Getting Started with the Arduino GSM Shield

This is a retired product.

The Arduino GSM shield allows an Arduino board to connect to the internet, send and receive SMS, and make voice calls using the GSM library.

The shield will work with the Arduino Uno out of the box. The shield will work with the Mega, Mega ADK, Yun, and Leonardo boards with a minor modification. The Due is not supported at this time.

The GSM library is included with Arduino IDE 1.0.4 and later.

What is GSM

GSM is an international standard for mobile telephones. It is an acronym that stands for Global System for Mobile Communications. It is also sometimes referred to as 2G, as it is a second-generation cellular network.

To use GPRS for internet access, and for the Arduino to request or serve webpages, you need to obtain the Access Point Name (APN) and a username/password from the network operator. See the information in Connecting to the Internet for more information about using the data capabilities of the shield.

Among other things, GSM supports outgoing and incoming voice calls, Simple Message System (SMS or text messaging), and data communication (via GPRS).

The Arduino GSM shield is a a GSM modem. From the mobile operator perspective, the Arduino GSM shield looks just like a mobile phone. From the Arduino perspective, the Arduino GSM shield looks just like a modem.

What is GPRS

GPRS is a packet switching technology that stands for General Packet Radio Service. It can provide idealized data rates between 56-114 kbit per second.

A number of technologies such as SMS rely on GPRS to function. With the GSM shield, it is also possible to leverage the data communication to access the internet. Similar to the Ethernet and WiFi libraries, the GSM library allows the Arduino to act as a client or server, using http calls to send and receive web pages.

Network operator requirements

To access a network, you must have a subscription with a mobile phone operator (either prepaid or contract), a GSM compliant device like the GSM shield or mobile phone, and a Subscriber Identity Module (SIM) card. The network operator provides the SIM card, which contains information like the mobile number, and can store limited amounts of contacts and SMS messages.

To use GPRS for internet access, and for the Arduino to request or serve webpages, you need to obtain the Access Point Name (APN) and a username/password from the network operator. See the information in Connecting to the Internet for more information about using the data capabilities of the shield.

SIM cards

In addition to the GSM shield and an Arduino, you need a SIM card. The SIM represents a contract with a communications provider. The communications provider selling you the SIM has to either provide GSM coverage where you are, or have a roaming agreement with a company providing GSM coverage in your location.

It's common for SIM cards to have a four-digit PIN number associated with them for security purposes. Keep note of this number, as it's necessary for connecting to a network. If you lose the PIN associated with your SIM card, you may need to contact your network operator to retrieve it. Some SIM cards become locked if an incorrect PIN is entered too many times. If you're unsure of what the PIN is, look at the documentation that came with your SIM.

Using a PUK (PIN Unlock Code), it is possible to reset a lost PIN with the GSM shield and an Arduino. The PUK number will come with your SIM card documentation.

Look at the PIN Management example in the "tools" folder, bundled with the GSM library for an example of how to manage your PIN number with the PUK.

There are a few different sizes of SIM cards; the GSM shield accepts cards in the mini-SIM format (25mm long and 15mm wide).

Notes on the Telefonica/Movilforum SIM included with the shield

The GSM shield comes bundled with a SIM from Telefonica/Movilforum that will work well for developing machine to machine (M2M) applications. It is not necessary to use this specific card with the shield. You may use any SIM that works on a network in your area.

The Movilforum SIM card includes a roaming plan. It can be used on any supported GSM network. There is coverage throughout the Americas and Europe for this SIM, check the Movilforum service availability page for specific countries that have supported networks.

Activation of the SIM is handled by Movilforum. Detailed instructions on how to register and activate your SIM online and add credit are included on a small pamphlet that comes with your shield. The SIM must be inserted into a powered GSM shield that is mounted on an Arduino for activation.

These SIM card come without a PIN, but it is possible to set one using the GSM library's GSMPIN class.

You cannot use the included SIM to place or receive voice calls.

