Skip to content

Lab 3: Button Test

Pixel says...

Pixel waves hello Buttons are how you talk back to your program. Let's find out how a press looks to the Pico!

Program file: 03-button-test.py

What you'll learn

  • Why a button reads 0 when you press it
  • How button.value() reads a pin
  • How to notice a change by remembering the last value
  • How to use if to print a message only when something happens

What you'll need

  • Your kit, with the buttons wired to GP14 and GP15 as shown in the Kit Guide
  • The matrix wired to GP0 (the corner pixels light up when you press)
  • config.py saved on the Pico
  • Thonny open and connected to your Pico

The program

This program prints a message each time you press or let go of a button. It also lights a pixel in the top row while you hold a button down.

03-button-test.py
# Test 03: Button Test
# Filename: 03-button-test.py
# Version: 1.0.0
#
# Prints a message when each button is pressed or released, and lights
# a corner pixel while the button is held.
# Not yet tested on hardware.

from machine import Pin
from neopixel import NeoPixel
from utime import sleep
import config

print("Test 03: Button Test (version 1.0.0)")

# hardware settings from config.py
NEOPIXEL_PIN = config.NEOPIXEL_PIN
NUMBER_PIXELS = config.NUMBER_PIXELS
MATRIX_WIDTH = config.MATRIX_WIDTH
BUTTON_PIN_1 = config.BUTTON_PIN_1
BUTTON_PIN_2 = config.BUTTON_PIN_2

strip = NeoPixel(Pin(NEOPIXEL_PIN), NUMBER_PIXELS)

# the buttons connect the pin to GND, so a press reads 0
button1 = Pin(BUTTON_PIN_1, Pin.IN, Pin.PULL_UP)
button2 = Pin(BUTTON_PIN_2, Pin.IN, Pin.PULL_UP)

# button 1 lights the first pixel, button 2 lights the last pixel of the first row
PIXEL_1 = 0
PIXEL_2 = MATRIX_WIDTH - 1

last1 = 1
last2 = 1
print("Press each button. Press Ctrl-C to stop.")
while True:
    now1 = button1.value()
    now2 = button2.value()
    if now1 != last1:
        print("Button 1 (GP%d):" % BUTTON_PIN_1, "pressed" if now1 == 0 else "released")
        last1 = now1
    if now2 != last2:
        print("Button 2 (GP%d):" % BUTTON_PIN_2, "pressed" if now2 == 0 else "released")
        last2 = now2
    strip[PIXEL_1] = (0, 20, 0) if now1 == 0 else (0, 0, 0)
    strip[PIXEL_2] = (0, 0, 20) if now2 == 0 else (0, 0, 0)
    strip.write()
    sleep(0.02)

Run it and press each button a few times. The Shell shows a message for every press and release. Button 1 lights the first pixel green while you hold it. Button 2 lights the last pixel of the top row blue.

Test 03: Button Test (version 1.0.0)
Press each button. Press Ctrl-C to stop.
Button 1 (GP14): pressed
Button 1 (GP14): released
Button 2 (GP15): pressed
Button 2 (GP15): released

How it works

Pressed means zero

button1 = Pin(BUTTON_PIN_1, Pin.IN, Pin.PULL_UP)

Pin.IN means this pin listens instead of sending. Pin.PULL_UP turns on a tiny spring-like resistor inside the Pico. It pulls the pin up to 3.3 volts, so an untouched pin reads 1. Your button connects the pin to ground. When you press, the pin drops to 0.

That feels backwards, so say it out loud once: pressed means zero.

Remember the last value

last1 = 1
last2 = 1

To notice a change, the program needs to remember what the button said last time. Both buttons start at 1, which means not pressed.

Spot a change

now1 = button1.value()
now2 = button2.value()
if now1 != last1:
    print("Button 1 (GP%d):" % BUTTON_PIN_1, "pressed" if now1 == 0 else "released")
    last1 = now1

button1.value() reads the pin right now. The != sign means "is not equal to". So the line if now1 != last1: asks a question. It asks, did the button change since last time? If yes, we print a message. The last part, "pressed" if now1 == 0 else "released", picks the word: 0 means pressed, and anything else means released.

Then last1 = now1 saves the new value for the next time around the loop.

Light a pixel

strip[PIXEL_1] = (0, 20, 0) if now1 == 0 else (0, 0, 0)
strip.write()

A color is three numbers: red, green, and blue, from 0 to 255. The color (0, 20, 0) is a dim green. The color (0, 0, 0) is off. Each trip through the loop picks the color from the button, then strip.write() sends it to the matrix.

Key idea

Reading a button over and over, and comparing it with the last reading, is how a program notices a press. Every game controller does something like this.

Try it yourself

  1. Change the color of Button 1's pixel from green to red. Which of the three numbers do you change?
  2. Press both buttons at the same time. What do you see in the Shell? What do you see on the matrix?
  3. Hold a button down and watch the Shell. Does it print over and over, or just once? Why?

Check your understanding

  1. What does a button read when nobody is pressing it? What does it read when you press?
  2. What does Pin.PULL_UP do?
  3. Why does the program keep the variables last1 and last2?
  4. What does the != sign mean?

Lab complete!

Pixel celebrates Your buttons work! Now your kit can listen as well as shine.

What's next: In Lab 4: First Pixel, you will light one pixel in three colors.