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The MAX98357A I2S Amplifier Project Kit

Why the MAX98357A Amplifier Rocks

Most beginner robots make sound with a tiny piezo speaker wired straight to a GPIO pin. The Pico turns that pin on and off very fast, using a trick called PWM, or Pulse Width Modulation. The piezo speaker turns that switching into a single, thin beep. It works, but a beep is the only sound it can ever make.

The MAX98357A is a completely different kind of chip. Audio engineers call this design a Class D amplifier — a chip that switches power on and off very efficiently instead of wasting it as heat. This small chip costs about $10, yet it can drive a real speaker loud enough to fill a room, powered by nothing more than three AA batteries.

The MAX98357A can also play real recorded sounds, not just a single tone. It reads digital audio files, so it can play full sentences, sound effects, even music — like the R2D2 sounds in this kit. A simple piezo beeper can never do that. It only knows one note at a time.

The MAX98357A also keeps the sound clean. It receives audio as a purely digital signal, called I2S, all the way until the very last step inside the chip. That means electrical noise and hum from motors, batteries, and the Pico's own power supply never gets a chance to sneak into your sound. A piezo wired straight to a GPIO pin has no such protection — every bit of electrical noise on that pin comes out as noise in the beep.

Ever hear a buzzer hum along with the motors?

Sparky thinking That hum is electrical noise leaking straight into the sound. Keeping audio digital until the very last moment, the way the MAX98357A does, is how real audio engineers keep motors and buzzers from fighting over the same wires.

This kit also lets you control volume with a real knob, called a potentiometer. Turning it smoothly changes how loud the speaker plays, just like a volume knob on a stereo. A simple piezo buzzer has no volume control at all — it is either on or off, at one fixed loudness.

Explore an interactive pinout diagram of the MAX98357A board - hover over the speaker terminal, the amplifier chip, and each of the seven pins to see what it does.

The MAX98357A I2S Amplifier is a mono amplifier with an input voltage of 2.5 to 5.5 volts and a default gain of 9dB.

Welcome, maker!

Sparky waving This little box can talk, glow, and groove — a speaker, a round color screen, a button, and a knob, all wired to one tiny Pico. Let's crack it open and make it come alive, one lab at a time!

Project Box Components

  1. Raspberry Pi Pico
  2. 1 MAX98357A breakout board
  3. 1/2 size breadboard
  4. Ribbon cable from MAX98357A breakout board to breadboard - note BCLK and LRC pins are reversed
  5. Smartwatch display
  6. Ribbon cable from breadboard to display
  7. Momentary push button
  8. 3 AA batteries
  9. Power toggle switch
  10. Clear plastic case (Container Store)
  11. LED for power indicator with 330 ohm resistor

It is designed to be used with 4-8 ohm speakers and it is rated at 3W.

Pico Connections

  • BCLK connect to GPIO 11
  • LRC connect to GPIO 12
  • DIN connect to GPIO 13
  • GND connect to GND
  • GAIN connect to GPIO 14

  • SD connect to GPIO 15 (lower left corner)

  • VIN connected to the VBUS at 5v - not the Pico's 3V3 OUT pin. The onboard 3.3V regulator only supplies a few hundred mA shared with the rest of the board; on 3V3 this amp browned out and audio cut off after about 1.2 seconds of playback.

  • Momentary push button GPIO 16 (lower right corner of the Pico)

Don't power me from 3V3 OUT

Sparky warning I learned this one the hard way — wiring VIN to the Pico's 3V3 OUT pin starved the amp of power and cut my audio off after about a second. Always connect VIN to VBUS (5V) instead.

This pin choice satisfies one hard constraint and one board-specific caution:

  1. MicroPython's machine.I2S driver on the RP2040 requires the ws (LRC) pin to be exactly one GPIO number higher than the sck (BCLK) pin.
  2. If this kit is later run on a Cytron Maker Pi RP2040 instead of a plain Pico, GPIO 8-11 are hardwired to that board's onboard DC motor driver and GPIO 12-15 to its onboard servo headers (see the MAKER-PI-RP2040 datasheet), so those pins aren't safe to reuse for I2S there. GPIO 2-6 are free Grove-port pins on that board too, so this wiring works on either.

Note. Our early testing had problems initially looked broken (clean I2S execution, but no sound on two separate Pico boards), which led to real doubt about whether those pins were somehow bad. That turned out to be a testing artifact, not a pin problem: a very short audio clip (~0.44s) played with almost no delay after enabling the amp, likely swallowed by the amp's own brief power-on mute. Adding a ~200ms delay after enabling the amp (before sending real audio) resolved it - see SETTLE_MS in the kit's scripts. The lesson: always test a new pin group with a clip of at least a second or two and a short settle delay, not the shortest file available, or you can easily mistake a playback-timing artifact for a wiring/pin fault.

A short clip can trick you

Sparky tip If a new sound seems to vanish right after I power on, it's probably not a wiring problem — it's my own power-on mute swallowing a clip that's too short. Test new pins with a clip a second or two long.

Labs

  1. Lab 1: Meet Your Kit - turn it on and try every part, no computer needed
  2. Lab 2: Your First Sound - connect to Thonny and play a musical tone
  3. Lab 3: Press the Button - the R2D2 sound board and how it cycles through sounds
  4. Lab 4: Light Up the Display - draw your own text and colors on the round screen
  5. Lab 5: Turn the Dial - the potentiometer, analog input, and the gauge ring
  6. Lab 6: Volume Control Lab - the full program that combines the button, knob, display, and speaker

Uploading the Code

The source code for this kit, plus a shared config.py, lives in src/kits/max98357a-amp/. The sound files it plays live in sounds/ at the root of the repository. To copy the whole kit (code, display driver, and sounds) onto the Pico in one step, run upload-code.sh from a terminal:

1
./upload-code.sh

Any single lab script can also be opened and run directly from Thonny, which is how each lab above is written to be used.