You can only place and receive SMS with other SIMs on the Movilforum network.

It's not possible to create a server that accepts incoming requests from the public internet. However, the Movilforum SIM will accept incoming requests from other SIM cards on the Movilforum network.

For using the voice, and other functions of the shield, you'll need to find a different network provider and SIM. Operators will have different policies for their SIM cards, check with them directly to determine what types of connections are supported.

Connecting the Shield

If you are using an Arduino Uno, follow the instructions below. If you are using an Arduino Mega, Mega ADK, Yun, or Leonardo, you must follow these instructions. The GSM shield is not currently supported on the Due.

To use the shield, you'll need to insert a SIM card into the holder. Slide the metal bracket away from the edge of the shield and lift the cradle up.

Insert the SIM in the plastic holder so the metal contacts are facing the shield, with the notch of the card at the top of the bracket.

Slide the SIM all the way into the bracket

Push the SIM to the board and slide the metal bracket towards the edge of the shield to lock it in place.

Once the SIM is inserted, mount it on top of an Arduino board.

To upload sketches to the board, connect it to your computer with a USB cable and upload your sketch with the Arduino IDE. Once the sketch has been uploaded, you can disconnect the board from your computer and power it with an external power supply.

Digital pins 2, 3 and 7 are reserved for communication between the Arduino and modem and cannot be used by your sketches. Communication between the moden and Arduino is handled by the Software Serial library on pins 2 and 3. Pin 7 is used for the modem reset.

When the yellow status LED turns on, it means the modem is powered, and you can try connecting to the network.

Developer versions of the GSM shield required you to press press the Power button on the shield for a few moments to turn the modem on. If you have an early version of the shield, and it does not turn on automatically, you can solder a jumper to the CTRL/D7 pad on the reverse side of the board, and it will turn on when an attached Arduino receives power.

The shield should work in any area with GSM coverage. Before buying the shield please verify that there is this kind of coverage where you plan to use it.

GSM Library

The GSM library handles communication between Arduino and the GSM shield. The majority of functions are for managing data, voice, and SMS communication. There are also a number of utilities for managing information about the modem and the SIM card's PIN. See the library reference pages for more information and a complete set of examples.

Testing the modem and network connection

This sketch will check the modem's IMEI number. This number is unique to each modem, and is used to identify valid devices that can connect to a GSM network. Once the number has been read from the modem, the Arduino will print out the network carrier it is connected to, and the signal strength of the network over the serial port.

Copy
1// import the GSM library
2#include <GSM.h>
3
4// PIN Number
5#define PINNUMBER ""
6
7// initialize the library instance
8
9GSM gsmAccess(true);     // include a 'true' parameter for debug enabled
10
11GSMScanner scannerNetworks;
12
13GSMModem modemTest;
14
15// Save data variables
16
17String IMEI = "";
18
19// serial monitor result messages
20
21String errortext = "ERROR";
22
23void setup()
24{
25
26  // initialize serial communications
27
28  Serial.begin(9600);
29
30  Serial.println("GSM networks scanner");
31
32  scannerNetworks.begin();
33
34  // connection state
35
36  boolean notConnected = true;
37
38  // Start GSM shield
39
40  // If your SIM has PIN, pass it as a parameter of begin() in quotes
41
42  while(notConnected)
43
44  {
45
46    if(gsmAccess.begin(PINNUMBER)==GSM_READY)
47
48      notConnected = false;
49
50    else
51
52    {
53
54      Serial.println("Not connected");
55
56      delay(1000);
57
58    }
59
60  }
61
62  // get modem parameters
63
64  // IMEI, modem unique identifier
65
66  Serial.print("Modem IMEI: ");
67
68  IMEI = modemTest.getIMEI();
69
70  IMEI.replace("\n","");
71
72  if(IMEI != NULL)
73
74    Serial.println(IMEI);
75
76  // currently connected carrier
77
78  Serial.print("Current carrier: ");
79
80  Serial.println(scannerNetworks.getCurrentCarrier());
81
82  // returns strength and ber
83
84  // signal strength in 0-31 scale. 31 means power > 51dBm
85
86  // BER is the Bit Error Rate. 0-7 scale. 99=not detectable
87
88  Serial.print("Signal Strength: ");
89
90  Serial.print(scannerNetworks.getSignalStrength());
91
92  Serial.println(" [0-31]");
93}
94
95void loop()
96{
97
98  // scan for existing networks, displays a list of networks
99
100  Serial.println("Scanning available networks. May take some seconds.");
101
102  Serial.println(scannerNetworks.readNetworks());
103
104    // currently connected carrier
105
106  Serial.print("Current carrier: ");
107
108  Serial.println(scannerNetworks.getCurrentCarrier());
109
110  // returns strength and ber
111
112  // signal strength in 0-31 scale. 31 means power > 51dBm
113
114  // BER is the Bit Error Rate. 0-7 scale. 99=not detectable
115
116  Serial.print("Signal Strength: ");
117
118  Serial.print(scannerNetworks.getSignalStrength());
119
120  Serial.println(" [0-31]");
121
122}

