Practical Guide to LEDs 1 - Pick Your LED!

You've heard about LEDs. Chances are you've already tinkered with them. But there are so much details you probably don't know about. Sadly the resources available are often incomplete or just unpractical. This guide takes you all the way from a beginner level to adept skills!

This is chapter 1 of a short series. Use the table of contents below to browse the content I've already published.

In this chapter we'll focus on how to choose the right LED for your project. Along the way you'll learn a few nifty things that'll come in handy later on.

Chapters:

Step 1: Brief Introduction to LEDs

LEDs are everywhere, and not without reason. Compared to traditional light sources they are highly efficient, resulting in less power consumption and less heat for the same brightness. They are available in all sizes, whether it be a tiny package suitable for mobile devices or in large clusters to illuminate rooms. Instead of fading on and off like lightbulbs they can be switched instantly, making them suitable for displays and precise brightness control. Powered correctly, they can last for decades without replacing.

All those benefits come at the cost of increased complexity of the circuit. The term "LED" is short for "Light Emitting Diode" and as all semiconductor based components they show highly non-linear characteristics. This means you can not treat an LED like an ordinary lightbulb and connect it straight to a power supply!

The good news: The circuit required is not that complicated either.

The circuit you will build must be tailored to the LED it is supposed to drive, so it is wise to pick first which LED is best suited for your project.

Step 2: Polarity

LEDs, like any other diode, allow current flow only in one direction. This is why you need to pay close attention to the polarity of every LED. The anode needs to be connected to positive (+), the cathode to negative (-) potential. There are several ways of identifying the polarity (follow in order):

Check the datasheet, if you have any
The cathode has a shorter pin
The case at the side of the cathode is flat
There's a '+' or '-' mark somewhere close to a pin
The cathode is usually the bigger metal part inside of the LED for manufacturing reasons

Very small LEDs have other forms of marks, such as tiny dots, printing on the bottom or asymmetric cases. Unfortunately this is not standardized so you have to read the datasheet.

If you are still not sure you can always connect a CR2032 battery directly to it and try it out! In fact this is a popular way to build simple projects such as LED Throwies. This works because a CR2032 can not provide enough power to destroy an LED.

Step 3: Properties & Units

For your design you may require to pick specific LEDs and develop a circuit for them. Datasheets often contain a ton of information so it can get hard to pick out the most important bits. Here's a short overview about the key parameters:

Name	Symbol	Unit	Explanation (Summary)
forward voltage	U_F	V	typical voltage across the LED when I_F flow through it
forward current	I_F	A	recommended max. current through the LED
peak current	I_PEAK	A	maximum current for a fraction of a second
reverse voltage	U_R	V	maximum voltage when connected in reverse without damage
viewing angle	φ	°	the width of the light beam
luminous intensity	I_v	mcd	brightness of the brightest spot
luminous flux	F	lm (lumen)	amount of light produced
dominant wavelength	λ_D	nm	measure for the dominant color of the light
color temperature		K	indicates the 'warmness' of a white LED
color rendering index	CRI		indicates how natural colors of illuminated objects are

Step 4: Colors

To get colored light with traditional bulbs a color filter (such as dyed glass) is required.

LEDs don't need such a filter, they produce only one wavelength (= color) in the first place. The color depends on the type of semiconductor used. Different semiconductors have also different electric properties, most notably the forward voltage.

The forward voltage can vary a lot, so it's always best to look in the datasheet for the exact values. When no data about the LED is available, the forward voltage can be approximated with the following table.

Color	Typ. forward voltage
IR	1.0-1.6V
Red	1.7-2.2V
Yellow	2.0-2.3V
Green	2.1-2.4V
Bright Green	3.2-3.8V
Blue	3.2-3.8V
White	3.3-3.8V
UV	3.3-4.0V

You should never connect an LED directly to a voltage source, even if the voltage is identical to the LEDs forward voltage. Read "Chapter 2: Essential Ciruits" to learn how to connect an LED properly.

