A Primer on LEDs in Electronics Design

Light emitting diodes (LEDs) have become increasingly popular as opposed to other light sources. They are durable, have a long lifespan, are energy efficient and are very flexible in their use. Here are a few introductory facts about LEDs.

LEDs are very commonly used as indicator lights on electronics. Typically, these are green, red and yellow, but other colors are occasionally used. If an indicator will only be one color such as red or green, that specific color can be used. Sometimes, the color changes – an example would be for a battery charge:  green for charged, yellow for charging and red for a very low charge. The way this is done is to use a tri-color LED. A tri-color LED actually has two LEDs in it – a red LED and a green LED. If only red or green is needed, only that LED is turned on. For yellow light, both red and green is turned on, which produces yellow.

Two of the most common LED packages are surface mount device (SMD) LEDs, which come in a variety of footprints, and radial LEDs. One of the common radial LEDs is a 5mm (T 1 3/4) which is enclosed in a plastic dome. To display them, the SMT LED usually has a light pipe that extends from the top of the LED to the case of the electronics where it is installed. This is usually seen as a see-through piece of plastic or it may be under a translucent label.  The radial LED typically is installed just below the case with its dome protruding through the case, usually seen as a curved surface.

Some of the parameters that may be important when selecting a LED are shown below. Most of the parameters not shown are not that important when selecting an LED but need to be addressed in the design.

Current draw: This is especially important if the device is a battery-powered device. Many times, the design will blink the LED which saves battery life.

Brightness (radiant intensity): On an indicator light, it’s not that important to be bright, so a LED with minimal brightness is usually selected. On a flashlight or light, brightness is much more important.

Viewing angle: A quick search on a distributor site shows viewing angles from 4? to 290?. For an indicator LED a smaller angle is fine because it will look like a point on a surface. Other applications will need more of a viewing angle – picking one will depend on where it needs to be seen from.

Wavelength: This determines what color the LED will be in the visible spectrum or even if it will be an ultraviolet (UV) LED or an infrared (IR) LED. As a sidenote, IR LEDs can’t be seen by the human eye, but some cell phone camera modes can see them (unless they have an active IR filter).

LED lightbulbs are basically one (or usually more) LEDs and some electronics that have been put in the shape of a traditional lightbulb, using the plastic casing as a diffuser. Interestingly, with LED lightbulbs, the LED itself rarely fails. When they go bad, it’s usually one of the electronic components that fail.

There are many ways to dim a LED. One of the most popular is by a method called pulse width modulation (PWM). This is just a fancy way of saying that the LED switches from ON to OFF. This is done at a frequency high enough that the human eye can’t detect that it is turning ON and OFF. To get full intensity, it is on 100% of the time, to get half intensity it pulses so that it is on 50% of the time and off 50% of the time and so on. This is another method that can be used to save battery life when needed.

Laser diodes are similar to LEDs but usually have higher power needs and a much narrower beam.

Some common uses for LEDS and laser diodes are:

• Indicator lights
• Light source
• Medical tools
• CD/DVD drives
• Rangefinders
• Used in conjunction with a light detector to sense objects
• Data communication through light pipes

There is a lot more information about LEDs and how to design and use them available. Have an electronics manufacturing project that needs LEDs? Contact us to partner on your program.

Bob Didonato
Engineering Program Manager