Sim900 GSM Module Arduino Tutorial

Programming Sim900 with Arduino

In this tutorial we will show Arduino can be programmed to send and receive AT command to/from Sim900 GSM module. The following is the Arduino program code to send/receive AT command to/from GSM module.

 #include <SoftwareSerial.h>
    
    SoftwareSerial sim900(8,7); // Rx,Tx
    char c;
  
    void setup() {
    // set serial communication with Terminal
    Serial.begin(9600);
    // set serial communication with Sim900
    sim900.begin(9600); 
    while(!Serial);
      Serial.println("Arduino is ready.");
    }
    
    void loop(){
        //Read from SIM800 and send to Terminal Serial Monitor
        if (sim900.available()){ 
          c = sim900.read();
          Serial.write(c);
          }
          //Read from Terminal Serial Monitor and send to sim900
          if (Serial.available()){ 
            c = Serial.read();
            sim900.write(c); 
          }        
    }

Since Arduino Tx and Rx pins are used for communication with PC for USB serial program transfer we cannot use it. Hence we create another serial port using SoftwareSerial library. Using SoftwareSerial library we have configured pin 8 and pin 7 of Arduino as Rx and Tx pins. We have setup the serial baud rate for communication with the terminal and for communication with the sim900 module. Both the baud rate is configured as 9600.

AT command

 Once the code is uploaded we can perform AT communication with Sim900 GSM shield. Though we can use any terminal, the Arduino serial monitor is not suitable for doing AT communication because it does not support key controls like Ctrl+Z and Esc which are often required during AT communication such as in SMS mode. Therefore here we will use Tera Term terminal software which is free for download(just google it). You can also use other terminal like Putty which is also free for download. 

After you open the terminal select the proper serial COM port. With tera term this is as shown below.

Then you should also make sure you have set up your terminal baud rate to 9600.

After this we are ready to perform AT tests. In the following examples we will show briefly some AT basic command to see settings, send/receive SMS message and call receive and making call using Sim900 GSM module.

AT commands to check Network and Sim status and other setting

The usual way to start AT command is to actually see whether there is any successful AT communication, that is whether Sim900A gives response to AT command. For this type AT and hit enter in the terminal, if the reply is OK then AT command works and you are good to go.

We can then inquire many information related to GSM network and Sim card. For example we can inquire whether the sim card in the sim900 module is registered or not using the AT command AT+CREG? We can get signal strength parameter using the AT command AT+CSQ. We can get sim information using the AT+CCID command. The command AT+CPIN can be used to check whether the sim card is inserted or not. We can also ask for baud rate setting using the AT+IPR=? command.

All these AT commands entered and response from Sim900 module is shown in the picture below.

Send SMS using AT command

To send SMS using Sim900 GSM module using AT command follow the steps below.

1. Set SMS mode 

Command: at+cmgf=1 (press enter)

Reply: OK

2. Enter the phone number to send SMS message

Command: at+cmgs="+XXXXXXXXXXXX"

Reply: >

3. Write message 

For Example: >ee-diary.blogspot.com

4. To send message and exit from SMS mode press Ctrl+Z on the keyboard. To escape from SMS mode pess Esc key on keyword 

Below picture shows how to send SMS message using Sim900 

Receive SMS using AT command

To receive SMS on Sim900 follow the following steps.

1. Set SMS mode 

Command: at+cmgf=1(Enter)

Reply: OK

2. Set SMS message reading mode to read newly arrived message

Command: at+cnmi=2,2,0,0 (press enter)

Reply:OK

3.If there is any new message it will displayed

For example, sms message "This is my sms msg" was sent from mobile phone to Sim900 and the following picture shows how to receive SMS message using Sim900 and the SMS message that was received.

AT command to start voice call

To call a number type the command atd+[YourNumber]; in the terminal, you will get OK as reply if there is a successful connection. 

command: atd+[YourNumber];
reply: OK

If the dialed user cut off the incoming call then busy will displayed:

reply: BUSY

See the picture below to make a call using Sim900 via tera term terminal.

AT command to receive voice call

 When you dial the sim900 number, you will see RING message in the terminal. To receive the voice call type ata and to hang up the call type ath. This is illustrated with tera term terminal below.

 
In Summary, we have shown here how to program Arduino to communicate to the Sim900 using terminal by sending AT commands. We have shown how to send AT command to get network and other setting from the Sim900 module. We have shown how to send and receive SMS using Sim900A module. We have also shown how to make phone call and how to receive phone call using Sim900 GSM shield. GSM module such as Sim900A, Sim900 can be used for application where voice or data communication over mobile network is required. For example, Sim900A can be used to create Internet of Things(IoT) application where sensors or actuators are controlled over the mobile network. Although such wireless solution can be achieved using ESP8266 WiFi module WiFi communication the advantage of using GSM based IoT over WiFi IoT is that GSM wireless coverage will be larger and there will be more bandwidth available to send/receive data and voice controlled application can be build easily.

See next the tutorial on how to Send and Receive SMS with Sim900 & Arduino.