How to Interface GSM Module to Arduino-Send and Receive SMS

jojo December 23, 2016 61 Comments

In this article, we are going to see how to interface GSM Module to Arduino. There are different kinds of GSM modules available in market. We are using the most popular module based on Simcom SIM900 and Arduino Uno for this tutorial. Interfacing a GSM module to Arduino is pretty simple. You only need to make 3 connections between the gsm module and arduino. So lets get to business!

A GSM Module is basically a GSM Modem  (like SIM 900) connected to a PCB with different types of output taken from the board – say TTL Output (for Arduino, 8051 and other microcontrollers) and RS232 Output to interface directly with a PC (personal computer). The board will also have pins or provisions to attach mic and speaker, to take out +5V or other values of power and ground connections. These type of provisions vary with different modules.

Lots of varieties of GSM modem and GSM Modules are available in the market to choose from. For our project of connecting a gsm modem or module to arduino and hence send and receive sms using arduino – its always good to choose an arduino compatible GSM Module – that is a GSM module with TTL Output provisions.

GSM Module – Buyers Guide – are you looking to buy a GSM module? There are a handful of product variants for GSM module – like SIM900, SIM300, SIM800 etc. We have created this buyers guide to help you select the right GSM module for your project needs.

Notes on GSM Module

1. We use SIM900 GSM Module – This means the module supports communication in 900MHz band. We are from India and most of the mobile network providers in this country operate in the 900Mhz band. If you are from another country, you have to check the mobile network band in your area. A majority of United States mobile networks operate in 850Mhz band (the band is either 850Mhz or 1900Mhz). Canada operates primarily on 1900 Mhz band. Please read this wiki entry on GSM Frequency Bands around the World.

2. Check the power requirements of GSM module – GSM modules are manufactured by different companies. They all have different input power supply specs. You need to double check your GSM modules power requirements. In this tutorial, our gsm module requires a 12 volts input. So we feed it using a 12V,1A DC power supply. I have seen gsm modules which require 15 volts and some other types which needs only 5 volts input. They differ with manufacturers. If you are having a 5V module, you can power it directly from Arduino’s 5V out.

Note:- GSM Modules are manufactured by connecting a particular GSM modem to a PCB and then giving provisions for RS232 outputs, TTL outputs, Mic and Speaker interfacing provisions etc. The most popular modem under use is SIM 900 gsm modem from manufacturer SIMCom. They also manufacture GSM Modems in bands 850, 300 and other frequency bands.

3. Check for TTL Output Pins in the module – You can feed the data from gsm module directly to Arduino only if the module is enabled with TTL output pins. Otherwise you have to convert the RS232 data to TTL using MAX232 IC and feed it to Arduino. Most of the gsm modules in market are equipped with TTL output pins. Just ensure you are buying the right one.

So that’s all about the gsm module basics. Now lets power it up!

Arduino to GSM Modem - Sim 900 Modem


Booting the GSM Module!

1. Insert the SIM card to GSM module and lock it.

2. Connect the adapter to GSM module and turn it ON!

3. Now wait for some time (say 1 minute) and see the blinking rate of ‘status LED’  or ‘network LED’ (GSM module will take some time to establish connection with mobile network)

4. Once the connection is established successfully, the status/network LED will blink continuously every 3 seconds. You may try making a call to the mobile number of the sim card inside GSM module. If you hear a ring back, the gsm module has successfully established network connection.

Okay! Now let’s see how to connect a gsm module to Arduino!

Connecting GSM Module to Arduino

There are two ways of connecting GSM module to arduino. In any case, the communication between Arduino and GSM module is serial. So we are supposed to use serial pins of Arduino (Rx and Tx). So if you are going with this method, you may connect the Tx pin of GSM module to Rx pin of Arduino and Rx pin of GSM module to Tx pin of Arduino. You read it right ? GSM Tx –> Arduino Rx and GSM Rx –> Arduino Tx. Now connect the ground pin of arduino to ground pin of gsm module! So that’s all! You made 3 connections and the wiring is over! Now you can load different programs to communicate with gsm module and make it work.

