Receive an array of characters using SIM900 module
#use this code and then inform me"
#include &lt;SoftwareSerial.h&gt;
SoftwareSerial SIM900(7, 8);

void setup()
{
    pinMode(3,OUTPUT);
    Serial.begin(19200); // for serial monitor
    SIM900.begin(19200); // for GSM shield
    SIM900power(); // turn on shield
    delay(20000); // give time to log on to network. ` 
    SIM900.print("AT+CMGF=1\r"); // set SMS mode to text
    SIM900.write(0x22);
    SIM900.write(0x0D);
    SIM900.write(0x0A);
    delay(100);
    SIM900.print("AT+CNMI=2,2,0,0,0\r");
    // blurt out contents of new SMS upon receipt to the GSM shield's serial out
    delay(100);
} 

void SIM900power()
// software equivalent of pressing the GSM shield "power" button24 
{
    digitalWrite(9, HIGH);
    delay(1000);
    digitalWrite(9, LOW);
    delay(7000);
}

void loop()
{
    // Now we simply display any text that the GSM shield sends out on the serial monitor
    if(SIM900.available() &gt;0)
    {
        char *in = SIM900.read();
        if (in =="abcd") digitalWrite(3,HIGH);
        if (in == "00") digitalWrite(3,LOW);
    }
}