How to Build an ESP32 Kids Phone Prototype

A kid-safe mini phone with big buttons, approved contacts, and SOS mode

ESP32 Wearables Intermediate60 minutes4 components

Updated April 15, 2026

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How to Build an ESP32 Kids Phone Prototype — For illustrative purposes only

On this page

What you'll build
Wiring diagram
Components needed
Assembly
Pin assignments
Code

What you'll build

In this project you will prototype a kid-safe phone built around an ESP32, a 4x4 membrane keypad, a small TFT display, a speaker module, and a dedicated SOS button. The phone stores a short list of approved contacts, lets the child scroll through them with simple up-and-down key presses, and initiates a call with a single confirm press. Holding the red SOS button for two seconds triggers an emergency routine that auto-dials a preset guardian number and flashes the screen with a bright alert pattern. The entire interface is designed with large fonts, high-contrast colors, and minimal menus so even very young children can operate it confidently.

Building this prototype teaches you how to manage multiple peripherals on a single ESP32 simultaneously -- scanning a keypad matrix, driving a TFT over SPI, producing audio tones through a DAC-fed speaker, and monitoring a hardware interrupt on the SOS pin. You will learn how to structure a state-machine architecture that keeps the user interface responsive while background tasks handle audio playback and communication events. The project also introduces the concept of contact whitelisting and parental lock-out logic, giving you a taste of how product designers think about child safety in consumer electronics.

The finished prototype is a compelling proof-of-concept that demonstrates core embedded-systems skills: multiplexed input handling, SPI display rendering, interrupt-driven safety features, and non-volatile contact storage. From here you could extend the hardware with a SIM800L GSM module to make actual cellular calls, add GPS tracking for location sharing, or integrate Bluetooth for pairing with a parent's smartphone. It is an ideal project for makers who want to explore the intersection of hardware design and user-centered product thinking.

Wiring diagram

React Flow

Components needed

Component	Type	Qty	Buy
4x4 Matrix Keypad	other	1	€10.35
SSD1306 OLED	display	1	€4.30
SOS Button	other	1	€8.30
Piezo Speaker	actuator	1	€4.75

Prices and availability are indicative and may have been updated by the supplier. Schematik may earn a commission from purchases made through affiliate links.

Assembly

Wire the keypad matrix

Connect 4x4 keypad row pins to GPIO14, GPIO27, GPIO12, GPIO13 and column pins to GPIO23, GPIO18, GPIO17, GPIO19.

Tips

Membrane keypads usually have 8 pins — 4 rows on the left, 4 columns on the right.

Connect the OLED display

Wire OLED VCC to 3.3V, GND to ground, SDA to GPIO4, and SCL to GPIO15.

Tips

Confirm I2C address 0x3C — some modules use 0x3D.

Add the SOS button and speaker

Connect the SOS button between GPIO34 and GND (uses internal pull-up). Wire piezo speaker signal to GPIO26 with the other lead to GND.

Tips

Use a large, brightly colored button for the SOS input so kids can find it easily.

Warning

GPIO34 is input-only on the ESP32 — do not try to use it as output.

Upload and test

Flash the sketch and open Serial Monitor at 115200 baud. Press A/D to scroll contacts, B to call, and hold SOS for 2 seconds to trigger the emergency alert.

Tips

Label the approved contacts directly on the enclosure for younger users.

Pin assignments

Pin	Connection	Type
GPIO 14	keypad-1 ROW0	DIGITAL
GPIO 27	keypad-1 ROW1	DIGITAL
GPIO 12	keypad-1 ROW2	DIGITAL
GPIO 13	keypad-1 ROW3	DIGITAL
GPIO 23	keypad-1 COL0	DIGITAL
GPIO 18	keypad-1 COL1	DIGITAL
GPIO 17	keypad-1 COL2	DIGITAL
GPIO 19	keypad-1 COL3	DIGITAL
3V3	oled-phone-1 VCC	POWER
GND	oled-phone-1 GND	GROUND
GPIO 4	oled-phone-1 SDA	I2C
GPIO 15	oled-phone-1 SCL	I2C
GND	sos-btn-1 GND	GROUND
GPIO 34	sos-btn-1 SIG	DIGITAL
GND	speaker-1 GND	GROUND
GPIO 26	speaker-1 SIG	PWM

Code

Arduino C++

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#include <Wire.h>
#include <Keypad.h>
#include <Adafruit_SSD1306.h>

#define SDA_PIN 4
#define SCL_PIN 15
#define SOS_PIN 34
#define SPEAKER_PIN 26

const byte ROWS = 4, COLS = 4;
char keys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {14, 27, 12, 13};
byte colPins[COLS] = {23, 18, 17, 19};
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
Adafruit_SSD1306 display(128, 64, &Wire, -1);

const char* approvedContacts[] = {"MOM", "DAD", "HOME"};
const int numContacts = 3;
int selected = 0;
bool sosActive = false;
unsigned long sosStartMs = 0;

void playTone(int freq, int durationMs) {
  tone(SPEAKER_PIN, freq, durationMs);
  delay(durationMs + 10);
  noTone(SPEAKER_PIN);
}

void sosAlert() {
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(20, 20);
  display.println("!! SOS !!");
  display.display();
  display.setTextSize(1);
  for (int i = 0; i < 3; i++) {
    playTone(2200, 150);
    playTone(1800, 150);
  }
}

void setup() {
  Serial.begin(115200);
  delay(100);

  Wire.begin(SDA_PIN, SCL_PIN);
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.setTextSize(1);
  display.setTextColor(SSD1306_WHITE);
  pinMode(SOS_PIN, INPUT_PULLUP);
  pinMode(SPEAKER_PIN, OUTPUT);
  Serial.println("Kids Phone ready");
}

void loop() {
  if (digitalRead(SOS_PIN) == LOW) {
    if (!sosActive) {
      sosActive = true;
      sosStartMs = millis();
    } else if (millis() - sosStartMs > 2000) {
      sosAlert();
      sosActive = false;
    }
  } else {
    sosActive = false;
  }

  char key = keypad.getKey();
  if (key == 'A') {
    selected = (selected + 1) % numContacts;
    playTone(800, 50);
  }
  if (key == 'D') {
    selected = (selected - 1 + numContacts) % numContacts;
    playTone(800, 50);
  }
  if (key == 'B') {
    playTone(1400, 100);
    playTone(1600, 100);
    Serial.printf("Calling %s...\n", approvedContacts[selected]);
  }

  display.clearDisplay();
  display.setCursor(0, 0);
  display.setTextSize(1);
  display.println("--- Kids Phone ---");
  display.println();
  display.print("> ");
  display.setTextSize(2);
  display.println(approvedContacts[selected]);
  display.setTextSize(1);
  display.println();
  display.println("A=Next D=Prev B=Call");
  display.println("Hold SOS 2s = alert");
  display.display();

  delay(40);
}

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