Note:- The problem with this connection is that, while programming Arduino uses serial ports to load program from the Arduino IDE. If these pins are used in wiring,  the program will not be loaded successfully to Arduino. So you have to disconnect wiring in Rx and Tx each time you burn the program to arduino. Once the program is loaded successfully, you can reconnect these pins and have the system working!

To avoid this difficulty, I am using an alternate method in which two digital pins of arduino are used for serial communication. We need to select two PWM enabled pins of arduino for this method. So I choose pins 9 and 10 (which are PWM enabled pins). This method is made possible with the SoftwareSerial Library of Ardunio. SoftwareSerial is a library of Arduino which enables serial data communication through other digital pins of Arduino. The library replicates hardware functions and handles the task of serial communication.

I hope you understood so far!  Lets get to the circuit diagram! So given below is the circuit diagram to connect gsm module to arduino – and hence use the circuit to send sms and receive sms using arduino and gsm modem.

GSM Mdule to Arduino - Circuit

Make the connections as shown! Now lets get to the coding part. The program has two objectives as described below:-

1) Send SMS using Arduino and GSM Module – to a specified mobile number inside the program

2) Receive SMS using Arduino and GSM Module – to the SIM card loaded in the GSM Module.

The Program
 #include <SoftwareSerial.h>

SoftwareSerial mySerial(9, 10);

void setup()
  mySerial.begin(9600);   // Setting the baud rate of GSM Module  
  Serial.begin(9600);    // Setting the baud rate of Serial Monitor (Arduino)

void loop()
  if (Serial.available()>0)
    case 's':
    case 'r':

 if (mySerial.available()>0)

 void SendMessage()
  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"+91xxxxxxxxxx\"\r"); // Replace x with mobile number
  mySerial.println("I am SMS from GSM Module");// The SMS text you want to send
   mySerial.println((char)26);// ASCII code of CTRL+Z

 void RecieveMessage()
  mySerial.println("AT+CNMI=2,2,0,0,0"); // AT Command to receive a live SMS

so that’s the program/code to make arduino send sms and receive sms using gsm module! Let’s get to explanation of program!

The Program Explanation

We begin by including SoftwareSerial library into the program.  In the next line, we create a constructor of SoftwareSerial with name mySerial and we pass the digital pin numbers as parameters. The actual format is like SoftwareSerial mySerial (Rx, Tx);

So in our code, pin number 9 will act as Rx of Arduino and 10 will act as Tx of Arduino.  Lets get to the configuration part of program inside setup. The first task is to set baud rates of SoftwareSerial library to communicate with GSM module. We achieve this by invoking mySerial.begin function. Our second task is to set the baud rate of Arduino IDE’s Serial Monitor. We do this by invoking Serial.begin function. Both should be set at the same baud rate and we use 9600 bits/second here in our tutorial.  Configuration part is over with setting baud rates and its good to give a small delay of 100 milli seconds.

Now lets get to the actual program inside loop(). To make things simpler, I have developed a user input based program. The program seeks user input via serial monitor of Arduino. If the input is ‘s’ the program will invoke function to send an sms from GSM module. If the user input is ‘r’, the program will invoke the function to receive a live SMS from GSM module and display it on serial monitor of Arduino. The whole program is as simple as that!

Serial.available() – checks for any data coming through serial port of arduino. The function returns the number of bytes available to read from serial buffer. If there is no data available, it returns a -1 (value less than zero). – Reads all the data available on serial buffer (or incoming serial data if put otherwise). Returns the first byte of incoming serial data.

mySerial.available() – checks for any data coming from GSM module through the SoftwareSerial pins 9 and 10. Returns the number of bytes available to read from software serial port. Returns a -1 if no data is available to read. – Reads the incoming data through software serial port.

Serial.write() – Prints data to serial monitor of arduino. So the function Serial.write( – prints the data collected from software serial port to serial monitor of arduino.

