Story Horse?
LEDs are everywhere.
From the tiny status LEDs on a Raspberry Pi to massive displays in stadiums and modern smart lighting systems, LEDs have completely transformed the way we produce and use light.
If you’re building projects with Raspberry Pi, Java, Pi4J, Arduino, or electronics in general, understanding LEDs is one of the most important skills you can learn.
In this guide, we’ll explore how LEDs work, the different sizes and colors available, SMD LEDs, resistor calculations, brightness ratings, common beginner mistakes, and some fascinating facts about these amazing components.
What is an LED?
LED stands for Light Emitting Diode.
An LED is a semiconductor device that emits light when an electrical current flows through it.
Unlike traditional incandescent bulbs, which generate light by heating a filament, LEDs convert electrical energy into light much more efficiently.
Because of their efficiency, low power consumption, and long lifespan, LEDs are used in countless applications, including:
- Raspberry Pi projects
- Consumer electronics
- Automotive lighting
- Home lighting
- Traffic signals
- Industrial equipment
- Digital displays
Picture 01: Led anatomy
How Does an LED Work?
An LED is a type of diode, meaning it only allows current to flow in one direction.
Every LED has two terminals:
- Anode (+) – positive terminal
- Cathode (-) – negative terminal
When connected correctly, current flows through the LED and light is produced.
If the LED is connected backwards, it will not light up.
For traditional through-hole LEDs:
- The longer leg is usually the anode.
- The shorter leg is usually the cathode.
- The flat edge on the LED body typically indicates the cathode.
Video 01: How LED Works
Common LED Colors and Forward Voltage
Different LED colors require different forward voltages.
Table 01: Led colors
This is why resistor calculations may differ depending on the LED color you are using.
Picture 02: Led color
Common LED Sizes
The most popular LEDs used by hobbyists are through-hole LEDs.
3 mm LEDs
Small and compact.
Commonly used for:
- Status indicators
- Small electronic projects
- Low-profile designs
5 mm LEDs
The most popular LED size.
Commonly used for:
- Raspberry Pi projects
- Breadboard circuits
- Educational electronics kits
8 mm and 10 mm LEDs
Larger LEDs with greater visibility.
Commonly used for:
- Decorative lighting
- Signage
- High-visibility indicators
The size refers to the diameter of the LED's lens and primarily affects its appearance, viewing angle, and perceived brightness rather than its electrical behavior. Smaller 3 mm LEDs are compact and ideal for indicator lights, while 5 mm LEDs are the most widely used due to their balance of size and visibility. Larger 10 mm LEDs offer increased visibility and are often chosen for projects where the LED needs to be seen from a greater distance. Regardless of size, LEDs should always be used with an appropriate current-limiting resistor, and their electrical specifications should be verified using the manufacturer's datasheet.
Picture 03: Led sizes
LEDs in a variety of shapes
Picture 04: Formats
What Does SMD Mean?
SMD stands for Surface Mount Device.
Unlike traditional LEDs that have wire leads passing through a circuit board, SMD LEDs are soldered directly onto the surface of a PCB.
Because they are smaller and easier to manufacture in large quantities, SMD LEDs are found in nearly every modern electronic device.
You can find SMD LEDs in:
- Smartphones
- TVs
- Computer monitors
- Raspberry Pi boards
- LED strips
- Smart lighting systems
Popular examples include:
- 3528
- 5050
- 2835
- 5730
The numbers usually represent the physical dimensions of the LED package.
For example:
- 5050 = 5.0 mm × 5.0 mm
- 3528 = 3.5 mm × 2.8 mm
Picture 05: SMD
LED Brightness: Lumens, Millicandela and Viewing Angle
One of the most confusing LED specifications for beginners is brightness.
Lumens (lm)
Lumens measure the total amount of visible light produced by a light source.
The higher the lumen rating, the more light the LED produces overall.
Millicandela (mcd)
Millicandela measures light intensity in a specific direction.
A high mcd value does not always mean the LED is brighter overall.
Sometimes it simply means the light is concentrated into a narrow beam.
Viewing Angle
The viewing angle describes how widely the light is distributed.
Table 02: Viewing Angle
A narrow-angle LED often appears brighter because the light is concentrated into a smaller area.
