Tutorial: Arduino and GSM Cellular – Part Two

This is the revised, 2012 edition of chapter twenty-seven of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – A tutorial on the Arduino universe. The first chapter is here, the complete series is detailed here.

Welcome back fellow arduidans!

Assumed understanding for this article is found in part one. If you have not already done so, please read and understand it.

Today we are going to harness the awesome power of the telephone network to control an Arduino board via a SM5100B Cellular Shield:

Are you using an Arduino Mega board?

Mega users – things will be slightly different for you. Please make sure the TX and RX pins of your GSM shield DO NOT plug into your Mega. Furthermore, run a jumper wire from GSM shield pin D2 to Mega pin 19, and a jumper from GSM shield pin D3 to Mega pin 18, as shown below:

Finally, the example sketches will be different. Mega will not use the NewSoftSerial library, instead we use the Serial1 port. Please use the following Mega-equivalent sketches for this article: Example 27.1 and 27.2.

Reach out and control something

First we will discuss how to make something happen by a simple telephone call. And the best thing is that we don’t need the the GSM module to answer the telephone call (thereby saving money) – just let the module ring a few times. How is this possible? Very easily. Recall example 26.1 – we monitored the activity of the GSM module by using our terminal software. In this case what we need to do is have our Arduino examine the text coming in from the serial output of the GSM module, and look for a particular string of characters.

When we telephone the GSM module from another number, the module returns the text as shown in the image below (click to enlarge):

We want to look for the text “RING”, as (obviously) this means that the GSM shield has recognised the ring signal from the exchange. Therefore need our Arduino to count the number of rings for the particular telephone call being made to the module. (Memories – Many years ago we would use public telephones to send messages to each other. For example, after arriving at a foreign destination we would call home and let the phone ring five times then hang up – which meant we had arrived safely). Finally, once the GSM shield has received a set number of rings, we want the Arduino to do something.

From a software perspective, we need to examine each character as it is returned from the GSM shield. Once an “R” is received, we examine the next character. If it is an “I”, we examine the next character. If it is an “N”, we examine the next character. If it is a “G”, we know an inbound call is being attempted, and one ring has occurred. We can set the number of rings to wait until out desired function is called. In the following example, when the shield is called, it will call the function doSomething() after three rings.

The function doSomething() controls two LEDs, one red, one green. Every time the GSM module is called for 3 rings, the Arduino alternately turns on or off the LEDs. Using this sketch as an example, you now have the ability to turn basically anything on or off, or call your own particular function. Another example would be to return some type of data, for example you could dial in and have the Arduino send you a text message containing temperature data.

Example 27.1 (Mega version)

/*  Example 27.1  Turn something on or off by telephoning the GSM shield for a set number of rings

tronixstuff.com/tutorials > chapter 27  CC by-sa-nc

NOT for Arduino Mega */

#include <NewSoftSerial.h>  //Include the NewSoftSerial library to send serial commands to the cellular module

char inchar;       // Will hold the incoming character from the Serial Port

NewSoftSerial cell(2,3);    //Create a 'fake' serial port. Pin 2 is the Rx pin, pin 3 is the Tx pin

int numring=0; // used to count the number of 'rings' received by the GSM module

int comring=3; // this is the number of rings to wait for until calling the function doSomething();

int onoff=0; // used to track LED status - 0 = off, 1 = on

void setup()

{

pinMode(12, OUTPUT);

pinMode(13, OUTPUT);  // LEDs - off = red, on = green

digitalWrite(12, HIGH);

digitalWrite(13, LOW);  //Initialize serial port for communication.

cell.begin(9600);

}

void doSomething()

{

if (onoff==0)

{

onoff=1;

digitalWrite(12, HIGH);

digitalWrite(13, LOW);

}   else

if (onoff==1)

{

onoff=0;

digitalWrite(12, LOW);

digitalWrite(13, HIGH);

}

}

void loop()

{  //If a character comes in from the cellular module...

if(cell.available() >0)

{

inchar=cell.read();

if (inchar=='R')

{

delay(10);

inchar=cell.read();

if (inchar=='I')

{

delay(10);

inchar=cell.read();

if (inchar=='N')

{

delay(10);

inchar=cell.read();

if (inchar=='G')

{

 delay(10);

// So the phone (our GSM shield) has 'rung' once, i.e. if it were a real phone

// it would have sounded 'ring-ring'

numring++;

if (numring==comring)

{

numring=0; // reset ring counter

doSomething();

}

}

}

}

}

}

}

And now for a quick video demonstration. The first call is made, and the LEDs go from red (off) to green (on). A second call is made, and the LEDs go from green (on) to red (off). Although this may seem like an over-simplified example, with your existing Ardiuno knowledge you now have the ability to run any function by calling your GSM shield.