Sending a SMS message

Once you have connected to your network with the sketch above, you can test some of the other functionality of the board. This sketch will connect to a GSM network and send a SMS message to a phone number of your choosing.

Copy
1#include <GSM.h>
2
3#define PINNUMBER ""
4
5// initialize the library instance
6
7GSM gsmAccess; // include a 'true' parameter for debug enabled
8
9GSM_SMS sms;
10
11// char array of the telephone number to send SMS
12// change the number 1-212-555-1212 to a number
13// you have access to
14char remoteNumber[20]= "12125551212";
15
16// char array of the message
17char txtMsg[200]="Test";
18
19void setup()
20{
21
22  // initialize serial communications
23
24  Serial.begin(9600);
25
26  Serial.println("SMS Messages Sender");
27
28  // connection state
29
30  boolean notConnected = true;
31
32  // Start GSM shield
33
34  // If your SIM has PIN, pass it as a parameter of begin() in quotes
35
36  while(notConnected)
37
38  {
39
40    if(gsmAccess.begin(PINNUMBER)==GSM_READY)
41
42      notConnected = false;
43
44    else
45
46    {
47
48      Serial.println("Not connected");
49
50      delay(1000);
51
52    }
53
54  }
55
56  Serial.println("GSM initialized");
57
58  sendSMS();
59}
60
61void loop()
62{
63// nothing to see here
64}
65
66void sendSMS(){
67
68  Serial.print("Message to mobile number: ");
69
70  Serial.println(remoteNumber);
71
72  // sms text
73
74  Serial.println("SENDING");
75
76  Serial.println();
77
78  Serial.println("Message:");
79
80  Serial.println(txtMsg);
81
82  // send the message
83
84  sms.beginSMS(remoteNumber);
85
86  sms.print(txtMsg);
87
88  sms.endSMS();
89
90  Serial.println("\nCOMPLETE!\n");
91}

Connecting to the internet

In addition to the SIM card and a data plan, you will need some additional information from your cellular provider to connect to the internet. Every cellular provider has an Access Point Name (APN) that serves as a bridge between the cellular network and the internet. Sometimes, there is a username and password associated with the connection point. For example, the Movilforum APN is sm2ms.movilforum.es, but it has no password or login name.

This page lists a number of carrier's information, but it may not be up to date. You may need to get this information from your service provider.

The sketch below will connect to arduino.cc/latest.txt and print out its contents.

NB: Some network operators block incoming IP traffic. You should be able to run client functions, such as the sketch below, with no issues.