Clear vs Diffused LEDs
In addition to different sizes, LEDs are also available with different lens finishes, the most common being diffused and clear (transparent). Diffused LEDs have a frosted lens that scatters light in multiple directions, producing a softer and more uniform glow that is visible from a wider viewing angle. Clear LEDs, on the other hand, use a transparent lens that focuses the light more directly, often making them appear brighter when viewed head-on. Because of this focused beam, clear LEDs are commonly used when maximum brightness or visibility is desired, while diffused LEDs are often preferred for indicators and projects where an even distribution of light creates a more pleasing visual effect. The choice between the two is usually based on the desired appearance rather than electrical characteristics, as both types are connected and controlled in the same way.
Clear LEDs
Characteristics:
- Transparent lens
- Focused beam
- Higher apparent brightness
- Better for long-distance visibility
Diffused LEDs
Characteristics:
- Frosted lens
- Softer light output
- Wider distribution
- Better for indicators and decorative projects
For Raspberry Pi projects, diffused LEDs are often easier on the eyes.
RGB LEDs
RGB LEDs combine three colors in a single package:
- Red
- Green
- Blue
By controlling the brightness of each color, thousands of color combinations can be created.
RGB LEDs are commonly used in:
- Decorative lighting
- Gaming setups
- Smart home projects
- Raspberry Pi projects
Examples include:
- Standard RGB LEDs
- WS2812B LEDs
- NeoPixels
- SK6812 LEDs
Picture 06: Led types
Why Do LEDs Need Resistors?
One of the most common beginner mistakes is connecting an LED directly to a power source.
Without a resistor, too much current can flow through the LED, causing permanent damage.
A resistor limits the current to a safe value.
The resistor value can be calculated using:
R = (Vsupply − Vled) ÷ I
Where:
- Vsupply = power source voltage
- Vled = LED forward voltage
- I = desired current
For example:
- Supply Voltage = 5V
- Red LED Voltage = 2V
- Current = 20mA
Result:
150 Ω resistor
In practice, hobbyists often use:
- 220 Ω
- 330 Ω
Both values work well for most Raspberry Pi LED projects.
Picture 07: Resistor
Video 02: LED Circuit Design
Recommended Resistors for Raspberry Pi
Since Raspberry Pi GPIO pins operate at 3.3V, a 220 Ω resistor is often a safe and simple choice.
Table 03: Resistors
Always verify the specifications of your LED before connecting it.
LED Polarity Tricks
Not sure which side is positive or negative?
Here are some easy ways to identify the cathode:
- The shorter leg is usually the cathode
- The flat edge indicates the cathode
- The larger internal metal piece is usually connected to the cathode
Always double-check before powering a circuit.
Why Do LEDs Come in Different Colors?
Many people assume the color comes from a colored plastic lens.
In reality, the color is determined by the semiconductor material used inside the LED.
Table 04: colors
This is why different LED colors require different forward
voltages.
Infrared and Ultraviolet LEDs
Not all LEDs produce visible light.
Infrared LEDs (IR)
Infrared LEDs emit light outside the visible spectrum.
Common applications:
- TV remote controls
- Security systems
- Motion sensors
- Communication devices
Ultraviolet LEDs (UV)
UV LEDs emit ultraviolet light.
Common applications:
- Currency validation
- Sterilization equipment
- Resin curing
- Scientific instruments
Connecting Multiple LEDs
When building larger projects, you may need multiple LEDs.
Series Connection
LEDs are connected one after another.
Advantages:
- Same current flows through all LEDs
- Efficient use of power
Disadvantages:
- If one LED fails, the entire chain may stop working
Parallel Connection
LEDs are connected side by side.
Advantages:
- Individual LEDs operate independently
Disadvantages:
- Current balancing becomes important
- Each branch may require its own resistor
For many projects, series connections are preferred when possible.
Picture 08: Series & Parallel
How Much Power Does an LED Consume?
LED power consumption depends on the type of LED.
Table 05: power consume
Modern LEDs are often much more efficient than older designs.
LEDs and Raspberry Pi GPIO Limits
When using LEDs with a Raspberry Pi, it is important to understand GPIO limitations.
GPIO pins:
- Operate at 3.3V
- Can only provide limited current
- Should not drive large loads directly
For larger LED arrays, LED strips, or high-power LEDs, use:
- Transistors
- MOSFETs
- Dedicated drivers
This protects both the Raspberry Pi and your LEDs.
