We’ll be building a smart device utilizing the TTGO T-Call development board with the ESP32 and SIM800L chip. You'll learn about the essential components required, including a 2.4GHz Wi-Fi antenna, a nano SIM card, an XL4015 step-down DC-DC converter, and a 100A solid-state relay.

How to Control Electrical Devices via SMS | ESP32 TTGO T-Call & SIM800L Tutorial

#define TINY_GSM_MODEM_SIM800  // Define SIM800 GSM modem
#define RELAY_PIN 14           // Pin connected to relay

// Include the necessary libraries
#include <Wire.h>
#include <TinyGsmClient.h>

// TTGO T-Call pins
#define MODEM_RST            5
#define MODEM_PWKEY          4
#define MODEM_POWER_ON       23
#define MODEM_TX             27
#define MODEM_RX             26

// Define serial communication for the modem and debugging
#define SerialMon Serial
#define SerialAT Serial1

// SIM card PIN (leave empty if not defined)
const char simPIN[] = "";

// Your phone number to send SMS (use + and country code)
#define SMS_TARGET  "+989393569424"

// Initialize modem
TinyGsm modem(SerialAT);

void setup() {
  // Initialize serial communication
  SerialMon.begin(115200);
  SerialAT.begin(115200, SERIAL_8N1, MODEM_RX, MODEM_TX);

  // Initialize relay pin
  pinMode(RELAY_PIN, OUTPUT);

  // Power up the modem
  pinMode(MODEM_PWKEY, OUTPUT);
  pinMode(MODEM_RST, OUTPUT);
  pinMode(MODEM_POWER_ON, OUTPUT);
  digitalWrite(MODEM_PWKEY, LOW);
  digitalWrite(MODEM_RST, HIGH);
  digitalWrite(MODEM_POWER_ON, HIGH);

  // Initialize the modem
  SerialMon.println("Initializing modem...");
  if (!modem.restart()) {
    SerialMon.println("Failed to restart modem");
    while (true);
  }
  SerialMon.println("Modem initialized successfully");

  // Unlock SIM card if necessary
  if (strlen(simPIN) && modem.getSimStatus() != 3) {
    modem.simUnlock(simPIN);
  }

  // Set SMS text mode
  SerialAT.println("AT+CMGF=1");
  delay(100);
  
  // Set SMS handling mode to deliver immediately to serial
  SerialAT.println("AT+CNMI=2,2,0,0,0");
  delay(100);

  SerialMon.println("Setup completed, waiting for SMS...");
}

void loop() {
  // Handle incoming SMS messages
  if (SerialAT.available()) {
    String sms = SerialAT.readStringUntil('\n');
    sms.trim();
    SerialMon.println("Received SMS: " + sms);

    // Check for "ON" or "OFF" commands in the SMS
    if (sms.indexOf("ON") >= 0) {
      digitalWrite(RELAY_PIN, HIGH);  // Relay ON (adjust if active-low)
      SerialMon.println("Relay turned ON via SMS");
    } else if (sms.indexOf("OFF") >= 0) {
      digitalWrite(RELAY_PIN, LOW);   // Relay OFF (adjust if active-low)
      SerialMon.println("Relay turned OFF via SMS");
    }
  }

  // Manual control from Serial Monitor
  if (Serial.available()) {
    String input = Serial.readString();
    input.trim();
    SerialMon.println("Received input: " + input);

    if (input.equalsIgnoreCase("ON")) {
      digitalWrite(RELAY_PIN, HIGH);  // Relay ON (adjust if active-low)
      SerialMon.println("Relay turned ON");
    } else if (input.equalsIgnoreCase("OFF")) {
      digitalWrite(RELAY_PIN, LOW);   // Relay OFF (adjust if active-low)
      SerialMon.println("Relay turned OFF");
    } else {
      SerialMon.println("Unknown command. Please type ON or OFF.");
    }
  }
  
  delay(1000);
}