Copy
1// include the GSM library
2#include <GSM.h>
3
4// PIN number if necessary
5#define PINNUMBER ""
6
7// APN information obrained from your network provider
8#define GPRS_APN       "GPRS_APN" // replace with your GPRS APN
9#define GPRS_LOGIN     "login"    // replace with your GPRS login
10#define GPRS_PASSWORD  "password" // replace with your GPRS password
11
12// initialize the library instances
13
14GSMClient client;
15
16GPRS gprs;
17
18GSM gsmAccess;
19
20// This example downloads the URL "http://arduino.cc/latest.txt"
21
22char server[] = "arduino.cc"; // the base URL
23char path[] = "/latest.txt"; // the path
24int port = 80; // the port, 80 for HTTP
25
26void setup()
27{
28
29  // initialize serial communications
30
31  Serial.begin(9600);
32
33  Serial.println("Starting Arduino web client.");
34
35  // connection state
36
37  boolean notConnected = true;
38
39  // Start GSM shield
40
41  // pass the PIN of your SIM as a parameter of gsmAccess.begin()
42
43  while(notConnected)
44
45  {
46
47    if((gsmAccess.begin(PINNUMBER)==GSM_READY) &
48
49        (gprs.attachGPRS(GPRS_APN, GPRS_LOGIN, GPRS_PASSWORD)==GPRS_READY))
50
51      notConnected = false;
52
53    else
54
55    {
56
57      Serial.println("Not connected");
58
59      delay(1000);
60
61    }
62
63  }
64
65  Serial.println("connecting...");
66
67  // if you get a connection, report back via serial:
68
69  if (client.connect(server, port))
70
71  {
72
73    Serial.println("connected");
74
75    // Make a HTTP request:
76
77    client.print("GET ");
78
79    client.print(path);
80
81    client.println(" HTTP/1.0");
82
83    client.println();
84
85  }
86
87  else
88
89  {
90
91    // if you didn't get a connection to the server:
92
93    Serial.println("connection failed");
94
95  }
96}
97
98void loop()
99{
100
101  // if there are incoming bytes available
102
103  // from the server, read them and print them:
104
105  if (client.available())
106
107  {
108
109    char c = client.read();
110
111    Serial.print(c);
112
113  }
114
115  // if the server's disconnected, stop the client:
116
117  if (!client.available() && !client.connected())
118
119  {
120
121    Serial.println();
122
123    Serial.println("disconnecting.");
124
125    client.stop();
126
127    // do nothing forevermore:
128
129    for(;;)
130
131      ;
132
133  }
134}

Making voice calls

Through the modem, it is possible to make voice calls. In order to speak to and hear the other party, you will need to add a speaker and microphone.

On the underside of the shield, there are through-holes labeled M1P and M1N. These are the positive and negative voice input pins for a microphone. The through-holes labeled S1P and S1N are the positive and negative voice output pins, to which you need to connect a speaker.

On page 43 of the modem documentation, there is an example voice and sound circuit that will connect to an earphone:

The following sketch allows you to place a voice call. Using the serial monitor, you can enter the remote phone number and terminate the call. When you see the READY message, type a phone number. Make sure the serial monitor is set to send a just newline when you press return.

