Interfacing SIM900A GSM Modem with Arduino

This post is intended to help beginners interfacing SIM900A to Arduino.

 

 

Power supply requirement:

For RS232 type modem mentioned in this link, requires adapter/battery of 12 Volt 1-2 Ampere current rating for reliable operation. DC Jack can be used to feed the input power.

For TTL Modem mentioned in this link, use 5 Volt, 1-2 Ampere adapter or Power bank.

 

Note : Watch-out for the network led pattern.

Status                               SIM900 behavior
Off                                    SIM900 is not running
64ms On/ 800ms Off       SIM900 not registered the network
64ms On/ 3000ms Off     SIM900 registered to the network
64ms On/ 300ms Off       GPRS communication is established

The code is tested with SIM900A and Arduino UNO. Connect the Tx and Rx pins of arduino pins to Rx and Tx pins of GSM Modem.

Note: The uart pins should be cross connected, ie. GSM Tx –> Arduino Rx and GSM Rx –> Arduino Tx. Make the ground common between Arduino and GSM modem.

 

 

TTL pins can be seen just behind the 9 pin DB9 connector. If you are using DB9 connector for interfacing you need MAX232 circuitary for converting RS232 output of DB9 to TTL levels.

Copy and paste the following code to your Arduino IDE as a starting point for GSM-Arduino interfacing test.

Note : Before powering SIM900A module user should insert the SIM into the module.

Change the phone number in the below code before running the code.

/*
Author: Dhanish
Created on: 19-08-2014
Company: Elementz Engineers Guild Pvt Ltd
Run this code only after SIM900A Module has powered and connected to a Network.
Please make a call to the module knowing whether the connection is established.
If connected a ring will be heard at the caller end
*/

void setup() {
// initialize serial communications at 9600 bps:
Serial.begin(9600);

delay(10000);
// print the serial data to GSM
Serial.print("ATD9020XXXXXX;" ); // change here to call a number using SIM900A
// wait 10 seconds before the next loop
delay(10000);
Serial.print("ATH" ); // hold the call
}

void loop() {
}

Another code using Software serial is as seen below

Note: Connect as follows
         digital pin 10 — connect to TX of GSM Modem
         digital pin 11 — connect to RX of GSM Modem

/*
The circuit:
* RX is digital pin 10 (connect to TX of GSM Modem)
* TX is digital pin 11 (connect to RX of GSM Modem)
*/
#include <SoftwareSerial.h>

SoftwareSerial mySerial(10, 11); // RX, TX

void setup()
{
// Open serial communications and wait for port to open:
Serial.begin(9600);

Serial.println("Calling through GSM Modem");

// set the data rate for the SoftwareSerial port
mySerial.begin(9600);
delay(2000);
mySerial.println("ATD81290255XX;"); // ATD81290255XX; -- watch out here for semicolon at the end!!

Serial.println("Called ATD81290255XX");
}

void loop() // run over and over
{
// print response over serial port
if (mySerial.available())
Serial.write(mySerial.read());
}

Arduino sample code can also be downloaded from the following Link.

Arduino – GSM Sample code.