Light Emitting Diodes, How Do They Work?
Light-emitting diodes were first introduced in 1962, but at the time were very expensive and impractical for most uses.
A light-emitting diode is a semiconductor component that emits light when a small current is allowed to flow through it. Light is created when the electrons excite the chemical compound inside it. The chemical compound combined with the amount of energy required to produce the light is what determines the colour.
Red was the first colour of LEDs available, followed shortly after by green and yellow. Blue came quite a bit later. Now, you can get orange and even pink ones.
Most recently, bright white became available, which in turn brought in a whole new source of lighting. Now, the LED is no longer just used as an indicator, but a popular source of artificial light, replacing traditional light bulbs.
Light-emitting diodes are now mass-produced and can cost as little as 20 cents each. The price of an LED will vary depending on its physical size and its colour output. Common colours such as red and green are less expensive than pink and orange.
Types of LEDs
Not only are LEDs available in different colours, but also in different types, lens types and physical sizes.
The most common sizes for LEDs are 3 mm, 5 mm, and 10 mm. There are also 8 mm ones, but they are rare. The size used most often is 5 mm.
There are two types of lenses used on LEDs. A diffused lens distributes the colour more evenly. Since a diffused lens is the same colour as the output, you can see the colour when the LED is not illuminated There are also clear lenses. With this type of lens, you need to look directly at the LED to experience the full illumination, Also, you can not tell what colour is going to be displayed until the LED is powered on.
Some have multiple LEDs built into one housing. These can be either two or three colours that are either steady or cycling.
Common cathode/common anode: These have multiple connectors and each one will illuminate a specific steady colour of the LED. The most common are red/green, red/blue, and RGB. There is no difference between a common cathode and a common anode in its output but the connection process is different.
Polarity-reversible: Usually an LED will only work if connected one way. The positive is indicated with a longer lead. However, this is not true with a polarity-reversible LED. These are simply two different colour LEDs in one housing but each connecting in the opposite polarity from the other. Simply turning the LED around and connecting it "backwards" will produce a different colour than previous one. Usually, these are available in red/green.
Colour-cycling: These are generally always RGB, but will cycle through many different colours as one, two, or all three LEDs in the housing will illuminate at any one time. You can get ones that cycle through the spectrum quickly or ones that slowly dissolve from one colour to the next.
Blinking: Blinking LEDs have a tiny circuit in them to make them blink. Generally, these are only available in a single colour, but there are a few that blink alternate colours.
Arrays and Panels
Other types of LEDs are arrays and panels.
Segment array: This type of LED contains several LEDs on a chassis. These are commonly used to display alpha-numeric characters. These generally only come in red, but there are other colours available such as blue and white. 7-segment and 28-segment arrays are the most common. These types of LEDs need a controller circuit.
Matrix panel: These are a series of LED dots used to display alpha-numeric and special characters. They are commonly available in red, blue, and green, but are also known to be available in white and orange as well. These matrix panels are commonly used as bus destination signs and road signs. These matrix panel LEDs require a microprocessor to work properly.
The colour of the LED determines the voltage it needs to operate. LEDs operate on voltages anywhere from 1.5 volts up to 3.5 volts. Red ones will begin to illuminate at about 1.5 volts, while the bright white ones won't start to light up until at least 3 volts. The cycling LEDs are designed to run on a single voltage despite having multiple colours. These generally start working at about 2.8 volts.
Almost all LEDs have a current draw of about 20 mA, but they are a current-hog. Meaning that they can not be connected directly to a power supply as the LED will try to draw as much current as possible, much more than 20 mA, to the point it will destroy iself. Therefore, an LED must be only connected as part of a circuit or in series with a resistor.
These are the basics of light-emitting diodes.
Look for more on electronics projects and components in future articles from "The Electronics Guy".
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