Button Box Phone BLUEPRINT V2.0

REV 2.0 • ESP32 + MAX98357A • 10 KEYS • USB POWERED

Overview

A recycled phone case turned into a sound box: each of the 10 keypad keys (1 to 9 and 0) is a rubber push button that plays its own short audio clip through a small speaker.

How it works

An ESP32 dev board reads the 10 buttons (active-low, internal pull-ups) and plays WAV files stored in its flash (LittleFS). The audio goes out over I2S to a MAX98357A amplifier that drives an 8 Ω speaker directly.

Trigger keys

WAV sample rate

16 kHz

Mono, 16-bit PCM recommended. About 87 s of audio fit in total at that format.

Playback rules

At power-up a short 3-note chime plays.
Press a key while idle: its clip plays to the end.
Press any key while a clip plays: playback stops (no new clip starts).
The LED blinks while audio plays and is off when idle.

Power

Always on from a 5 V USB adapter through the ESP32 board's USB port; there is no power switch. The amplifier takes 5 V from the board's VIN pin.

Firmware

The PlatformIO project lives in the firmware/ folder of the repository, with build, flashing and audio replacement instructions in firmware/README.md.

Bill of Materials (BOM)

Everything needed for the finished phone. Click 'View' to focus the 3D camera on a part.

Component	Qty	Details
ESP32 dev board	1	ESP32-WROOM-32, 4 MB flash ("esp32dev"), no PSRAM needed
Push buttons	10	Rubber momentary buttons, normally open, one per key 1-9 and 0
MAX98357A	1	I2S class D mono amplifier breakout
Speaker 8 Ω	1	0.5-3 W, wired straight to the amplifier
LED + resistor	1 + 1	Any 3 or 5 mm LED, 220-330 Ω series resistor
USB cable + adapter	1	5 V USB adapter, 1 A or more; always-on power and flashing
Recycled phone case	1	Housing, keypad and handset
Breadboard + jumpers	1 set	For testing before installing in the case

Wiring & Pinout

Same pin table as firmware/src/main.cpp. Buttons are active-low: one leg to the GPIO, the other to GND (internal pull-ups, no resistors needed).

Keypad buttons

Key	ESP32 GPIO	Other leg	File

Key 0 plays audio10.wav.

MAX98357A amplifier

VIN → ESP32 5V (VIN) pin

+5V USB

GND → ESP32 GND

BCLK → I2S bit clock

GPIO 27

LRC → I2S word select

GPIO 26

DIN → I2S data

GPIO 25

GAIN → not connected

9 dB default

SD → not connected

(L+R)/2 mono

SPK + / - → 8 Ω speaker

never to GND

Status LED & power

LED → 220-330 Ω → LED anode, cathode to GND

GPIO 33

USB → ESP32 board USB port

5V always on

Sound Test

Each key plays a generic test tone (the same tones as the firmware placeholders). The demo follows the firmware rules: a press while a tone plays only stops it, and * and # are silent. Load a local audio file on any key to preview your own clip; it stays in this browser tab only.