Now although turning one thing on or off is convenient, how can we send more control information to our GSM module? For example, control four or more digital outputs at once? These sorts of commands can be achieved by the reception and analysis of text messages.

Doing so is similar to the method we used in example 27.1. Once again, we will analyse the characters being sent from the GSM module via its serial out. However, there are two AT commands we need to send to the GSM module before we can receive SMSs, and one afterwards. The first one you already know:

AT+CMGF=1

Which sets the SMS mode to text. The second command is:

AT+CNMI=3,3,0,0

This command tells the GSM module to immediately send any new SMS data to the serial out. An example of this is shown in the terminal capture below (click to enlarge):

Two text messages have been received since the module was turned on. You can see how the data is laid out. The blacked out number is the sender of the SMS. The number +61418706700 is the number for my carrier’s SMSC (short message service centre). Then we have the date and time. The next line is the contents of the text message – what we need to examine in our sketch.

The second text message in the example above is how we will structure our control SMS. Our sketch will wait for a # to come from the serial line, then consider the values after a, b, c and d – 0 for off, 1 for on. Finally, we need to send one more command to the GSM module after we have interpreted our SMS:

AT+CMGD=1,4

This deletes all the text messages from the SIM card. As there is a finite amount of storage space on the SIM, it is prudent to delete the incoming message after we have followed the instructions within. But now for our example. We will control four digital outputs, D9~12. For the sake of the exercise we are controlling an LED on each digital output, however you could do anything you like. Although the sketch may seem long and complex, it is not – just follow it through and you will see what is happening.

Example 27.2 (Mega version)

/*    Example 27.2   Control four digital outputs via SMS

tronixstuff.com/tutorials > chapter 27   CC by-sa-nc

NOT for Arduino Mega   */

#include <NewSoftSerial.h>  //Include the NewSoftSerial library to send serial commands to the cellular module.

char inchar;                //Will hold the incoming character from the Serial Port.

NewSoftSerial cell(2,3);    //Create a 'fake' serial port. Pin 2 is the Rx pin, pin 3 is the Tx pin.

int led1 = 9;

int led2 = 10;

int led3 = 11;

int led4 = 12;

void setup()

{    // prepare the digital output pins

pinMode(led1, OUTPUT);

pinMode(led2, OUTPUT);

pinMode(led3, OUTPUT);

pinMode(led4, OUTPUT);

digitalWrite(led1, LOW);

digitalWrite(led2, LOW);

digitalWrite(led3, LOW);

digitalWrite(led4, LOW);    //Initialize GSM module serial port for communication.

cell.begin(9600);

delay(30000); // give time for GSM module to register on network etc.

cell.println("AT+CMGF=1"); // set SMS mode to text

delay(200);

cell.println("AT+CNMI=3,3,0,0"); // set module to send SMS data to serial out upon receipt

delay(200);

}

void loop()

{    //If a character comes in from the cellular module...

if(cell.available() >0)

{

inchar=cell.read();

if (inchar=='#') // OK - the start of our command

{

delay(10);

inchar=cell.read();

if (inchar=='a')

{

delay(10);

inchar=cell.read();

if (inchar=='0')

{

digitalWrite(led1, LOW);

}

else if (inchar=='1')

{

digitalWrite(led1, HIGH);

}

delay(10);

inchar=cell.read();

if (inchar=='b')

{

inchar=cell.read();

if (inchar=='0')

{

digitalWrite(led2, LOW);

}

else if (inchar=='1')

{

digitalWrite(led2, HIGH);

}

delay(10);

inchar=cell.read();

if (inchar=='c')

{

inchar=cell.read();

if (inchar=='0')

{

digitalWrite(led3, LOW);

}

else if (inchar=='1')

{

digitalWrite(led3, HIGH);

}

delay(10);

inchar=cell.read();

if (inchar=='d')

{

delay(10);

inchar=cell.read();

if (inchar=='0')

{

digitalWrite(led4, LOW);

}

else if (inchar=='1')

{

digitalWrite(led4, HIGH);

}

delay(10);

}

}

cell.println("AT+CMGD=1,4"); // delete all SMS

}

}

}

}

}

And now for a video demonstration:

So there you have it – controlling your Arduino digital outputs via a normal telephone or SMS. Now it is up to you and your imagination to find something to control, sensor data to return, or get up to other shenanigans.

If you enjoyed this article, you may find this of interest – controlling AC power outlets via SMS.

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