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Lab 33: Button and Built-in LED

Pixel says...

Pixel waves hello Here's where your code stops watching and starts reacting! You press a button, and a light answers. That's the heart of every interactive project.

Program file: 33-button-led-test.py

What you'll learn

  • How to turn an output on and off with led.on() and led.off()
  • How to use if and else to react to a button
  • How old_value lets a program print only when something changes
  • Where the Pico's built-in LED lives, and how to reach it on a Pico W

What you'll need

  • Your base kit: a Pico, a breadboard, and a push button wired as shown in The Two Buttons. This lab uses Button 1 only.
  • The config.py file saved on the Pico (see Getting Code onto the Kit)
  • Thonny open and connected to your Pico

This lab builds on Lab 32: Button Test, which explains why a pressed button reads 0. It does not use the LED strip.

The program

This program turns the Pico's built-in LED on while you hold Button 1. It also prints the button's value each time the value changes.

33-button-led-test.py
# Lab 33: Button and Built-in LED
# Filename: 33-button-led-test.py
# Version: 1.0.0
#
# The Pico's built-in LED turns on while Button 1 is pressed.

from machine import Pin
import time
import config

# hardware settings from config.py
BUTTON_PIN_1 = config.BUTTON_PIN_1
# the LED built onto the Pico board (not set in config.py)
BUILT_IN_LED_PIN = 25

# input on the lower left of the Pico
# the other side of the button goes to ground
# if the switch is open the value is pulled up to 3.3 volts

# Create a button object on the pin
button = Pin(BUTTON_PIN_1, Pin.IN, Pin.PULL_UP)

# create an LED object
led = Pin(BUILT_IN_LED_PIN, Pin.OUT)

button_value = 0
old_value = 0
while True:
    # get the new value
    button_value = button.value()

    # change the LED to on when the button is pressed
    if button_value == 1:
        led.off()
    else:
        led.on()

    # only print on if there is a change
    if button_value != old_value:
        print(button_value)
        old_value = button_value

Run it. The small green LED on the Pico board glows while you press Button 1 and goes dark when you let go. The Shell prints 1 at the start, 0 when you press, and 1 when you release.

How it works

One pin listens, one pin sends

button = Pin(BUTTON_PIN_1, Pin.IN, Pin.PULL_UP)
...
led = Pin(BUILT_IN_LED_PIN, Pin.OUT)

The button is an input, as in Lab 32. The led is an output: a pin the Pico controls, so it can turn something on. BUILT_IN_LED_PIN is 25, the pin that connects to the LED on the board. Every standard Pico uses the same pin, so it lives in the program and not in config.py.

Heads up

A Pico W is different. Its built-in LED is not on pin 25. On a Pico W, replace the led = ... line with the version below.

Your change
led = Pin("LED", Pin.OUT)

React to the button

This part reads the button and picks what the LED does.

    button_value = button.value()

    # change the LED to on when the button is pressed
    if button_value == 1:
        led.off()
    else:
        led.on()

The == sign asks "are these two things equal?" A single = stores a value instead. If button_value is 1, the button is released, so led.off() runs. Otherwise the value must be 0, so the button is pressed and led.on() runs. The LED is on exactly while you hold the button.

The program starts with two variables.

button_value = 0
old_value = 0

Then the last part of the loop compares them.

    if button_value != old_value:
        print(button_value)
        old_value = button_value

The != sign means "is not equal to". The old_value variable remembers the value from the last time we printed. If the new value is different, the program prints it and updates old_value. Without this check, the Shell would fill with 1s or 0s as fast as the loop can run.

You may wonder why a 1 appears before you press anything. old_value starts at 0, but a released button reads 1. On the first pass they differ, so the program prints 1. That is the program noticing the button's real value, not a press.

Unlike Lab 32, this loop has no sleep(). It checks the button as fast as it can, so it catches even a quick tap.

Key idea

Because the loop is so fast, a button can print an extra pair of numbers now and then. The reason is bounce: the metal contacts inside touch and separate a few times in a few thousandths of a second. Lab 34 shows how to handle it.

Try it yourself

  1. Make the LED normally on and off while you press. Swap the two lines led.off() and led.on().
  2. Print words instead of numbers. Replace the if button_value != old_value: block with this version. Keep it indented inside the loop.
Your change
if button_value != old_value:
    if button_value == 0:
        print("pressed")
    else:
        print("released")
    old_value = button_value

Check your understanding

  1. Which pin is the built-in LED on for a standard Pico? How do you name it on a Pico W?
  2. Why does the LED turn on when button_value is 0?
  3. Why does the Shell print a 1 before you press anything?
  4. What would happen if we removed the old_value check?

Lab complete!

Pixel celebrates A press went in and a light came out! That input-to-output idea is inside every interactive project you'll build.

What's next: In Lab 34: Two Buttons, you'll add a second button and meet a faster way to catch a press.