GSM модем SIM800L V2.0 Первый взгляд

#include "SIM900.h"
#include <SoftwareSerial.h>
#include "sms.h"
SMSGSM sms;

int numdata;
boolean started=false;
char smsbuffer[160];
char n[20];

void setup() 
{
                pinMode(13, OUTPUT);//Вывод, на котором установлен диод
                digitalWrite(13, LOW);  //Выключаем диод
  //Serial connection.
  Serial.begin(9600);
  Serial.println("GSM Shield testing.");
  //Start configuration of shield with baudrate.
  //For http uses is raccomanded to use 4800 or slower.
  if (gsm.begin(2400)){
    Serial.println("\nstatus=READY");
    started=true;  
  }
  else Serial.println("\nstatus=IDLE");
  
  if(started){
    //Enable this two lines if you want to send an SMS.
    //if (sms.SendSMS("3471234567", "Arduino SMS"))
      //Serial.println("\nSMS sent OK");
  }

};

void loop() 
{
  if(started){
    //Read if there are messages on SIM card and print them.
    if(gsm.readSMS(smsbuffer, 160, n, 20))
    {
      //Serial.println(n);
         Serial.println(smsbuffer);
         if (smsbuffer[0]=='f')       //если первая буква смс f
         {digitalWrite(13, LOW);} //выключаем диод
        if (smsbuffer[0]=='o')       //если первая буква смс o
         {digitalWrite(13, HIGH);}//включаем диод
//туда можно написать любой симфол на латиннице, с русским шрифтом не дружит   
 }
    delay(1000);
  }
};

#include "SIM900.h"
#include <SoftwareSerial.h>

#include "sms.h"
SMSGSM sms;

int numdata;
boolean started=false;
char smsbuffer[160];
char n[20];
        char mes1[2]={'O','n'};    //сообщения, которые ждем
        char mes2[3]={'O','f','f'};
void setup() 
{
  pinMode(13, OUTPUT);
  digitalWrite(13, LOW);
  //Serial connection.
  Serial.begin(9600);
  Serial.println("GSM Shield testing.");
  //Start configuration of shield with baudrate.
  //For http uses is raccomanded to use 4800 or slower.
  if (gsm.begin(2400)){
    Serial.println("\nstatus=READY");
    started=true;  
  }
  else Serial.println("\nstatus=IDLE");
  
  if(started){
  }

};

void loop() 
{
  if(started){
    //Read if there are messages on SIM card and print them.
    if(gsm.readSMS(smsbuffer, 160, n, 20))
    {
      Serial.println(smsbuffer);
             int b=0;
             int c=0;
             for (int i=0;i<2;i++)
      {
      if (smsbuffer[i]==mes1[i])
      {b++;}
      if(b==2){digitalWrite(13, HIGH);}
      }
            for (int i=0;i<3;i++)
      {
      if (smsbuffer[i]==mes2[i])
      {c++;}
      if(c==3){digitalWrite(13, LOW);}
      }
    }
    delay(1000);
  }
};

Это последняя эволюция кода с подключением библиотеки string.h
#include <string.h>

#include "SIM900.h"
#include <SoftwareSerial.h>

#include "sms.h"
SMSGSM sms;

int numdata;
boolean started=false;
char smsbuffer[160];
char n[20];
char mes1[]="On\r";    //сообщения, которые ждем
char mes2[]="Off\r";
void setup() 
{
  pinMode(13, OUTPUT);
  digitalWrite(13, LOW);
  //Serial connection.
  Serial.begin(9600);
  Serial.println("GSM Shield testing.");
  //Start configuration of shield with baudrate.
  //For http uses is raccomanded to use 4800 or slower.
  if (gsm.begin(2400)){
    Serial.println("\nstatus=READY");
    started=true;  
  }
  else Serial.println("\nstatus=IDLE");
  
  if(started){
  }

};

void loop() 
{
  if(started){
    //Read if there are messages on SIM card and print them.
    if(gsm.readSMS(smsbuffer, 160, n, 20))
    {
      Serial.println(smsbuffer);
      if(strcmp(smsbuffer, mes1) == 0){
          digitalWrite(13, HIGH);
      }
      if(strcmp(smsbuffer, mes2) == 0){
          digitalWrite(13, LOW);
      }
    }
    delay(1000);
  }
};