Step 5: Appearances

LEDs come in a huge variety of styles. We'll take a look at the most common features:

Mounting Types
There are three common way in which LEDs can be mounted:

Through Hole (THT)
Surface Mounted (SMD)
Screw mounted or thermal adhesive

THT components might be what you are most familiar with: Their pins go through holes in a PCB and are soldered on the other side.
SMD parts are soldered directly to the surface of a PCB. With this method they can be made much smaller than THT parts and are perfect for tiny projects, such as wearables. On the flipside they are more difficult to solder.
Some LEDs, especially power LEDs are mounted on an aluminum or ceramic substrate to improve the cooling. They are meant to be mounted with either screws or a thermal adhesive.

Optics
The LED chip itself produces a wide beam of light (about 120°). To make it suitable for spotlights a clear dome is placed right above the chip to act as a lens. Some LEDs, so called diffused LEDs, don't have any beam at all, they evenly scatter the light in all directions. This is very useful for indication LEDs as the light is visible from any direction.

Multi-Color/ RGB LEDs
You may have seen LEDs with multiple chips for different colors into one case. With such LEDs you can not only select the color you want, but also mix the light to get any color in between! We'll come back to this in "Chapter 3: Switching & Dimming".
Often it is not possible to have two dedicated pins for each LED chip, in such case either the anodes or cathodes are connected internally to only one common pin.

LED Matrix
Similar to Multi-Color LEDs these contain multiple LEDs in one package, but usually all the same color. They are also available as both, common anode and common cathode configurations. They can be used to display text or numbers and are readable from quite afar. However the high LED count usually requires a micro-controller and multiplexing. In "Chapter 4: Matrix & Multiplexing" we'll dig deeper into this.

7 Segment Display
While numbers can be displayed with a simple LED matrix, a seven segment display provides a much more elegant solution with way less LEDs you need to control. Alphanumeric displays are based on the same idea, but with a total of 14 or 16 segments they can show in addition all uppercase English letters. Electrically this kind of display is identical to the common LED matrix.

Integrated circuit
No matter the light effect you want, with some effort everything is possible. For the most common effects, such as blinking or color cycling, you don't even need to invest that much time. Some LEDs come equipped with a tiny circuit to do just that.
A very special LED is the WS2812/ WS2812B, sold as "Neopixel" by Adafruit. The chip inside takes serial data from a micro-controller (such as an arduino) and controls the RGB LED to match that!

No matter which LED you decide to get, check the datasheet briefly, so you don't miss anything important.

Step 6: Power Rating

The most common LEDs are made to handle around 20mA. Depending on their purpose LED with different current requirements are also available.

Low current LEDs are specifically made for battery powered devices, such as wearables. They are made to be highly efficient, even at 2mA they are still fairly bright. Be aware that even slightly larger currents might damage them.

When normal LEDs can't provide the amount of light you need, consider stepping up to high power LEDs. The very common 1W LED delivers as much light as 20 5mm THT LEDs at a fraction of the price. If that's not enough there are modules with dozens of LEDs in one package rated at a up to 100W total!
All that light comes at a cost, though: It will get hot. So hot that it will damage the LEDs (and burn your fingers). An appropriate heatsink is required for cooling . Be sure to get LEDs pre-soldered on an aluminum PCB for easy mounting. Driving the LED gets also more complicated. The circuit must not only be rated for higher currents and voltages, but it also often requires its own cooling. The whole 'Chapter 5: High Power & Lighting' will be dedicated to this topic.

Quite often you'll see circuits which exceeding the LEDs current rating to get a little more light. While this might work well for some time it will reduce the LEDs efficiency, reliability and lifetime. Gross overloads might result in a "burnt" LED as shown in the comparison picture above. The picture was taken from this instructable, thanks @openproducts for letting me use it!