🛠 Nœud MQTT : télémétrie IMU et commandes à distance

Objectives

Turn the M5StickC Plus2 into a complete IoT node: periodic publication of telemetry (acceleration, battery voltage via the G38 detection pin) to MQTT topics, and subscription to a command topic to trigger the buzzer remotely. Using the PubSubClient library on 2.4 GHz Wi-Fi.

Hardware

• 1× M5StickC Plus2 (v2)
• Accessible MQTT broker (e.g., test.mosquitto.org for testing, or a local broker)
• A test client (Mosquitto CLI, MQTT Explorer) on the PC
• Arduino IDE / PlatformIO: M5Unified + PubSubClient

Complete code

// Noeud MQTT : publie accel + batterie, recoit des ordres de bip
#include <M5Unified.h>
#include <WiFi.h>
#include <PubSubClient.h>

const char* SSID = "VOTRE_WIFI_24GHZ";
const char* PASS = "motdepasse";
const char* BROKER = "test.mosquitto.org";   // port 1883
const char* BASE = "stickc2/atelier/";
WiFiClient wifiNet;
PubSubClient mqtt(wifiNet);
uint32_t dernierEnvoi = 0;

void callback(char* topic, byte* payload, unsigned int len) {
  String msg((char*)payload, len);
  M5.Display.setCursor(0, 90);
  M5.Display.printf("<-%s", msg.c_str);
  int freq = msg.toInt;
  if (freq > 40 && freq < 8000) M5.Speaker.tone(freq, 400);  // buzzer G2
}

void setup {
  auto cfg = M5.config;
  M5.begin(cfg);
  M5.Display.setRotation(1);
  M5.Display.setTextSize(1);
  WiFi.mode(WIFI_STA);
  WiFi.begin(SSID, PASS);
  while (WiFi.waitForConnectResult != WL_CONNECTED) delay(2000);
  mqtt.setServer(BROKER, 1883);
  mqtt.setCallback(callback);
  while (!mqtt.connect("stickc2plus2")) delay(1000);
  String cmd = String(BASE) + "cmd";
  mqtt.subscribe(cmd.c_str);
}

void loop {
  M5.update;
  mqtt.loop;
  if (!mqtt.connected) {            // reconnexion + re-abonnement
    if (mqtt.connect("stickc2plus2")) {
      String cmd = String(BASE) + "cmd";
      mqtt.subscribe(cmd.c_str);
    }
  }
  if (millis - dernierEnvoi > 2000) {
    dernierEnvoi = millis;
    float ax, ay, az;
    M5.Imu.getAccel(&ax, &ay, &az);
    String t = String(BASE) + "accel";
    char json[96];
    snprintf(json, sizeof json, "{\"ax\":%.2f,\"ay\":%.2f,\"az\":%.2f}", ax, ay, az);
    mqtt.publish(t.c_str, json);
    // Tension batterie : broche de detection G38 (ADC1) — la conversion
    // brute -> pourcentage n'est pas documentee :
    // > 
    uint32_t batt = mqtt.publish((String(BASE) + "batt").c_str,
                                 String(analogRead(38)).c_str);
    (void)batt;
    M5.Display.setCursor(0, 20);
    M5.Display.printf("pub accel OK  RSSI %d\n", WiFi.RSSI);
  }
  vTaskDelay(20 / portTICK_PERIOD_MS);
}

Expected verification

With mosquitto_sub -h test.mosquitto.org -t 'stickc2/atelier/#', you will see the acceleration JSON and the raw battery value arrive every 2 seconds. Publishing 880 to stickc2/atelier/cmd makes the buzzer sound for 400 ms and displays the command on the screen.

Common errors

• Publishing without calling mqtt.loop regularly: ACKs and command reception do not work.
• Forgetting to resubscribe after a reconnection: telemetry resumes, but commands remain silent.
• Using a broker with mqtts:// (TLS, port 8883) with a simple WiFiClient: you need WiFiClientSecure.
• Reading the battery on G25/G26: these ADC2 pins are unusable when Wi-Fi is active; official battery detection is on G38 (ADC1).
• Converting raw ADC values to percentage without a reference curve: the official documentation does not provide the discharge curve for the 200 mAh battery — display the raw value or calibrate it yourself.
• Flooding publications at 10 Hz on a public broker: throttle to 0.5–1 Hz and preserve battery margin.