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Lab 1: Blink the Onboard LED

Pixel says...

Pixel waves hello Every big light show starts with one little blink! I'm an LED (a light-emitting diode) too, so this one is close to my heart. Let's light this up!

Program file: 01-blink-onboard-led.py

What you'll learn

  • How a Python program loads tools with import
  • How a variable (a name stuck on a number) makes code easier to change
  • How to turn a pin on and off to blink the LED
  • How while True repeats code forever
  • How the sleep time sets the speed

What you'll need

  • Your Raspberry Pi Pico and a USB data cable
  • Thonny open and connected to your Pico (see the Desktop Setup page)
  • No wires and no config.py for this lab. This program uses the small LED that is already built onto the Pico board

The program

This program blinks the Pico's own LED over and over. It checks that your Pico is connected to Thonny and running Python.

01-blink-onboard-led.py
# Test 01: Blink the Onboard LED
# Filename: 01-blink-onboard-led.py
# Version: 1.0.0
#
# Use this program to test that your Pico is connected to Thonny and
# running MicroPython. No wiring needed - this uses the LED already
# built onto the Pico board.

from machine import Pin
from utime import sleep

print("Test 01: Blink the Onboard LED (version 1.0.0)")

BUILT_IN_LED_PIN = 25    # every Pico has an LED wired to this pin
BLINK_DELAY = 0.25       # seconds the LED stays on or off - change me!

led = Pin(BUILT_IN_LED_PIN, Pin.OUT)

while True:
    led.toggle()          # switch the LED on if it's off, or off if it's on
    sleep(BLINK_DELAY)

Open the file in Thonny and click the green Run button. The small LED near the USB connector blinks twice every second. The Shell at the bottom of Thonny shows the name and version of the program.

Click the red Stop button when you want it to end.

How it works

Load the tools

from machine import Pin
from utime import sleep

Python comes with a toolbox of ready-made code. The import word brings tools into your program. The machine toolbox has tools that talk to the Pico's pins. Pin is the tool for one pin. The utime toolbox has sleep, which makes the Pico wait.

Give your numbers names

BUILT_IN_LED_PIN = 25    # every Pico has an LED wired to this pin
BLINK_DELAY = 0.25       # seconds the LED stays on or off - change me!

A variable is a name stuck on a value. BUILT_IN_LED_PIN is a name for the number 25. BLINK_DELAY is a name for 0.25, a number with a decimal point. Names written in capital letters are a habit programmers use for settings. When you want a slower blink, you change one number at the top and nothing else.

Make the LED object

led = Pin(BUILT_IN_LED_PIN, Pin.OUT)

This line creates a Pin that controls pin 25. The word Pin.OUT means the pin sends a signal out, so it can turn a light on. We keep the result in a variable named led.

Repeat forever

while True:
    led.toggle()          # switch the LED on if it's off, or off if it's on
    sleep(BLINK_DELAY)

The while True: line starts a loop, which is code that repeats. Because True is always true, this loop never ends. The two indented lines belong to the loop. The spaces at the start of a line tell Python which lines are inside it.

led.toggle() flips the LED. If it was off, it turns on. If it was on, it turns off. Then sleep(BLINK_DELAY) waits a quarter of a second. One trip through the loop changes the LED once, so the LED needs two trips to finish one blink.

Key idea

A program on a microcontroller usually never ends. It checks, changes, and waits, again and again, for as long as the power stays on.

Try it yourself

  1. Change the speed. Set BLINK_DELAY = 0.1, then BLINK_DELAY = 1. Predict what each one does before you run it.

  2. Make a double blink. Replace the two lines inside the loop with the lines below. Run it and watch for two quick blinks and then a long pause.

    while True:
    led.on()           # turn the LED on
    sleep(0.1)
    led.off()          # turn the LED off
    sleep(0.1)
    led.on()
    sleep(0.1)
    led.off()
    sleep(1)           # a long pause before the next double blink

  3. Find the pause. How many seconds is one full blink (on, then off) when BLINK_DELAY is 0.25?

Check your understanding

  1. What does the import word do?
  2. What is a variable? Name one variable in this program.
  3. Why does the while True: loop never stop on its own?
  4. What does led.toggle() do each time it runs?
  5. How could you make the LED blink four times every second?

Lab complete!

Pixel celebrates Your Pico runs Python and your LED obeys! That's the first step in every project.

What's next: In Lab 2: Hardware Probe, your Pico tests all of your wiring and tells you what it finds.