The light emitting diode (LED) comes in a wide variety of sizes, shapes, and colors, and is something every ham should be familiar with.  There are a number of exam questions related to LEDs.

LEDs are more common than you may realize.  Not only used on ham radio equipment, they are typically found on most electronic gadgets, primarily as indicator lights, most often as power or status indicators.

Developed in the early 1960s, visible LEDs became practical replacements for miniature incandescent lamps in the 1980s.  Their main advantage is in efficiency (wasting little power), but they also last many years and illuminate instantly, all compared to incandescent light bulbs.

The above question phased out of the General class exam pool in 2019 but it speaks to the benefits of LEDs over other technology.  This example from the Tech license exam pool shows an LED being used in a traditional power supply circuit as a power indicator:

Useful as more than just on/off indicators, LEDs, when grouped, really open up interesting applications.  When formed as bars in a figure 8 arrangement, the classic seven segment display is created:

This revolutionized electronics to display numbers quickly and inexpensively and were an early use of the technology starting in the 1970s.

Also, when different color LEDs are bunched together their combined light can form a different color, often to achieve white.  This is the principle of LED back lighting for flat panel displays and for the recent adoption for home and general lighting (replacing light bulbs).

White LEDs (or a blend of other colors to make white) weren’t practical until the mid-1990s, and it took years to become economical.  The widespread use of LEDs for display back lighting and general lighting is now less than 15 years old.

For those interested in the technology, we’ll dive into some of the details of LEDs now.  As the name implies, an LED is a form of diode, the simplest possible semiconductor device formed by a P-N junction.  This makes it a polarized component, having a positive (+) terminal (anode) and a negative (-) one (cathode).  With diodes current flows only when voltage is applied accordingly; we say this is forward biased.

A simple schematic of an active LED circuit is shown below:

Every diode has a nominal voltage drop across it in the forward biased condition and LEDs are no exception.  LEDs typically have a forward voltage (Vf) of 2 to 3.5V, depending on the particular device (usually varies with color and intensity).  The power supply must provide at least the minimum voltage to get the LED to illuminate and then the brightness can be adjusted by increasing within the device limits (max Vf).

LEDs also have a nominal (If) and maximum forward current (max If), typically 20-30mA for common indicator types.  Lacking a current controlled source, many users simply use current limiting resistors with the LED as shown in the schematic above.  Exceeding max If  by much will usually destroy an LED.

LEDs are designed to radiate photons at a certain spectral wavelength which yields different colors.  This is determined by the semiconducting junction and device construction.  You can find LEDs which emit light from UV to infrared and all the colors in between.

Brightness of each LED also depends on device composition, and viewing angle differs with lens type.  Many specifications with LEDs to consider; they are not quite as simple as you might think.

High power LEDs used for flashlights, room and street lighting are a popular new application of the technology.  These run much hotter at higher currents and typically use current controlled circuits and require heat sinks to dissipate the waste energy (as efficient as they are, it still adds up).

If you’re not already experienced with LEDs, buy an assortment of them and some resistors to play with after learning the basics.  It is good to know how this fundamental and common electronics component works.

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Interesting and Useful Web Links

How Light Emitting Diodes (LEDs) Work – howstuffworks

What is an LED? – LEDs Magazine

Light-Emitting Diodes -sparkfun

LEDs and OLEDs – Edison Tech Center

LED Basics – YouTube video