Picture 09: Raspberry PI
Special Types of LEDs
Not all LEDs are simple indicator lights.
Flashing LEDs
These LEDs contain an internal circuit that automatically flashes the light.
Bi-Color LEDs
Contain two colors inside a single package.
Examples:
- Red/Green
- Red/Yellow
RGB LEDs
Contain three LEDs in one package and can create millions of colors.
Picture 10: RGB
Addressable LEDs
Each LED can be controlled individually.
Examples:
- WS2812B
- NeoPixel
- SK6812
These are perfect for animations and visual effects.
High-Power LEDs
Standard LEDs typically consume around 20mA.
High-power LEDs can operate at:
- 1W
- 3W
- 5W
- 10W
- 50W or more
These LEDs often require:
- Heat sinks
- Constant-current drivers
- Thermal management
Applications include:
- Floodlights
- Automotive lighting
- Industrial illumination
The Evolution of LEDs
LED technology has evolved dramatically over the last few decades.
1962
The first practical red LED was developed.
1970s
Yellow and green LEDs became available.
1990s
Efficient blue LEDs were invented.
2000s
White LED lighting became commercially viable.
Today
Smart RGB LEDs, addressable LED strips, and highly efficient lighting systems dominate the market.
LED Applications You Use Every Day
You probably interact with LEDs hundreds of times per day.
LEDs can be found in:
- TVs
- Monitors
- Smartphones
- Cars
- Traffic lights
- Routers
- Raspberry Pi boards
- Keyboards
- Smart watches
- Home appliances
Modern life would look very different without LEDs.
LED Myth Busting
Myth: All LEDs Need 20mA
False.
Many modern LEDs operate perfectly at lower currents.
Myth: Bigger LEDs Are Always Brighter
False.
Brightness depends on efficiency, design, and viewing angle.
Myth: LEDs Never Burn Out
False.
LEDs gradually degrade over time and eventually lose brightness.
Fun LED Facts
LEDs Can Last Over 50,000 Hours
A quality LED can operate continuously for years before failing.
Blue LEDs Changed the World
The invention of efficient blue LEDs made modern white LED lighting possible.
This breakthrough earned the inventors the 2014 Nobel Prize in Physics.
Video 03: Why It Was Almost Impossible to Make the Blue LED
LEDs Are Extremely Efficient
Modern LEDs convert much more energy into light than traditional incandescent bulbs.
This results in:
- Lower power consumption
- Less heat generation
- Longer lifespan
LEDs Can Also Detect Light
A little-known fact is that LEDs can act as primitive light sensors under certain conditions.
While not as effective as photodiodes, they can detect changes in light levels.
Common Beginner Mistakes
When starting with LEDs, avoid these common errors:
- Connecting LEDs backwards
- Forgetting the resistor
- Using the wrong resistor value
- Exceeding Raspberry Pi GPIO current limits
- Miswiring RGB LEDs
- Assuming all LEDs have the same voltage requirements
Picture 11: Mistakes
What’s Next?
To help you continue experimenting, all of the code from this tutorial is available as part of the Pi4J LED Playground project, a collection of LED-based examples designed to demonstrate different GPIO concepts and Pi4J features.
The project serves as a hands-on playground where you can explore progressively more advanced scenarios, including different blinking patterns, multiple LEDs, traffic light simulations, animations, and other creative ways of interacting with Raspberry Pi hardware.
Whether you're new to physical computing or looking to deepen your understanding of Pi4J, the playground provides a practical environment for learning through experimentation.
You can find the project on GitHub: Pi4J LED Playground
Feel free to clone the repository, modify the examples, and create your own LED effects. The best way to learn GPIO programming is by experimenting, and this project is designed to be a starting point for exactly that journey.
Stay tuned for the upcoming Pi4J LED Playground project series.
Links
https://en.wikipedia.org/wiki/Light-emitting_diode
https://en.wikipedia.org/wiki/LED_circuit
https://www.digikey.ie/en/resources/conversion-calculators/conversion-calculator-led-series-resistor
https://www.omnicalculator.com/physics/led
https://uk.rs-online.com/web/content/discovery/tools-and-calculators/led-resistor-calculator
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