1. How LEDs Work

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 stations
An LED is a small lamp that uses electricity to generate light without getting hot
LEDs operate with low voltages and currents, although some can have built in electronics to allow them to be used on the mains
In 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 white
LEDs can be made that emit invisible Infrared and Ultra Violet light
LEDs are now used in cars, traffic lights, domestic and industrial lighting and many other applications
White LEDs are already more efficient than compact fluorescent lamps
Research indicates that they can still improve by a factor of 5 beyond this
The intensity of the light is proportional to the current flowing
An LED can continue to emit light for over 20 years with little change
The 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 currents
electrons
An 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 supply
An LED can be made from two semiconductors joined in this way.
Light is emitted as each electron falls down the step
BATTERY
BATTERY

9. BASIC OPERATING PRINCIPLE OF A BLUE LED BLUE LIGHT NEEDS A LARGE 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 RED LED RED LIGHT NEEDS A SMALLER 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 Violet
Red<Orange<Yellow<Green< Turquoise< Blue<Violet
LEDs can also emit invisible light
Infrared light needs a smaller step than Red light
Ultra violet light needs a larger step than Blue light
White LEDs actually emit Blue light but include a material called a PHOSPHOR that converts some of this to Red and Green light
Red + Green + Blue light is seen as White light

13. Polarity of Battery or DC Supply
For an LED to emit light
it must be correctly connected to the battery, or power supply
The voltage applied must be more than the minimum for the colour emitted
It will not emit light (and may be damaged) if the voltage is connected the wrong way round, as shown below
The circuit symbol for an LED is
The head of the “arrow” shape must always point to the negative terminal for light to be emitted
The 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?

15. Summary An LED converts electrical energy to light energy.
They can be made in many colours, including white
They already have many useful applications
They are much more efficient than filament lamps and “environmentally friendly” compact fluorescent lamps
They generate much less heat and can last for >20 years
They generate their light by electrons falling down a step.
They will become much more affordable as more of them are sold