Solución

Solo estoy publicando la solución si alguien la necesita. Todo el mundo parece rechazar el problema en lugar de dar soluciones.

• En primer lugar, quite todos los puentes [ver la imagen]

• LuegoagreguedoscablesmachoahembraalS-TXyalS-RX[AquíSsignificaSIM900,puedeencontrarlospinesdespuésdequitarlospuentes.]

• CortalosmotivosdelosmódulosarduinoyATK-SIM900A

• S-TXdebeestarconectadoalarduinoRXyS-RXdebeestarconectadoalarduinoTX.

• Normalmenteenciendaelmódulo[NoseolvidedepresionarPWRKeydespués]

Código

#include<SoftwareSerial.h>#defineBAUD9600#defineRX10#defineTX11SoftwareSerialgsm(RX,TX);int8_tsendATcommand(char*ATcommand,char*expected_answer,unsignedinttimeout){uint8_tx=0,answer=0;charresponse[100];unsignedlongprevious;memset(response,'
#include<SoftwareSerial.h>#defineBAUD9600#defineRX10#defineTX11SoftwareSerialgsm(RX,TX);int8_tsendATcommand(char*ATcommand,char*expected_answer,unsignedinttimeout){uint8_tx=0,answer=0;charresponse[100];unsignedlongprevious;memset(response,'%pre%',100);//Initializethestringdelay(100);while(gsm.available()>0)gsm.read();//Cleantheinputbuffergsm.println(ATcommand);//SendtheATcommandx=0;previous=millis();//thisloopwaitsfortheanswerdo{//iftherearedataintheUARTinputbuffer,readsitandchecksfortheasnwerif(gsm.available()!=0){response[x]=gsm.read();x++;Serial.println(response);//checkifthedesiredanswerisintheresponseofthemoduleif(strstr(response,expected_answer)!=NULL){answer=1;}}//Waitsfortheasnwerwithtimeout}while((answer==0)&&((millis()-previous)<timeout));returnanswer;}voidsetup(){//putyoursetupcodehere,torunonce:Serial.begin(BAUD);gsm.begin(BAUD);}//Codeforsendingsms,replacethestarsandcountrycodewithyourcorrespondingphonenumbervoidloop(){sendATcommand("AT+CMGF=1\r\n", "OK", 500);
  sendATcommand("AT+CMGS=\"+880******582\"\r", "OK", 500);
  delay(500);
  gsm.println("Hello world"); //The message to be sent
  delay(500);
  gsm.println((char)26); // Done
  delay(500);
}
', 100);    // Initialize the string

  delay(100);

  while ( gsm.available() > 0) gsm.read();   // Clean the input buffer

  gsm.println(ATcommand);    // Send the AT command


  x = 0;
  previous = millis();

  // this loop waits for the answer
  do {
    // if there are data in the UART input buffer, reads it and checks for the asnwer
    if (gsm.available() != 0) {
      response[x] = gsm.read();
      x++;
      Serial.println(response);
      // check if the desired answer is in the response of the module
      if (strstr(response, expected_answer) != NULL)
      {
        answer = 1;
      }
    }
    // Waits for the asnwer with time out
  } while ((answer == 0) && ((millis() - previous) < timeout));

  return answer;
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(BAUD);
  gsm.begin(BAUD);
}


//Code for sending sms, replace the stars and country code with your corresponding phone number

void loop() 
{
  sendATcommand("AT+CMGF=1\r\n", "OK", 500);
  sendATcommand("AT+CMGS=\"+880******582\"\r", "OK", 500);
  delay(500);
  gsm.println("Hello world"); //The message to be sent
  delay(500);
  gsm.println((char)26); // Done
  delay(500);
}

Gracias a todos.