Copy
1#include <GSM.h>
2
3// PIN Number
4#define PINNUMBER ""
5
6// initialize the library instance
7
8GSM gsmAccess; // include a 'true' parameter for debug enabled
9
10GSMVoiceCall vcs;
11
12String remoteNumber = "";  // the number you will call
13char charbuffer[20];
14
15void setup()
16{
17
18  // initialize serial communications
19
20  Serial.begin(9600);
21
22  Serial.println("Make Voice Call");
23
24  // connection state
25
26  boolean notConnected = true;
27
28  // Start GSM shield
29
30  // If your SIM has PIN, pass it as a parameter of begin() in quotes
31
32  while(notConnected)
33
34  {
35
36    if(gsmAccess.begin(PINNUMBER)==GSM_READY)
37
38      notConnected = false;
39
40    else
41
42    {
43
44      Serial.println("Not connected");
45
46      delay(1000);
47
48    }
49
50  }
51
52  Serial.println("GSM initialized.");
53
54  Serial.println("Enter phone number to call.");
55
56}
57
58void loop()
59{
60
61  // add any incoming characters to the String:
62
63  while (Serial.available() > 0)
64
65  {
66
67    char inChar = Serial.read();
68
69    // if it's a newline, that means you should make the call:
70
71    if (inChar == '\n')
72
73    {
74
75      // make sure the phone number is not too long:
76
77      if (remoteNumber.length() < 20)
78
79      {
80
81        // show the number you're calling:
82
83        Serial.print("Calling to : ");
84
85        Serial.println(remoteNumber);
86
87        Serial.println();
88
89        // Call the remote number
90
91        remoteNumber.toCharArray(charbuffer, 20);
92
93        // Check if the receiving end has picked up the call
94
95        if(vcs.voiceCall(charbuffer))
96
97        {
98
99          Serial.println("Call Established. Enter line to end");
100
101          // Wait for some input from the line
102
103          while(Serial.read()!='\n' && (vcs.getvoiceCallStatus()==TALKING));
104
105          // And hang up
106
107          vcs.hangCall();
108
109        }
110
111        Serial.println("Call Finished");
112
113        remoteNumber="";
114
115        Serial.println("Enter phone number to call.");
116
117      }
118
119      else
120
121      {
122
123        Serial.println("That's too long for a phone number. I'm forgetting it");
124
125        remoteNumber = "";
126
127      }
128
129    }
130
131    else
132
133    {
134
135      // add the latest character to the message to send:
136
137      if(inChar!='\r')
138
139        remoteNumber += inChar;
140
141    }
142
143  }
144}

Connecting to Yún, Mega & Leonardo

The GSM shield communicates with an attached Arduino through the Software Serial library. By default, this communication uses digital pins 2 and 3. On the Uno this works without modification, but to use the Leonardo, Yun, or Mega boards, some slight changes are required.

The GSM_TX pin, pin 2 on the shield, sends information to the Arduino. The Arduino relies on an interrupt on this pin to know when information is available to read. On the Uno, this interrupt capability is on pin 2. The Leonardo, Yun, and Mega have interrupt capabilities on different pins.

You do not need to change any code to program the shield for use with the Leonardo, Yun, or Mega, as the library will change the Arduino's RX pin automatically depending on the board selected in the "Tools" menu of the IDE. You do, however, need to re-route the GSM_TX data to the appropriate pin on the Arduino.

Arduino Leonardo and Arduino Yun

The GSM library uses digital pin 8 to communicate with the Leonardo or the Yun. Thus, you need to route the signal from pin 2 of the GSM shield to pin 8 of the Arduino, and at the same time prevent this signal from connecting to pin 2 of the Arduino. The following two steps accomplish this:

On the GSM shield, connect a jumper wire between digital pins 2 and 8. This routes the signal from pin 2 of the GSM shield to pin 8 of the Arduino.

To prevent this signal from interfering with or being interfered by pin 2 of the Arduino, bend the male header attached to pin 2 on the GSM shield to the side so it does not connect to the Leonardo or Yun.

Arduino Mega

The GSM library uses digital pin 10 to communicate with the Mega. Thus, you need to route the signal from pin 2 of the GSM shield to pin 10 of the Arduino, and at the same time prevent this signal from connecting to pin 2 of the Arduino. The following two steps accomplish this:

On the GSM shield, connect a jumper wire between digital pins 2 and 10. This routes the signal from pin 2 of the GSM shield to pin 10 of the Arduino.

To prevent this signal from interfering with or being interfered by pin 2 of the Arduino, bend the male header attached to pin 2 on the GSM shield to the side so it does not connect to the Mega.

Next steps

Now that you have tested the basic functionality of the board, see the GSM library pages for information about the library's API and additional examples.

The text of the Arduino getting started guide is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the guide are released into the public domain.