OTP based Lock system to work with SIM900 and NEOSIM GPS. Regularly send GPS coordinates to the phone that asked for OTP (starting from when unlocked till the lock is not armed again). Open to suggestions, please do tag me (@brute4s99). :)

withGPS.ino

/*
  Connect 5VT to D9 and 5VR to D10
  Feed GSM SIM900A with Arduino's 5V
  Code by Piyush Aggarwal
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  A0 -
  A1
  A2
  A3
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  A5

*/

#include <SoftwareSerial.h>
#include <math.h>
#include <Keypad.h>
// pins reserved by Keypad
byte rowPins[4] = { 9, 7, 8, 6 };
byte colPins[4] = { 5, 4, 3, 2 };
char keys[4][4] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, 4, 4 );

// pin reserved by Lock
int Lock = A0;
// pins reserved by SIM900A
SoftwareSerial SIM900A(10, 11);
SoftwareSerial GPS(4, 3); // TX,RX (as written on module)
// FOR GPS
bool first_loop_exec;
#define GPS_INFO_BUFFER_SIZE 128
char GPS_info_char;
char GPS_info_buffer[GPS_INFO_BUFFER_SIZE];
unsigned int received_char;
int i; // counter
bool message_started;
char gps_loc[29];
char location[30];
int position = 0;
char ph_no[14] = {'\0'};
char check;
char otp[7] = "XXXXXX\0";
int number;
void LockedPosition(int lock_it)
{
  if (lock_it) digitalWrite(Lock, LOW);
  else digitalWrite(Lock, HIGH);
}
void set_gps_module() {
  GPS.begin(9600);
  GPS.println("Connected");

  first_loop_exec = true;
  i = 0;
  message_started = false;
}


void set_sim_module() {
  SIM900A.begin(9600);   // Setting the baud rate of GSM Module
  // command one : AT+COPS?
  SIM900A.println("AT+COPS?");
  delay(1000);
  // SIM900A speaks
  while ((SIM900A.available() > 0)) Serial.write(SIM900A.read());
  delay(100);

  // command two : AT+CLCC=1
  SIM900A.println("AT+CLCC=1");
  delay(1000);
  // SIM900A speaks
  while ((SIM900A.available() > 0)) Serial.write(SIM900A.read());
  delay(100);
  Serial.println("SIM900A ready!");
}

void set_otp_module() {
  randomSeed(analogRead(0)); // make sure this pin is UNCONNECTED AND ANALOG
}
// gets the phone number from incoming call as "+91XXXXXXXXXX"
void get_ph_no() {

  Serial.println("Inside get_ph_no()");
  while (true) {
    check = SIM900A.read(); // store each character in a char named `check`
    delay(50);
    if (check == '\"') {
      Serial.println("Hey I ran!");
      for (int i = 0; i < 13; i++) {
        ph_no[i] = SIM900A.read();
        delay(50);
      }
      break;
    }
  }

  ph_no[13] = '\0';
  // once you got the phone number, hang the call and flush the output from SIM900A
  SIM900A.println("ATH");
  delay(1000);
  while (SIM900A.available()) Serial.write(SIM900A.read());
}
// calculates a purely random OTP every time
void calc_otp() {

  Serial.println("Inside calc_otp()");
  number = random(99, 1000);
  int n = 6;
  int i;
  for ( i = 0; i < 3; i++)
  {
    otp[i] = number % 10 + '0';
    number /= 10;
  }
  number = random(99, 1000);
  for ( i = 3; i < 6; i++)
  {
    otp[i] = number % 10 + '0';
    number /= 10;
  }

  otp[n] = '\0';
  //  Serial.println("Sending you the OTP in 10...");
  delay(1000);
  Serial.println(otp);
}
// Sends an OTP to the caller
void send_otp() {

  Serial.println("Inside send_otp()");
  SIM900A.println("AT+CMGF=1");
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  char dest_set[21] = "AT+CMGS=\"XXXXXXXXXX\"\r";
  for (int i = 0; i < 10; i++) {
    dest_set[i + 9] = ph_no[i + 3];
  }
  SIM900A.println(dest_set);
  delay(1000);
  SIM900A.print("Your OTP for the scooter number DL-8SBV-4470 is: ");
  SIM900A.println(otp);
  delay(100);
  SIM900A.println((char)26);
  delay(1000);
}
void challenge() {
  Serial.println("Inside challenge()");
  while (SIM900A.available()) Serial.write(SIM900A.read());
  if (check_otp_input()) {
    LockedPosition(false);
    Serial.println("Lock has been unlocked!");
    unlock_sms(true);
  }
  else Serial.println("LOCK KEY NOT CORRECT!!");
  unlock_sms(false);
}
bool check_otp_input() {
  Serial.println("Inside check_otp_input()");
  //  Serial.println("Waiting for User OTP input");
  int t = 1200;
  position = 0;
  int pressed = 0;
  while (t--) {
    //// get keys from Keypad
    char key = Serial.read();
    if ((key >= '0' && key <= '9') || (key >= 'A' && key <= 'D') || key == '*' || key == '#') {
      //      Serial.print(key);
      pressed++;
      if (key == otp[position]) position++;
    }

    if (position == 6 && pressed == 6) {
      return true;
    }
    if (pressed >= 6) break;
    delay(100);
  }
  return false;
}
void unlock_sms(bool success) {