Lets get the functions SendMessage()  and RecieveMessage()

These are the functions in which we actually send commands to GSM module from Arduino. These commands to communicate with GSM module are called AT Commands. There are different commands to perform different tasks using the GSM module. You can read complete AT Commands Library to understand all that is possible with GSM module.

SendMessage() – is the function we created in our arduino sketch to send an SMS. To send an SMS, we should set our GSM module to Text mode first. This is achieved by sending an AT Command “AT+CMGF=1”  We send this command by writing this to SoftwareSerial port. To achieve this we use the mySerial.println() function. mySerial.println writes data to software serial port (the Tx pin of our Software Serial – that is pin 10) and this will be captured by GSM module (through its Rx pin). After setting the GSM module to Text mode, we should the the mobile number to which we shall send the SMS. This is achieved with AT command “AT+CMGS=\”+91xxxxxxxxxx\”\r” – where you may replace all x with the mobile number.

In next step, we should send the actual content of SMS. The end of SMS content is identified with CTRL+Z symbol. The ASCII value of this CTRL+Z is 26. So we send a char(26) to GSM module using the line mySerial.println((char)26); Each and every AT command may be followed by 1 second delay. We must give some time for GSM module to respond properly. Once these commands are send to GSM module, you shall receive an SMS in the set mobile number.

RecieveMessage() – is the function to receive an SMS (a live SMS). The AT command to receive a live SMS is “AT+CNMI=2,2,0,0,0” – we just need to send this command to GSM module and apply a 1 second delay. Once you send this command, try sending an SMS to the SIM card number put inside GSM module. You will see the SMS you had sent displayed on your Arduino serial monitor.

There are different AT commands for different tasks. If you want to read all SMS’s stored in your SIM card, send the following AT Command to gsm module – “AT+CMGL=\”ALL\”\r”

Okay! That’s all and you are done. And you have learnt how to use arduino to send sms and receive sms message with example code.

I shall summarize this tutorial on how to send/receive a text message using arduino and gsm module with the following notes:-

AT Commands to Send SMS using Arduino and GSM Module
AT+CMGF=1 // Set the GSM module in text mode
AT+CMGS=\"+YYxxxxxxxxxx\"\r // Input the mobile number| YY is country code
“the message” with stopping character (char)26 // ASCII of ctrl+z
AT Commands to Receive SMS using Arduino and GSM Module
AT+CMGF=1 // Set the GSM Module in text mode
AT+CNMI=2,2,0,0,0 // AT Command to receive live sms

Read the AT commands library and start playing with your GSM module and Arduino! If you have any doubts please ask in comments.

  • sanjeevi
    April 3, 2017

    Hi. I need to send sms for multiple users using the same program…?
    Kindly give me a solution as soon as possible

    • jojo
      April 7, 2017

      @ sanjeevi – Save mobile numbers of users into an array. Execute the AT Commands to send SMS in a loop and call mobile numbers from array via loop iteration.

  • jane
    July 19, 2016

    only the sending sms part works well but with the receiving part it doesn’t. my serial port doesnt show anything in it, its a plain blank space.

  • Navonita Sharma
    July 15, 2016

    Thanks for the tutorial and program. I have implemented your code but while running the program with my arduino UNO and GSM module SIM900A, no message is received in the provided mobile number. The serial monitor shows error in sending the message. Kindly suggest the possible cause for this error.

  • Sagarika
    June 28, 2016

    my serial monitors notifies me when a msg is received but doesnt show what the msg is?

  • Stan
    April 3, 2016

    Hi Jojo,
    I follow the steps you described.
    Everything works well.
    But when I try to plug the shield onto my UNO, nothing works..
    Currently I use this shield
    As I said, even with this shield, your tutorial does the job.
    I suppose I have to change the pins ?
    Any Idea ?

