiconductor material that convert electrical energy into light (the opposite of photovoltaic cells), but, unlike light bulbs they do it efficiently and produce almost no heat
electric motors (convert electrical energy into motion)
analogue galvanometers, ammeters and voltmeters (a special type of electric motor)
solenoids (convert electrical energy into movement to physically switch on or off another circuit. A solenoid is a useful way to couple a low power electronic circuit to a high energy (high currents and voltages) electric circuit containing, for example, a powerful electric motor or flood lights
aerials linked to radio and TV stations convert coded electric currents into coded electromagnetic signals that are broadcast into the atmosphere and space; magnetrons do that in microwave ovens

Semiconductor devices in electronic circuits have strict limits on the amount of current they can carry without being destroyed. A relay allows a small current in an electronic circuit to mechanically switch on a high current-carrying circuit.

The coil is well insulated from the soft iron core, armature and contacts linked to the other circuit (labeled OUT). When the current flows into the coil (IN) or solenoid, as it is sometimes called, it becomes an electromagnet. The soft iron core inside the coil concentrates the magnetic field. This magnetic field pulls the metal armature towards it (see dotted lines showing new position of armature) and brings it against the switch contacts (arrowheads) to complete another circuit (OUT). The pivot holds the armature in place and ensures a reliable connection with the switch contacts as long as the current flows in the coil. When the current in the coil is shut off, the spring pulls the armature back off the switch contacts, thus breaking the other circuit.
The other circuit (connected to OUT) may have a high voltage & high current (250 V and 10 A) to run an electric motor. Relays like this are also used to turn on the headlights in a car (12V, 15A circuits).

Some more information and diagrams of relays can be found at the Jag Lovers, USA website.

Two examples follow. You can work in reverse. Use the circuit diagram to tell the story of the two circuits.

Light emitting diodes (LEDs) are solid state devices that emit light when forward-biased. A layer of P-type and a layer of N-type semiconductors are made adjacent to each other by doping a suitable substrate semiconductor material. The diode with its connection leads is packaged in a plastic capsule which is transparent to the light frequency it is designed to produce.
Forward-biasing the diode pushes charge carriers across the P-N junction. The P-N junction emits energy in the form of light in a process called electroluminescence as they do so. The colour of the LED depends on the substrate semiconductor material and the doping chemicals used to make the P & N type semiconductor material.
Background information: A range of LED colours (frequencies), bias voltages and the composition of some semiconductors producing those colours is provided at EF Schubert, Cambridge University Press, UK

These problems are similar to ones seen in the potential (voltage) divider section. LEDs and the coil in the electromagnet behave as simple resistances in DC circuits. The LED resistance is finite and may vary slightly depending on the amount of forward bias.

LEDs are very efficient (all light and no heat) and reliable producers of light at a single frequency and they are virtually indestructible. They are used when using as little energy as possible is important and where reliability is paramount. Light output is being increased all the time and they are now used in traffic control signals and in car lighting systems to replace incandescent globes (the filaments burn out in these).