How to Connect SIM900-TTL GSM/GPRS Modem to Arduino Mega?

          SIM900-TTL GSM/GPRS modem works as a response for some AT commands given to modem. Various methods are there to give AT commands to modem. Either it can be directly given from a computer through serial port. A software interface is there to interact with modem when connected to computer. Microcontrollers can also be used. A simple method to connect microcontroller to SIM900-TTL GSM/GPRS modem is using an arduino board.

          Six pins of SIM900-TTL GSM/GPRS modem is used for connecting to arduino board. Those pins are Vin (5V), Gnd, Vinterface, RXD and TXD. Top view of front side of SIM900-TTL GSM/GPRS modem, purchased from rhydolabz is given below. Modem pins used for connecting to arduino is labelled in the image.

Top view of back side of same modem is given below.

         Circuit for connecting SIM900-TTL GSM/GPRS modem to arduino mega is done as shown in the following diagram.

          Vin (5V) is given from a 4.2V-12V power supply. This power supply should be capable of giving an output current of 1A. Ground of this power supply should be connected to the Gnd pin of modem next to the Vin (5V) pin. Vinterface pin of modem is connected to the 5V pin of arduino. RXD pin of modem is connected to the TX0 pin of arduino. TXD pin of modem is connected to the RX0 pin of arduino. Gnd pin next to the RXD pin of modem is connected to the Gnd pin of arduino. If the circuit is completed, upload the following program to your arduino board.

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void setup() 
{
  Serial.begin(2400);
  Serial.write("AT+CMGF=1\r"); // Set GSM to text mode
  delay(1500);

  Serial.write("+++");         // Test Command
  delay(1000);
  while(1)
  {
    if(Serial.available())
    {
      Serial.write(Serial.read());  
    }   
  }   
}

void loop() 
{

}

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          '+++' is the test command. If the circuit is correct, modem will reply 'OK'. This can be seen in serial monitor. Don't forget to change the baud rate to 2400 in serial monitor. Screenshot of serial monitor is given below.