  SIM900A.println("AT+CMGF=1");
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  char dest_set[21] = "AT+CMGS=\"XXXXXXXXXX\"\r";
  for (int i = 0; i < 10; i++) {
    dest_set[i + 9] = ph_no[i + 3];
  }
  SIM900A.println(dest_set);
  delay(1000);
  if (success)SIM900A.println("Time to ride!");
  else SIM900A.println("You entered incorrect OTP. Please call again.");
  delay(100);
  SIM900A.println((char)26);
  delay(1000);
  while (SIM900A.available()) Serial.write(SIM900A.read());
}

void send_gps() {

  SIM900A.println("AT+CMGF=1");
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  char dest_set[21] = "AT+CMGS=\"8010070210\"\r"; // OWNER NUMBER HARSH
  delay(1000);
  SIM900A.print("Location : ");
  SIM900A.print(location);
  SIM900A.println("Scooter : DL-8SBV-4470");
  delay(100);
  SIM900A.println((char)26);
  delay(1000);
  while (SIM900A.available()) Serial.write(SIM900A.read());

}
void setup() {

  // put your setup code here, to run once:
  Serial.begin(9600);    // Setting the baud rate of Serial Monitor (Arduino)
  //  set_sim_module();
  set_otp_module();
  set_gps_module();
  // pinMode(LED_BUILTIN, OUTPUT);
  //  pinMode(Lock, OUTPUT);
  //  LockedPosition(true);
  Serial.println("READY!!!!");
}
int timer = 0;
void loop() {
  set_gps_module();
  if (first_loop_exec == true) {
    delay(2000);
    GPS.println(F("$PUBX,40,RMC,0,0,0,0*47")); //RMC OFF
    delay(100);
    GPS.println(F("$PUBX,40,VTG,0,0,0,0*5E")); //VTG OFF
    delay(100);
    GPS.println(F("$PUBX,40,GGA,0,0,0,0*5A")); //CGA OFF
    delay(100);
    GPS.println(F("$PUBX,40,GSA,0,0,0,0*4E")); //GSA OFF
    delay(100);
    GPS.println(F("$PUBX,40,GSV,0,0,0,0*59")); //GSV OFF
    delay(100);
    GPS.println(F("$PUBX,40,GLL,0,0,0,0*5C")); //GLL OFF
    delay(1000);
    first_loop_exec = false;
  }
  GPS.println("$PUBX,00*33"); // data polling to the GPS
    delay(100);
  // MANAGES THE CHARACTERS RECEIVED BY GPS
  while (GPS.available()) {
    GPS_info_char = GPS.read();
    if (GPS_info_char == '$') { // start of message
      message_started = true;
      received_char = 0;
    } else if (GPS_info_char == '*') { // end of message
      int commas = 0;
      int loc = 0;
      // PROCESSING ON OUTPUT AND EXTRACTING COORDINATES INTO BEAUTIFUL STRING HERE !!!!
      for (i = 0; i < received_char; i++) {
        //        Serial.write(GPS_info_buffer[i]); // writes the message to the PC once it has been completely received
        if (GPS_info_buffer[i] == ',') commas++;
        if (commas >= 3) gps_loc[loc++] = GPS_info_buffer[i];
        if (commas == 7) break;
      }
      gps_loc[loc] = '\0';
      int j = 0;
      // PROCESSING ON CONVERSION INTO READABLE FORMAT HERE !
      location[j++] = gps_loc[1];
      location[j++] = gps_loc[2];
      location[j++] = '°';
      location[j++] = gps_loc[3];
      location[j++] = gps_loc[4];
      location[j++] = '\'';
      location[j++] = gps_loc[6];
      location[j++] = gps_loc[7];
      location[j++] = '.';
      location[j++] = gps_loc[8];
      location[j++] = gps_loc[9];
      location[j++] = '\"';
      location[j++] = gps_loc[12];
      location[j++] = ' ';
      location[j++] = gps_loc[14];
      location[j++] = gps_loc[15];
      location[j++] = gps_loc[16];
      location[j++] = '°';
      location[j++] = gps_loc[17];
      location[j++] = gps_loc[18];
      location[j++] = '\'';
      location[j++] = gps_loc[20];
      location[j++] = gps_loc[21];
      location[j++] = '.';
      location[j++] = gps_loc[22];
      location[j++] = gps_loc[23];
      location[j++] = gps_loc[26];
            location[j] = '\0';

//      Serial.println();
      message_started = false; // ready for the new message
    } else if (message_started == true) { // the message is already started and I got a new character
      if (received_char <= GPS_INFO_BUFFER_SIZE) { // to avoid buffer overflow
        GPS_info_buffer[received_char] = GPS_info_char;
        received_char++;
      } else { // resets everything (overflow happened)
        message_started = false;
        received_char = 0;
      }
    }
  }
//  set_sim_module();
  delay(100);
  //    if (Serial.read() == '#') LockedPosition(true);
  //    if (SIM900A.available()) { // read output of SIM900A char by char
  //      Serial.println("heyo imma runnin yo!");
  //      get_ph_no();
  //      calc_otp();
  //      send_otp();
  //      challenge();
  //    }
  timer++;
  if (timer >= 10*60) {
    timer = 0;
    Serial.println(location);
    //  send_gps();
  }
}