2 LEDs (Light Emitting Diodes) “New Lamps for Old”Most lamps emit light by using an electric current to heat a filament wire to a high temperature (white hot)This is very inefficient, so to generate the electricity needed a relatively large amount of carbon dioxide is released into the atmosphere from power stationsAn LED is a small lamp that uses electricity to generate light without getting hotLEDs operate with low voltages and currents, although some can have built in electronics to allow them to be used on the mainsIn an LED an electric current can generate light very efficiently, so that much less energy is consumed and less greenhouse gases are released
3 LEDs (Light Emitting Diodes) “New Lamps for Old”LEDs are available in many colours, including whiteLEDs can be made that emit invisible Infrared and Ultra Violet lightLEDs are now used in cars, traffic lights, domestic and industrial lighting and many other applicationsWhite LEDs are already more efficient than compact fluorescent lampsResearch indicates that they can still improve by a factor of 5 beyond thisThe intensity of the light is proportional to the current flowingAn LED can continue to emit light for over 20 years with little changeThe light from an LED can follow changes in the current very rapidly, up to millions of times a second (because they don’t need to heat up and cool down)As their numbers grow, then their price will fall substantially
4 Electric currentselectronsAn electric current is due to the flow of billions upon billions upon billions of tiny particles called electrons.Electrons flow from a region of low (or negative) voltage to one with a higher (or positive) voltage (this can be created by a battery).
5 Electrons Meet an electron. It is part of a vast family where everyone looks identical.Each electron is a tiny particle that causes electricity.Electrons are found in every material.Like most family members electrons need their own space and repel other family members if they get too close.
6 Electrons and conductors and insulators In an electrical conductor electrons are free to move about easily. Metals are electrical conductors.In an electrical insulator electrons are not able to move about easily. Materials such as gases, plastics, rubber and glass are common electrical insulators.
7 Electrons and semiconductors Some electrons live in materials called semiconductors.A semiconductor is a funny material where some electrons may have enough energy to travel about freely, but only if they are warm enough, or in a bright light.As its name suggests, a semiconductor has properties in between those of an electrical conductor and an electrical insulator.
8 Electrons and semiconductors When two semiconductors are joined, they can form a step.For the electrons to flow around the circuit they must gain energy to flow through the wire to the top of the step.This energy can come from a battery or power supplyAn LED can be made from two semiconductors joined in this way.Light is emitted as each electron falls down the stepBATTERYBATTERY
9 BASIC OPERATING PRINCIPLE OF A RED LED RED LIGHT NEEDS A SMALL STEP -LED+LIGHT
10 - BASIC OPERATING PRINCIPLE OF A GREEN LED LED + LIGHT GREEN LIGHT NEEDS A MEDIUM STEP-LED+LIGHT
11 BASIC OPERATING PRINCIPLE OF A BLUE LED BLUE LIGHT NEEDS A LARGE STEP -LED+LIGHT
12 Other types of LED LEDs can emit many different colours of light The height of the step required increases as the colour of the light moves through the spectrum from Red to Blue and VioletRed<Orange<Yellow<Green< Turquoise< Blue<VioletLEDs can also emit invisible lightInfrared light needs a smaller step than Red lightUltra violet light needs a larger step than Blue lightWhite LEDs actually emit Blue light but include a material called a PHOSPHOR that converts some of this to Red and Green lightRed + Green + Blue light is seen as White light
13 Polarity of Battery or DC Supply For an LED to emit lightit must be correctly connected to the battery, or power supplyThe voltage applied must be more than the minimum for the colour emittedIt will not emit light (and may be damaged) if the voltage is connected the wrong way round, as shown belowThe circuit symbol for an LED isThe head of the “arrow” shape must always point to the negative terminal for light to be emittedThe long lead on an LED should be connected to the positive terminal of the supply +In order to limit the current, a resistor is always used in series with an LED to prevent it overheating ( ) although this may be built into some lamps
14 What are LEDs used for? • White LEDs are used in torches, as backlights forLCDs, and as roomlights, underwater lights and vehicleheadlightsColouredare used in indicatorlights, in traffic lights, tail/signal lightsin vehicles and decorative lightsare also used indigital scanners and autofocuslights in digital camerasThey are also used in computer miceand simple displaysInfraredare used in remotecontrols, webcams, sensors & securitycamerasUltravioletare used to revealsecret signatures and securitymarkings on property & banknotesMany other things
15 Summary An LED converts electrical energy to light energy. They can be made in many colours, including whiteThey already have many useful applicationsThey are much more efficient than filament lamps and “environmentally friendly” compact fluorescent lampsThey generate much less heat and can last for >20 yearsThey generate their light by electrons falling down a step.They will become much more affordable as more of them are sold