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Lab 3: Press the Button

In Lab 1, pressing the button on your kit played a different R2D2 sound every time. In this lab, you will run that same program from Thonny, watch the code decide which sound to play, and change how many sounds are in the list.

Welcome back, maker!

Sparky waving Ready to see how I pick which sound to play next? We're diving into the code behind the button — a list, a counter, and a clever bit of math that keeps everything in order.

What You Need

  • Your MAX98357A kit, connected to your computer with a USB cable (power switch off, same as Lab 2)
  • Thonny, open and connected to the Pico

What You'll Learn

  • What a digital input is
  • What "debounce" means and why buttons need it
  • How a program can keep a list of items and remember its place in that list
  • How to read a simple if statement and a loop that never stops

Step-by-Step

Step 1: Open the File

In Thonny's file browser, open 03-play-sounds-on-button.py.

Step 2: Run It

Press Run (F5). Look at the Thonny console. You should see something like:

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2
Loaded 16 sound(s) from /sounds.
Press the button (GPIO16) to play the next one in order. Ctrl-C to stop.

Step 3: Press the Button and Watch the Console

Press the button on the side of your kit. Two things should happen:

  1. A sound plays through the speaker.
  2. A new line appears in the Thonny console, like Playing R2D2 Excited.

Press the button several more times. Watch the console print a different sound name each time, in the same order, every time you run the program.

Step 4: Find the List of Sounds

Near the bottom of the file, find this line:

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sound_files = sorted(name for name in os.listdir(SOUND_DIR) if name.endswith(".wav"))

This line asks the Pico: "look in the sounds folder, and give me every file that ends with .wav, sorted alphabetically." Every sound file on your kit ends in .wav — that's a common format for storing digital audio.

Step 5: Find the Counter

Just below that, find:

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sound_index = 0

sound_index is a number that starts at 0 and keeps track of which sound to play next. Every time you press the button, look for this line further down:

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sound_index = (sound_index + 1) % len(sound_files)

This line adds 1 to sound_index — and then wraps it back to 0 once it reaches the end of the list, using the % symbol (called modulo). That's why the sounds never repeat until you've heard every single one.

Step 6: Stop the Program

Click the red Stop button in Thonny, or press Ctrl-C in the console, to stop the program. You should see:

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Done - amp shut down.

Try It Yourself

  • Run the program again and count how many sounds play before the first one repeats. Does it match the number from Lab 1?
  • Find the line print("Loaded {} sound(s)... — that number tells you exactly how many .wav files are in the sounds folder without you having to count button presses.
  • Look at the if button.value() == 0: line. Can you find where the code checks the button value a second time, a few lines later? That second check is what makes sure you were really pressing the button, and not just picking up electrical noise for an instant.

What's Happening Under the Hood

The button is a digital input — a pin on the Pico that can only read one of two values: 1 (not pressed) or 0 (pressed). The pin is wired so it normally reads 1; pressing the button connects it straight to the ground wire, pulling it down to 0.

Real buttons are a little "bouncy" — when you press one, the electrical signal can flicker between 0 and 1 a few times in a few thousandths of a second before settling down. If the code checked the button only once, it might accidentally count one press as two or three. Waiting a tiny bit (time.sleep_ms(20)) and checking again is called debouncing, and it's a trick used in almost every button project you'll ever build.

Have you ever double-clicked by accident?

Sparky thinking That's the same bounce happening on a mouse button instead of mine. Debouncing is why a single, deliberate click almost always registers as exactly one click, not two.

Check Your Understanding

  1. What value does the button pin read when it is not pressed — 0 or 1?
  2. What does the % (modulo) operator do to sound_index once it reaches the end of the list?
  3. Why does the code check the button's value twice, with a short wait in between?
  4. If you added 4 more .wav files to the sounds folder, would the program automatically include them, or would you need to change the code?

Full Code

You can find the complete program at src/kits/max98357a-amp/03-play-sounds-on-button.py.

You just decoded a button's memory trick!

Sparky celebrating You read a real if statement, traced a list, and figured out how modulo keeps a counter from running off the end. That's exactly how programmers manage lists in every kind of software!