LEDs Revisited

LEDs are available in a number of different sizes and shapes with most being round topped cylinders that are either 3 mm or 5 mm in diameter. These sizes are also commonly referred to as T1 (3 mm) and T1 (5 mm). The body of most LEDs is made of plastic that is either the same color as the lit LED, generally referred to as a diffused LED, or of a completely transparent plastic.
LEDs Revisited - schematic

As a rule the clear LEDs are brighter than the diffused ones. The two LEDs on the left in this photo are diffused, the two on the right are clear. You can see the relative sizes of the 3mm and 5mm units, as well. Colors that are commonly available include red, orange, yellow or amber, green, blue and white. Infrared and ultraviolet LEDs are also available as are multicolored devices that can light with two or more colors. An LED`s color is frequently expressed as its wavelength. The shorter the wavelength, the closer the light is to the red end of the spectrum, the longer the wavelength the closer it is to the violet end. (Do you remember Roy G. Biv  ) A typical red LED has a wavelength of 660 nm (nanometers or billionths of a meter) Most LEDs have two metal wires extending from the back of the plastic body where they can be connected to a source of DC power. The longer lead is called the anode and is connected to the positive terminal of the power supply or battery. The shorter lead is the LED`s cathode and is connected to negative. The base of the LED generally has a flat area next to the cathode which helps with lead identification if the wires have been cut. You can also identify the cathode by looking carefully at the insides of the LED. The cathode connects to the part that contains a dish shaped lens. Note that this is much easier to identify on the clear, non-diffused LEDs. In the photo below the cathode is on the...

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