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But LED's are different colours because of the casing, so how can RGB LED's be different colours with a clear casing?
Some LEDs are tinted but that's only to make it easy to see what colour they are when unpowered. For the tint to act as a filter, this would require white light to be emitted, which requires a lot of energy since white light is made up of a mixture of every wavelength in the visible spectrum. It's only recently (since around 2005 I think) that we've been able to make blue LEDs, since blue light is high energy, it's easier to produce light of lower energy (greater wavelength) which is why if you look at old products that contain LEDs they are always red, which is the lowest energy light in the visible spectrum. Later we got green LEDs and that's when you started seeing LEDs that could emit red/orange/yellow/green light, this works by having two LEDs in the same package, one red, one green, and varying the intensity of each to replicate different colours. The same trick is used to replicate white light (and hence any colour) by adding a blue LED to the package.
So why did we have to wait so long to get red green and blue LEDs? Well, this comes down to way LEDs produce light. Unlike an incandescent bulb (those near-extinct bulbs with the thin curly wire filament between two prongs), which emits more and more of the EM (electromagnetic) spectrum as it heats up, LEDs only emit a specific wavelength of light, which is much more cost-effective and produces far less wasted light in wavelengths we can't even see.
The wavelength of light emitted by an LED depends on (stay with me here) the chemical composition of its doped semiconductor layers.
What is a semiconductor? (skip this paragraph if you already know) A semiconductor is a material that, as the name suggests, lies somewhere between a conductor and an insulator, in terms of electrical conductivity. But how can something "sort of" conduct electricity? To understand this we need to understand why some materials conduct electricity at all while others don't. The conduction of electricity is essentially the free movement of electrons; for an electron to move to another atom it requires a certain amount of energy (see Fermi level). In metals, electrons are already sat around this energy level so it's easy for them to leave their atom. In electrical insulators, electrons sit at energy levels far below the required energy, making the jump up to the conduction band would take an enormous amount of energy. So you've probably guessed that for semi-conductors the gap is much smaller, it wont conduct electricity unless enough energy is given to its electrons to make the jump. Transistors exploit this feature of semiconductors to allow current to be stopped or allowed through at will.
Quick explanation of transistors. (Not strictly necessary to understand LEDs but transistors are what really started this all so it helps to understand them.) The three pins on a transistor are called Base, Collector and Emitter, the collector and emitter pins are at either end of the pathway through the transistor, but a semiconductor bridges the gap. Current coming from the collector alone is not enough to make it over the gap to the emitter, current delivered to the base is what charges up the semiconductor bridge, giving it enough energy to allow electrons to pass when they come through from the collector.
Back to LEDs. I said the colour of the light emitted by an LED depends on how its semiconductor layers are doped. Doping is when you a little bit of one element into a material made of another element to change its behaviour—like how putting salt on your chips changes their taste—doping semiconductors with different elements can change the size of that energy gap we talked about earlier.
It is the electrons themselves that produce the light, or rather they emit photons when they lose energy, and the wavelength of light emitted depends on the energy of the photon, and the energy of the photon depends on the energy lost by the electron. Electrons must lose energy to drop into lower energy levels within an atom, electrons like to drop as low as they can so they quite happily emit photons to shed energy. The trick is finding the right doping balance to train the electrons to emit the wavelength of light that we want. That's why it took so long to get those shorter wavelength (higher energy) colour LEDs, because it's difficult to find doping that trains electrons to emit such high energy photons instead of lower energy ones.
This is a really nice, understandable post on this topic. Thanks!
how to make multi color led ?
A colour changing LED isn't one LED in a package but three LEDs along with a small computer to drive them. The LED is made up of red, green and blue LEDs each of which can be controlled by a microcontroller. Since the two legs on the LED that supply the power are connected to the microcontroller and not the LED elements a current limit resistor is not required.
The microcontroller is able to turn each of the colours on or off, so if the red LED is turned on then the output from the colour changing LED is red. When the blue LED is turned on it is blue, if both the blue and red LEDs are turned on then the colour changing LED is a shade of purple (called magenta). Similarly combining red with green gives yellow and blue & green gives cyan.
Although the colour changing LED uses the six colours mentioned above, it slowly changes from one to another. This is still done using the three basic red, green & blue elements. If the red LED is combined with the blue LED, but the blue LED is only driven at 50% of its normal brightness then a colour half way between red and magenta is generated.
Whilst the red LED is left turned on, if the blue LED is slowly taken from 0% brightness to 100% brightness then the colour will gradually change from red to magenta.
If a standard LED is turned on and off very quickly, say 100 times every second then as far as the human eye is concerned it looks like it is constantly on. If the amount of time the LED is on for is the same as the time it is off for then it will be on for 50% of the time and 50% of its full brightness.
This same method can be done with the three LED elements inside the colour changing LED. This means it is possible to combine any amount of the red, green and blue to give the desired colour. Looking once again at the change from red to magenta, if the blue LED starts mainly turned off, goes to being on and off in even amounts and then to mainly being on.
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Posted:Apr 20, 2010
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