  • Dibyendu Sur
    April 2, 2016

    we have used this code:

    char phone_no[]=”9062261355″;
    void setup()

    // put your setup code here, to run once:


    void loop() {
    // put your main code here, to run repeatedly:


    In serial monitor the following characters are printed


    But no phone calls are forwarded to the phone number 9062261355
    Please help in this matter

    • jojo
      April 2, 2016

      Please try adding country code. Also the delay seems to be very high. Just a 1 second or 2 second delay is enough.

      • Dibyendu Sur
        April 2, 2016

        I have just used country code(India +91)
        I have reduced the delay before ATH to 2000 (2 second) as well as 10000 (10 second)
        every time, only the text is printed in serial monitor, but no call is forwarded to the phone number

        • Dibyendu Sur
          April 2, 2016

          I am using Arduino Uno version 1.7.8

          • DIBYENDU SUR
            April 2, 2016

            Please reply soon

    • Sagarika
      June 28, 2016

      you need to put in command “ATD 98xxxxxxxxxx;”

  • April 1, 2016

    Hello sir , I have a querry what should i do if i wanted to replace the number in


    with a variable which can store the number , For example


  • raam
    March 29, 2016

    hai, your code is working perfectly with my arduino mega. but I need to send GPS coordinates via SMS. how can I do that cos softwareserial can’t be used two times at a time. Can you advise on this?? when I use hardware serial for GPS delay was not taken.

  • smruthi
    March 25, 2016

    Sir,im doing a project including arduino and gsm.i need the code for entering the phone number manually using keypad and send sms to it.will you please help me…..

  • March 20, 2016

    Hay sir,
    I already tested your code. it works perfectly.
    But, i want to modify to turn on/off the LED and not success.
    Could you help me, for this problem?
    Just simple, i want turn on LED 13, after recieve sms “on”.

    thank you,

    • jojo
      March 20, 2016

      @albert – It is very simple. I assume you want to turn an LED at pin 13 ‘ON’ after receiving an sms. First assign a variable name to pin 13 with command – const int LED = 13; In the next step configure pin as OUTPUT. Inside void setup() – write command – PinMode(LED,OUTPUT). Now write the following command inside RecieveMessage() method after delay(1000); – command – digitalWrite(LED,HIGH); delay(1000); For more details – refer –

      • albert
        March 21, 2016

        Dear Sir,
        I already know how to control on/off the LED in arduino. The problem is, if i want to turn on the LED by send sms “ON_LED” and then send sms “OFF_LED” to turn off the LED.
        This my code, still not working for turn on/off the LED.

        char inchar;

        void RecieveMessage()
        SIM900.println(“AT+CNMI=2,2,0,0,0”); // AT Command to receive a live SMS
        if (inchar==’ON_LED’)
        digitalWrite(led, HIGH);
        else if (inchar==’OFF_LED’)
        digitalWrite(led, LOW);

        whats the problem?
        thank you

  • hemanth
    March 18, 2016

    hi sir …..
    i am gud with ur program…i need some help in creating a new program in which i am using ultrasonic sensor hc sr-04 and interfacing sim900…arduino should send the mesg to the no. when the distance from the ultrasonic sensor reaches 15cms…can u please send me the code for that

  • siva
    March 18, 2016

    My gsm module responds to first received message but not for the second one. what exactly is the problem ?

    • jojo
      March 18, 2016

      @siva – the program we have written is an example for interfacing GSM Module to Arduino. The program is written in such a way that it scans any input from serial monitor of arduino continuously. Once you press ‘r’ on serial monitor, the program will call method to receive message from GSM module. This message is then written to serial monitor of arduino for display. Once that process is over, program again scans for a command (‘s’ or ‘r’) from serial monitor. So if you want to receive second message from gsm module, press ‘r’ again on serial monitor. If you to receive 3rd message, press ‘r’ again (after second message is displayed on serial monitor)

      Note:- You need to alter this program to receive messages continuously.

    March 18, 2016

    hai, i am from SriLanka. new to this field. I have tried this but this is not working for me. I am using Arduino Mega and SIM900A GSM/GPRS module which is shown in this link
    Please can you help me on this??

