🛠 Publier capteurs et batterie en MQTT via une passerelle

Objectives

Build a battery-powered sensor node that publishes temperature and battery level to an MQTT broker. The hardware architecture is fixed: the Nicla (no WiFi) acts as a BLE peripheral with a custom service, while the MKR WiFi 1010 host board serves as the BLE-to-MQTT gateway. On the Nicla side, the Nicla_System API provides an accurate battery level read from the BQ25120 charger registers (nicla::getBatteryVoltagePercentage, nicla::getCurrentBatteryVoltage), which is much more reliable than a simple voltage divider. This is the reference schema for Nicla-format wireless sensor networks as described by Arduino Pro.

Hardware

• Nicla Sense ME (ABX00050) + LiPo battery on J4 (with or without NTC depending on the battery: nicla::setBatteryNTCEnabled(false) if absent)
• MKR WiFi 1010 (or Portenta C33) as the gateway; ArduinoBLE libraries on the Nicla, WiFi101 + PubSubClient on the host
• Accessible MQTT broker (local or hosted)
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Complete Code

// Sketch NICLA : peripheral BLE batterie + temperature
#include "Nicla_System.h"
#include "Arduino_BHY2.h"
#include <ArduinoBLE.h>

Sensor temp(SENSOR_ID_TEMP);
BLEService batteryService("180F");
BLEUnsignedCharCharacteristic battLevel("2A19", BLERead | BLENotify);
BLEFloatCharacteristic tempChar("2A6E", BLERead | BLENotify);

void setup {
  Serial.begin(9600);
  nicla::begin;
  nicla::setBatteryNTCEnabled(false);
  nicla::leds.begin;
  nicla::leds.setColor(green);
  BHY2.begin;
  temp.begin;
  if (!BLE.begin) { while (1); }
  BLE.setLocalName("NiclaNode");
  BLE.setAdvertisedService(batteryService);
  batteryService.addCharacteristic(battLevel);
  batteryService.addCharacteristic(tempChar);
  BLE.addService(batteryService);
  BLE.advertise;
}

void loop {
  static auto t = millis;
  BHY2.update;
  BLE.poll;
  if (millis - t >= 10000) {
    t = millis;
    battLevel.writeValue((unsigned char)nicla::getBatteryVoltagePercentage);
    if (temp.dataAvailable) tempChar.writeValue(temp.value);
  }
}

// Sketch HOTE (MKR WiFi 1010) : passerelle BLE -> MQTT
#include <WiFi101.h>
#include <PubSubClient.h>

WiFiClient net;
PubSubClient mqtt(net);
BLEDevice node;

void setup {
  WiFi.begin("MON_WIFI", "MON_MOT_DE_PASSE");
  mqtt.setServer("192.168.1.40", 1883);
  BLE.begin;
  BLE.setScanTimeout(5000);
  node = BLE.scanForName("NiclaNode");
  while (!node) { node = BLE.scanForName("NiclaNode"); }
  node.connect;
  mqtt.connect("nicla-gateway");
}

void loop {
  if (!node.connected || !mqtt.connected) { return; } // reconnexion a gerer
  BLE.poll;
  static auto t = millis;
  if (millis - t >= 30000) {
    t = millis;
    unsigned char batt = node.characteristic("2A19").readValue;
    char payload[96];
    snprintf(payload, sizeof(payload), "{\"batt_pct\":%u}", batt);
    mqtt.publish("maison/nicla1", payload);
  }
}

Expected Verification

mosquitto_sub -t 'maison/nicla1' displays periodic JSON containing the battery percentage read from the charger. The green LED on the Nicla indicates power; using nRF Connect, you can audit the 2A19/2A6E characteristics between publications. As the LiPo discharges, the percentage should decrease regularly (recalibrate if the curve appears flat, as LiPo voltage is not very linear).

Common Errors

• Publishing blindly without handling disconnections: the Nicla's BLE sleeps, and the host's WiFi drops; plan for complete reconnection loops before leaving the node alone in production.
• Forgetting BLE.poll on the Nicla: central devices read frozen values; the same symptom occurs if BHY2.update is missing.
• Relying on a raw percentage without accounting for charging: nicla::getOperatingStatus and the charging voltage affect the reading; also display the raw voltage for debugging.
• Publishing too frequently: 10 s on the Nicla and 30 s on MQTT are comfortable for a 200 mAh LiPo; the datasheet specifies 1.15 mA typical during advertising + one measurement per hour, and every extra second of radio use costs battery life.
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