Maker Advent Calendar Day #2: Let’s Get Blinky!

Welcome to day two of your 12 Projects of Codemas Advent Calendar. Today we’ll be playing with one of the most popular components for makers - LEDs!

LEDs (Light Emitting Diodes) are a staple component for makers of all experience levels, handy for indication, feedback or just for a fun bit of blinky!

LEDs come in all sorts of shapes, sizes and ratings, and are everywhere in our daily lives – in your PC, your home, car dashboard, vending machines, spaceships and more.

Let's go ho ho!

Box #2 Contents

In this box you will find:

• 1x 5mm Red LED (with a clear lens)
• 1x 5mm Amber LED (with a clear lens)
• 1x 5mm Green LED (with a clear lens)
• 3x 330 ohm resistors
• 4x Male to male jumper wires

Today’s Projects

Today we’ll be programming LEDs to light up, flash and display sequences, building on yesterday’s code using GPIO pins to control them.

We'll also be learning some new functions in MicroPython to introduce some new elements to your code. The box includes three different colours to allow us to have some fun making light patterns.

What is an LED?

An LED is a Light Emitting Diode. These components emit light when electrical current flows through them.

Our LEDs have two legs, one longer than the other, as they have a specific polarity - which means that electrical current can only flow in one direction (and if not, the LEDs can be damaged!).

The long leg is the Anode (+) and the short leg is the Cathode (-). Current must always flow from the anode to the cathode, so be sure to read our wiring instructions below carefully to make sure you get this bit right.

Resistors

LEDs usually require a current-limiting resistor when using them with a microcontroller.

Resistors limit the amount of current that can pass through a circuit, which helps avoid the LED trying to pull more current than our Pico's GPIO pins can safely provide, and equally protects the LED from too much current.

It doesn't matter which side of the LED the resistor sits, as long as it's there limiting the flow of current for the entire circuit (take a look at the comment section for an example).

Raspberry Pi Pico Pin Map

Here’s a simplified map of the pins on the Pico (known as a ‘pinout’) to help you with the next steps and the rest of the calendar.

The numbers in the grey squares are the physical pin numbers which are numbered in order around the board. The GPIO pin numbers which we use in our code are in green. You can also download the full PDF version here.

Construct the Circuit

First, make sure your Pico is disconnected from the USB cable. You should always do this when amending a circuit. Then grab the set of jumper wires, resistors and LEDs.

Breadboards

We're going to be adding components to our breadboard. Breadboards allow you to connect and prototype a circuit without soldering, using wires with pins at the end called jumper wires (sometimes called DuPont wires).

These breadboards have two sets of horizontal channels (red/blue) on both sides. All the red pins are connected, as are the blue (but each side is disconnected from the other). We use these to create 'rails' of connections for us to use, such as Ground (GND) for the blue channel and 3.3V for the red.

The holes in the middle are connected in vertical lanes, with each lane having 5 connected pins either side of the divider. The divider stops both sides connecting together.

The Circuit

Place the three LEDs into the lower section of the breadboard, 1 hole apart, with the longest leg to the right as seen in the image below.

You won't know which colour is which yet as these LEDs have clear lenses, but that doesn't matter and we'll come back that in a moment. Your breadboard should look like this:

Tip: The short leg is the Cathode (-) and the long leg is the Anode (+).

We now need to add a resistor to limit the current that the LED can draw from our GPIO pins.

Place a resistor between the left leg of each LED and the lower blue channel, like below. We'll be connecting that blue channel to a Ground (-) pin shortly.

Now grab the jumper wires. You need one wire connecting the blue channel to a Ground pin on your Pico – we suggest using physical pin 38 like we’ve done in the diagram below (remember to refer to the Pico pinout if you need a reminder).

You also need to connect the right leg of each LED leg to a GPIO pin. Use GPIO18 (physical pin 24) for Red, GPIO19 (physical pin 25) for Amber, and GPIO20 (physical pin 26) for Green, like the diagram below.

Tip: It doesn't matter what colour jumper wires you use, they all work the same way. In an ideal world we would reserve black for ground connections and red for voltage, but for simple projects it's not essential.

Everything’s hooked up – let’s code some blinky!

Activity 1: Light each LED

A simple exercise first, using some of the code we learnt yesterday. We’re going to light each LED, wait 5 seconds, then turn them off. To do this we need to introduce a new module to you: time.

Note: assuming you turned off your computer after yesterday’s box, you may need to jump back into Thonny and select Run > Configure interpreter again to tell it which device we’re playing with.

The Time Module

The time module allows you to program delays in your code, making it wait for seconds or fractions of seconds before continuing. It’s another fundamental module you’ll use in most of your projects.

The Code

Our code imports pin again, and also imports time. We then define the pin number for each colour of LED (giving them a sensible name for each) and set them to outputs.

After that it’s a simple case of setting each LED pin HIGH, waiting for 5 seconds using time.sleep(5), then setting them all LOW again.

Here’s the code to copy into Thonny and then run in the usual way by hitting the green button. Once you've lit each LED and know which colour is which, unplug your USB cable and rearrange them to get them in the right order of green/amber/red:

from machine import Pin
import time

red = Pin(18, Pin.OUT)
amber = Pin(19, Pin.OUT)
green = Pin(20, Pin.OUT)

red.value(1)
amber.value(1)
green.value(1)

time.sleep(5)

red.value(0)
amber.value(0)
green.value(0)

Activity 2: Flashing LEDs

Let’s make things a little more interesting by making our LEDs flash!