    • jojo
      March 18, 2016

      I have seen your GSM Module image here –
      Here are my thoughts!

      1. You should connect it to TTL pins Rx, Tx and Ground. GSM Rx to Arduino Tx and GSM Tx to Arduino Rx. GSM Gnd to Arduino Gnd.
      2. Dont worry about other jumper pins. Just make the connection.
      3. After inserting SIM Card, Press and Hold Power button. Notice the Network Status LED marked as NET_STA on your GSM board. This LED should blink in 3 seconds delay continuously.
      4. Make a call to the sim card number inside GSM module. If you hear ringing sound on your phone,the interfacing of GSM module to Arduino is perfect.

      • SHAYAN
        March 18, 2016

        Hi again,
        I have followed your way already and I was able to hear ring back tone. But when try the code, Serial.available was not read in serial monitor. I don’t know what is wrong. Can you guess what will be the reason for this? but my Arduino serial ports are working properly.

        • jojo
          March 18, 2016

          Which pins are you using ? If you are using SoftwareSerial library for communicating with Arduino, then select any 2 PWM enabled pins instead of Rx and Tx of Arduino (like pins 9 and 10 in Arduino uno)

          • SHAYAN
            March 19, 2016

            Thank you for your information. i have tried but there is no response yet. could you please tell me that do i need to edit coding rather than change the pin (in mega2560 PWM pins can be 4-12) and the destination mobile number??

          • jojo
            March 19, 2016

            Note these points.

            Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15, 50, 51, 52, 53, A8 (62), A9 (63), A10 (64), A11 (65), A12 (66), A13 (67), A14 (68), A15 (69).

            Refer –

            1.Make sure pins are configured in software correctly.
            2. Try changing the delay given in program (inside SendMessage() method)
            3. Make sure your country code is given correctly in mobile number. +91 – is country code of India. Change it to your country.

          • SHAYAN
            March 19, 2016

            Hi Sir, there is improvement in sending a SMS. I have used 50,51 pins and my no is given correctly. How can I use Mega Rx,Tx directly to interface SIM900A with Arduino?? Could you please explain??

          • SHAYAN
            March 19, 2016

            Hi Sir, there is no improvement in sending a SMS. I have used 50,51 pins and my no is given correctly. How can I use Mega Rx,Tx directly to interface SIM900A with Arduino?? Could you please explain??

          • SHAYAN
            March 23, 2016

            Hello sir,
            your code works perfectly. really really thanks for guiding me. there is one more change that i wanted to do to get successful. finally works with pin 11 and 12.

  • nsimi
    March 18, 2016

    Kindly explain how to send this ‘s’ and ‘r’ variables as input in order to call receive or send message function.

    • jojo
      March 18, 2016

      @nismi – Open the “Serial Monitor”of Arduino. Type s and hit enter. Once you are finished with sending message, type r and press enter button.

  • March 17, 2016

    I am not getting serial commends for this program? what to do for getting

    • jojo
      March 18, 2016

      @suraj – What is the problem you are facing ? Serial commands are written in the program. To send ‘s’ and ‘r’ commands serially, open the serial monitor of Arduino.

  • Chandar
    March 14, 2016

    In my project I am out off all digital pin except Rx and Tx. Can I use the software serial library for Rx and Tx. Like instead of mentioning Softwareserial myserial (9,10) , can I mention Softwareserial myserial (Rx,Tx)?
    If not can you send the code for Rx,Tx.

    • jojo
      March 15, 2016

      @chandar – Software Serial actually makes use of the Rx and Tx hardware internally. You better connect directly to Rx and Tx and use the serial communication commands of Arduino. Refer –

  • chandra mouli
    March 13, 2016

    hi sir

    do we require any converter from rs232 to ttl in between the microcontroller and the sim900 modem

    • jojo
      March 14, 2016

      @chandra mouli – Check if your GSM module has TTL output provision. Most of the GSM modules available in market has TTL output provisions. If the module has only RS232 output provision, you need to use an RS232 to TTL converter to interface that particular module with Arduino.

