Experiment 7: GSM module, SMS and calls

1#include<SoftwareSerial.h>2#define Tx 103#define Rx 114 5SoftwareSerial sim (Tx,Rx);6 7int timeout;8String buffr;9String number = "+91XXXXXXXXXX";10 11void setup() {12  // put your setup code here, to run once:13  Serial.begin(9600);14  buffr.reserve(50);15  Serial.println("System Started");16  sim.begin(9600);17  delay(1000);18  Serial.println("Type s to send msg, r to recieve msg, c to make a call");19 20}21 22void loop() {23  // put your main code here, to run repeatedly:24  if(Serial.available()>0){25     switch(Serial.read())26      {27        case 's':28          sendMessage();29          break;30        case 'r':31          recieveMessage();32          break;33        case 'c':34          callNumber();35          break;36      }37   }38  if (sim.available()>0){39    Serial.write(sim.read());40  }41}42void sendMessage(){43  Serial.println("Sending message: ");44  sim.println("AT+CMGF=1"); //set the GSM modul in text mode45  sim.println("AT+CMGS=\"" + number + "\"\r");46  delay(200);47  String sms = "Hello from the IoT lab GSM module";48  sim.println(sms);49  delay(100);50  sim.println((char)26); //ASCII code of ctrl+Z51  delay(200);52  buffr=readSerial();53}54 55void recieveMessage(){56  Serial.println("SIM800L read an SMS");57  sim.println("AT+CMGF=1");58  delay(200);59  sim.println("AT+CNMI=2,2,0,0,0"); //AT command to read live msg60  delay(200);61  Serial.write("Unread msg done");62}63 64String readSerial(){65  timeout=0;66  while(!sim.available() && timeout < 12000){67    delay(13);68    timeout++;69  }70  if (sim.available()){71    return sim.readString();72  }73}74 75void callNumber(){76  sim.print(F("ATD"));77  sim.print(number);78  sim.print(F(";\r\n"));79  buffr=readSerial();80  Serial.println(buffr);81  }

1. Line 1
   A UART implemented by the CPU on ordinary pins.
2. Lines 2 to 3
   Named from the module’s side: pin 10 receives the module’s TX, pin 11 drives the module’s RX.
3. Line 5
   Constructor is (rxPin, txPin): Arduino RX = 10, TX = 11.
4. Lines 7 to 9
   Wait counter, reply buffer, and the target number (masked in your file).
5. Lines 13 to 15
   PC link; reserve(50) pre-allocates the String to reduce heap fragmentation.
6. Line 16
   Modem link at 9600 baud.
7. Line 18
   The menu.
8. Lines 24 to 33
   Dispatch: read one character and call the matching action.
9. Lines 38 to 39
   Relay: one byte from the modem to the PC per pass.
10. Lines 42 to 44
    Text mode.
11. Line 45
    Start the message to number; the modem will answer with >.
12. Lines 47 to 48
    The message body.
13. Line 50
    Ctrl+Z (byte 26) ends and sends the message. ESC (27) would cancel.
14. Line 52
    Collect the reply (+CMGS: n and OK on success).
15. Lines 55 to 59
    Text mode, then route new messages straight to serial; the relay in loop() prints them when they arrive.
16. Line 61
    Only a confirmation that the mode is set; messages arrive later.
17. Lines 64 to 71
    Wait up to 12000 × 13 ms ≈ 156 s for the modem to start replying, then read until the stream goes quiet.
18. Lines 75 to 78
    ATD<number>; dials a voice call. F() keeps the literal in flash instead of RAM.