🛠 Envoyer et recevoir des SMS

Objectives

Use the GSM_SMS class from the MKRGSM library to send an alert SMS to an international number and listen for incoming SMS messages to control the board remotely (text "LED ON" / "LED OFF"). SMS remains the ideal backup channel when GPRS is down: it works even when data is unavailable, as long as 2G coverage exists.

Hardware

• Arduino MKR GSM 1400 board + deployed U.FL antenna
• Micro-SIM card with a plan including SMS, PIN code disabled
• LiPo battery ≥ 1500 mAh connected
• A phone for round-trip testing

Complete code

#include <MKRGSM.h>
#include "arduino_secrets.h"
const char PINNUMBER[] = SECRET_PINNUMBER;
const char NUMERO_DEST[] = "+33612345678";  // format international

GSM     gsmAccess;
GSM_SMS sms;

void setup {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  while (!Serial) {}
  while (gsmAccess.begin(PINNUMBER) != GSM_READY) delay(1000);
  Serial.println("GSM pret, ecoute SMS...");

  // SMS de demarrage
  sms.beginSMS(NUMERO_DEST);
  sms.print("MKR GSM 1400 en ligne.");
  sms.endSMS;
}

void loop {
  if (sms.available) {
    String msg;
    char c;
    while ((c = sms.read) != -1) msg += c;
    msg.trim;
    Serial.println(msg);
    if (msg.equalsIgnoreCase("LED ON"))  digitalWrite(LED_BUILTIN, HIGH);
    if (msg.equalsIgnoreCase("LED OFF")) digitalWrite(LED_BUILTIN, LOW);

    sms.beginSMS(sms.remoteNumber);   // accuse de reception
    sms.print("Recu : " + msg);
    sms.endSMS;
  }
  delay(1000);
}

Expected verification

At startup, the phone receives "MKR GSM 1400 online.". Sending "LED ON" turns on LED D6 and returns an acknowledgment "Received: LED ON"; "LED OFF" turns it off. endSMS blocks until the message is sent or fails: the first send after registration may take several seconds.

Common errors

• Phone number in national format (06...): use the international format "+33...".
• Full SMS inbox on the SIM: available reads block or loop; clear the SIM memory (often 5 to 10 messages) or delete them from a phone.
• Long SMS (> 160 characters) or accents: Unicode encoding reduces the useful length; stick to ASCII for commands.
• Empty or incorrect sender number: depending on the carrier, the sender's number may be missing — do not rely on it to authorize a sender without verification.
• Simultaneous SMS + data attempt: the module is in circuit-switched mode during a call/SMS; design clear priorities between SMS and GPRS.
• Current spike during transmission: again, a LiPo battery is mandatory to avoid silent restarts.