GSM Smart Socket

/*
 * Date       : 6 March 2021 
 * Author     : Erick H Moshi
 * Description: GSM based switch
 */
 
#include <SoftwareSerial.h> // Library for using serial communication
SoftwareSerial SIM800(7, 8); // Pins 7, 8 are used as used as software serial pins

String incomingData;       // for storing incoming serial data
String message = "";       // A String for storing the message
String lineState = "IDLE"; // To initially set the state to idle once booted or in cases of power cuts
int relay_pin = 13;        // Initialized a pin for relay module

void setup()
{
  Serial.begin(115200); // baudrate for serial monitor
  SIM800.begin(19200);  // baudrate for GSM shield

  pinMode(relay_pin, OUTPUT);     // Setting erlay pin as output pin
  digitalWrite(relay_pin, HIGH);  // Making relay pin initailly low

  delay(30000);
  
  Serial.println("Initializing ...");
  delay(1000);

  // set SMS mode to text mode
  SIM800.print("AT+CMGF=1\r"); 
  delay(5000);

  SIM800.print("AT+CMGD=1,4\r"); // delete all sms on sim card
  delay(5000);
  
  // set gsm module to show the output on serial out
  SIM800.print("AT+CNMI=2,2,0,0,0\r"); 
  delay(5000);

  // Retrieve own mobile number from SIM
  Serial.println(F("Attempting to retrieve own MSISDN number from sim"));
  SIM800.print("AT+CNUM\r");
  delay(5000);

  // Enable caller ID on incoming calls
  Serial.println(F("Enabling Caller ID on incoming calls"));
  SIM800.print("AT+CLIP=1\r");
  Serial.println("Setup complete!");
  delay(5000);

  // Send text to alert that system is back online after a power cut/ when switched on
  SIM800.println("AT+CMGF=1");
  delay(100);
  SIM800.println("AT+CMGS=\"+ZZZxxxxxxxxx\""); // Send SMS to the phone number, this will alert you that the system is on
  delay(100);
  SIM800.println("SYSTEM ONLINE"); // The content of the SMS
  delay(100);
  SIM800.println((char)26);                    // ASCII code of CTRL+Z
  delay(100);
  SIM800.println();
  delay(1000);
  
}

void loop()
{
  //Function for receiving sms
  receive_message();

  // if received command is to turn on relay
  if(incomingData.indexOf("LINE_ON")>=0)
  {
    digitalWrite(relay_pin, HIGH);// set to high since we are using the Normarlly Closed side of the relay
    message = "Line is turned ON";
    lineState = "ON";
    // Send a sms back to confirm that the relay is turned on
    send_message(message);
  }
  
  // if received command is to turn off relay
  if(incomingData.indexOf("LINE_OFF")>=0)
  {
    digitalWrite(relay_pin, LOW);//Set to low since we are using the Normally Closed side of the relay and Low will activate the relay switching the line off
    message = "Line is turned OFF";
    lineState = "OFF"; 
    // Send a sms back to confirm that the relay is turned off
    send_message(message);
  }
  if(incomingData.indexOf("STATE")>=0)
  {
    message = "Line state is " + lineState;
    send_message(message);
    Serial.println("Line state resquest");
    message = "";
  }
}

void receive_message()
{
  if (SIM800.available() > 0)
  {
    incomingData = SIM800.readString(); // Get the data from the serial port.
    Serial.print(incomingData); 
    delay(10); 
  }
}

void send_message(String message)
{
  SIM800.println("AT+CMGF=1");                 //Set the GSM Module in Text Mode
  delay(100);  
  SIM800.println("AT+CMGS=\"+ZZZxxxxxxxxx\""); // Sends feedback text to the phone number
  delay(100);
  SIM800.println(message);                     // The SMS text you want to send
  delay(100);
  SIM800.println((char)26);                    // ASCII code of CTRL+Z
  delay(100);
  SIM800.println();
  delay(1000);  
}