To do this, we need to introduce while loops into our code. Technically you could just repeat the ON/OFF code above, but that would make your code very long indeed and would eventually end, so let's look at a better way (but perhaps not the best way...we'll show you another way in a future box once we've got the hang of MicroPython).

We also need to introduce variables, indentation and code commentary...

While Loops

While loops are used to repeat a block of code as long as the condition remains true.

Imagine you’re bouncing a ball whilst your friend counts how many bounces you’ve completed. Your condition might be “bounce the ball until you count five bounces”. After five bounces, you would stop.

We can tell a while loop to run a block of code until a count or certain condition is met, or we can just make it repeat forever by using 'while True' which is useful for projects intended to keep running.

So, we’re now going to make our LEDs flash 10 times then stop, and we'll show you how in just a moment.

Code Commentary

We’re also going to start adding commentary in our code examples.

You can add comments in MicroPython code by adding a # before the comment. This will be ignored by MicroPython.

Adding commentary makes explaining your code easier to others or even as a reminder to yourself when you return to a project a number of weeks or months later. It’s a very good habit to get in to and allows us to explain each line clearly:

Variables

A variable is just somewhere to store a value (a 'container'), which we can also update when we want to. When we need to use the variable in a line of code, we just use its name.

In our example below our variable is called counter. We create this before the while loop and set an initial value of 1.

In programming, a variable can be a whole number (an 'integer'), a number with a decimal place (a 'floating-point value' or 'float') or a word/text (a 'string').

Indentation

Our while loop says “If the counter is less than 11, run the the code indented below”.

Indentation is a gap at the front of a line of code, usually a tab space or a number of spaces. It's very important in MicroPython as it tells Thonny that the lines belong to a particular block of code (in our example below, we've indented the lines under the while loop as the code belongs to that loop).

The Code

The code below first imports the things we need as always, sets our pin numbers as we've done before, then creates a variable called ‘counter’ which we’ll use to count how many times we’ve run the block of code.

We start a while loop using while counter < 11: which is saying "while the counter variable is less than 11...". 

Our code prints the counter to help us see the value changing in Thonny, then simply turns all the LEDs off then on again, with half-second pauses in-between. Without the pauses, it would be so fast we wouldn’t see the flashes!

At the end of the block we add +1 to our counter, then it starts the loop again. As soon as that counter variable reaches 11, the loop stops running and the program ends.

Copy this code over to Thonny and run it in the usual way using the green button:

# Imports
from machine import Pin
import time

#Set up our LED names and GPIO pin numbers
red = Pin(18, Pin.OUT)
amber = Pin(19, Pin.OUT)
green = Pin(20, Pin.OUT)

counter = 1 # Set the counter to start at 1

while counter < 11: # While count is less than 11...
    
    print(counter) # Print the current counter
    
    # LEDs all on
    red.value(1)
    amber.value(1)
    green.value(1)
    
    time.sleep(0.5) # Wait half a second
    
    # LEDs all off
    red.value(0)
    amber.value(0)
    green.value(0)
    
    time.sleep(0.5) # Wait half a second
    
    counter += 1 # Add 1 to our counter

Try changing both of the time.sleep values to different numbers and see what happens. It should change the speed of the flashing LEDs.

You can also change the while loop to run forever. Swap the line out with the line below to give that a try - the counter will still continue to increase and print but won't change what the code does:

while True:

Activity 3: LED Sequence

We’re now going to make our LEDs flash one after the other in a sequence – yes, like a festive decoration!

Whilst there are more advanced ways of achieving this (using lists for example which we'll cover in a future box), we’re going to keep things simple whilst we continue to learn MicroPython.

The program for this is very similar, however we change the LED control section to turn each LED on one by one, turning off the others as we go.

Try the code below, paying attention to the OFF/ON comments next to each LED control line:

# Imports
from machine import Pin
import time

#Set up our LED names and GPIO pin numbers
red = Pin(18, Pin.OUT)
amber = Pin(19, Pin.OUT)
green = Pin(20, Pin.OUT)

counter = 1 # Set the counter to 1

while counter < 11: # While count is less than 11
    
    print(counter) # Print the current counter
    
    # Red ON
    red.value(1) # ON
    amber.value(0) # OFF
    green.value(0) # OFF
    
    time.sleep(0.5) # Wait half a second
    
    # Amber ON
    red.value(0) # OFF
    amber.value(1) # ON
    green.value(0) # OFF
    
    time.sleep(0.5) # Wait half a second
    
    # Green ON
    red.value(0) # OFF
    amber.value(0) # OFF
    green.value(1) # ON
    
    time.sleep(0.5) # Wait half a second
    
    counter += 1 # Add 1 to our counter

Day #2 Complete!

Another great day of learning how to code with the Raspberry Pi Pico and MicroPython! Today you have:

• Created your first circuit!
• Learnt how to use the time module to add delays to programs
• Learnt how to use while loops and conditions
• Learnt about variables (our counter)
• Learnt about code commentary
• Created a dashing, flashing sequence of LEDs

Please do not disassemble the circuit as we're going to be using it again with the contents in the next box...see you all tomorrow!

We used Fritzing to create the breadboard wiring diagram images for this page.