  • March 12, 2016

    hi iam trying your program but it not working

    the message not send automatically…..
    what can I do now

    • jojo
      March 13, 2016

      @Karthikeyan – We have tested this code many times repeatedly and it was working 100% all the time. Your issue may not be with the code. Double check hardware connections! Also check if you have input the mobile number correctly!

  • Ali
    March 9, 2016

    Good day sir,
    Please can you explain how can we send the input reading through GSM modem. For example i want ardinuo to take input reading and then send it through sms. How can we implement this please?

    • jojo
      March 9, 2016

      @Ali – Its very simple. Save the input to a variable or array. Now write the variable values one by one serially to GSM module using Serial.println() command. In our example we are using Software Serial method, so we use mySerial.println() method. If input variable is val_input, then write the following to GSM module – mySerial.println(val_input);

  • March 6, 2016

    sir we have to perform operation based on sms coming to that no
    eg: if on is come then blow a light
    otherwise off the light
    plzzz help me out for this

    • jojo
      March 6, 2016

      @Abdul Malik – Its actually simple. You first learn AT Commands to receive an SMS. Once you receive the SMS, save its content to a variable or array. Scan the array to check for required number/digit/character.

  • wooja
    March 1, 2016

    good day sir! i tried to display the text that the GSM shield sends out on the serial monitor. the output was this

    +CMT: “+639XXXXXXXX”,””,”16/03/02,8:31:09:+32″
    hello world

    what does +32 mean? and where is it from?

    • jojo
      March 1, 2016

      @wooja – I don’t have a good idea of the +32. It could be some number of characters sent or something like that. Refer AT Commands Datasheet.

  • February 16, 2016

    can you please help me with a simple example of how the aduino can respond to any remote number from a mobile phone using the sim 900 module without specifying any number

    • jojo
      February 16, 2016

      I did not understand your question. Please explain it again.

      • February 16, 2016

        am trying to make a system that responds to any user who sends an sms command to the sim900 module using the exact step that you had provided ,but using mySerial.println(“AT+CMGS=\”+91xxxxxxxxxx\”\r”); will only allow the system to send a response to one number , how can i implement a system that responds to multiple phone numbers wich send a command to the sim900 or a function that Retrieves the phone number an from an incoming SMS message and stores it in a named array , just like the function “remoteNumber()” wich is present in the aduino “GSM.h” library .

  • aiman ramlan
    November 12, 2015

    hello sir, im trying to use GSM sim900A in malaysia with using the exact step that you had provide. but the GSM still does not sending any text or receive text. but when im trying to establish the network connection of GSM, i had heard the ringing. so what is the problem ? can you help sir ? its important to me. thank you

    • jojo
      February 16, 2016

      @aiman – It can be an error with your program. Double check the program.

  • BS
    October 10, 2015

    thanks, it is working fine!!

  • Aman
    July 11, 2015

    We are using ” \” in “AT+CMGS=\”+91xxxxxxxxxx\”\r” to differentiate mobile no & AT command Stringset. I want to know how arduino reads this code?

  • Sai Abhinay
    June 3, 2015

    Could you please specify the syntax for call commands. I tried different combinations but I am unable to figure it out and Thank you for this SMS information, it worked perfect

  • June 3, 2015

    please i need the pdf format of this project. i will be very glad if you can send it through my e-mail address above. thanks

  • Victor
    May 9, 2015

    Yea thanks for this. I found out that output of the gsm module is static, i mean is only what we have input is what is doing. Is there no way it could check for out box messages and be able to re-send any in it to the controller. Maybe some other dynamic functions too.

    • jojo
      May 19, 2015

      @Victor Its possible to read inbox and many other functions using GSM module. Learn AT Commands thoroughly for that.

  • sajal nagwanshi
    May 2, 2015


    Thank you very much. The information about software serial was very helpful.

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