# Lesson 5: Blinking Two LEDs

In this tutorial, we will learn the difference between current sourcing and current sinking by revisiting our LED Blink example. We will also incorporate the use of a breadboard.

We are going to build two simple LED circuits:

1. LED Circuit 1 will be the exact same as before with the LED anode facing Pin 3 and the cathode facing ground. When we drive Pin 3 HIGH (5V), the current will travel through the LED to GND. In this circuit, Pin 3 is the current source.
2. LED Circuit 2 is similar but different. Here, we’ll hook up a second LED with the anode facing away from Pin 4 (instead, towards 5V) and the cathode facing toward Pin 4. When we drive Pin 4 HIGH (5V), the LED will turn off because no voltage difference exists between the two ends of our circuit. However, if we drive Pin 4 LOW (0V), the LED will turn on. In this circuit, Pin 4 is the current sink.

Yes, this can be a bit confusing at first (“wait, the LED turns off when Pin 4 is HIGH?!?!”). But you’ll gain understanding by completing this lesson. In the animation below, pay attention to the current direction in each circuit. Notice how they’re opposite!

## Materials

Our materials are almost the same as before but this time, we are going to make two separate LED circuits (with the same components). So, we need two red LEDs and two 220Ω resistors. Now that we’re using more components, we’ll also need a breadboard—which will make it easier to make a clean, organized circuit.

Breadboard Arduino LED Resistor
Breadboard Arduino Uno, Leonardo, or similar 2 Red LEDs 2 220Ω Resistors

## Before you begin: breadboarding circuits

We will increasingly be using our breadboards in these lessons so now is a good opportunity to revisit how to use them. If you’re unfamiliar please read our breadboarding guide and watch the following video:

## Making the circuit

### Step 2: Wire up the first LED circuit

Now let’s wire up the exact same circuit as before (e.g., LED Blink and LED Fade) but this time we’ll use a breadboard. Make sure the LED anode (the long leg) is facing Pin 3.

### Step 3: Wire up the second LED circuit

Now wire up the second LED circuit. This time, however, connect the LED cathode (short leg) to Pin 4 and the resistor to the 5V rail.

## Writing the code: blinking Pins 3 and 4

Let’s write code to blink the LEDs hooked up to Pins 3 and 4.

Importantly, the Pin 3 circuit (LED Circuit 1 i) will turn on with digitalWrite(3, HIGH) whereas the Pin 4 circuit (LED Circuit 2) will turn off with digitalWrite(4, HIGH). Why? Recall that current always flows from high voltage potential to low voltage potential.

When Pin 3 is HIGH (5V), there is a voltage difference between Pin 3 and GND so current flows from Pin 3 to ground. When Pin 4 is HIGH (5V), however, there is no voltage difference across the circuit (from Pin 4 to 5V) and thus, no current. This behavior is illustrated in the animation below.

Let’s write the code!

### Step 3: Compile, upload, and run the code!

We did it! Now compile and upload the code.

And here’s a top-down video with the code window:

## Our Blink2 code on GitHub

You can access our Blink2 code in our Arduino GitHub repository. It’s also displayed below:

This source code is on GitHub.

## Next Lesson

In the next lesson, we will use a new component—an RGB LED—to output a variety of LED colors beyond just red and learn about the difference and how to use Common Anode vs. Common Cathode RGB LED designs.

This website was developed by Professor Jon E. Froehlich and the Makeability Lab using Just the Docs. If you found the website useful or use it in your teaching, we'd love to hear from you: jonf@cs.uw.edu. This website and all code is open source (website GitHub, Arduino GitHub, p5js GitHub). You can find the MakeabilityLab_Arduino_Library here. Found an error? File a GitHub Issue.