Tutorial: Control AC outlets via SMS
Learn how to control AC outlets via SMS text message. This is chapter thirty-three 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.
Updated 02/03/2013
Assumed understanding for this article is found in part one. If you have not already done so, please read and understand it.
In this chapter we will continue with the use of the SM5100 cellular shield to turn digital outputs on and off via SMS. However please read chapters twenty-six and twenty-seven first if you are unfamiliar with using the GSM shield with Arduino. As an extension of chapter twenty-seven, we will use our Arduino to turn on or off AC outlets via a common remote-control AC outlet pack. Please note this is more of a commentary of my own experience, and not an exact tutorial. In other words, by reading this I hope you will gain some ideas into doing the necessary modifications yourself and in your own way.
Firstly, we need some remote-control AC outlets. Most electrical stores or giant retail warehouses may have something like this:
Nothing too original, just a wireless remote control that can switch on or off receiver outlets on a choice of four radio frequencies. Before moving forward I would like to acknowledge that this article was inspired by the wonderful book Practical Arduino – Cool Projects for Open Source Hardware by Jon Oxer and Hugh Blemings. In chapter two an appliance remote-control system is devised using a similar system.
At first glance the theory behind this project is quite simple – using the hardware in example 27.2, instead of controlling LEDs, activate the buttons on the wireless remote control for the AC outlets – leaving us with AC outlets controlled via SMS. However there are a few things to keep in mind and as discovered during the process, various pitfalls as well.
Before voiding the warranty on your remote control, it would be wise to test the range of the remote control to ensure it will actually work in your situation. I found this was made a lot easier by connecting a radio to the remote outlet – then you can hear when the outlet is on or off. If this is successful, make a note of the amount of time required to press the on and off buttons – as we need to control the delay in our Arduino sketch.
The next step is to crack open the remote control:
… and see what we have to work with:
Straight away there are two very annoying things – the first being the required power supply – 12 volts; and the second being the type of button contacts on the PCB. As you can see above we only have some minute PCB tracks to solder our wires to. It would be infinitely preferable to have a remote control that uses actual buttons soldered into a PCB, as you can easily desolder and replace them with wires to our Arduino system. However unless you can casually tear open the remote control packaging in the store before purchase, it can be difficult to determine the type of buttons in the remote.
As you can see in the photo above, there is an off and on pad/button each for four channels of receiver. In my example we will only use two of them to save time and space. The next question to solve is how to interface the Arduino digital outputs with the remote control. In Practical Arduino, the authors have used relays, but I don’t have any of those in stock. However I do have a quantity of common 4N25 optocouplers, so will use those instead. An optocoupler can be thought of as an electronic switch that is isolated from what is it controlling – see my article on optocouplers for more information.
Four optocouplers will be required, two for each radio channel. To mount them and the associated circuitry, we will use a blank protoshield and build the Arduino-remote control interface onto the shield. The circuitry for the optocoupler for each switch is very simple, we just need four of the following:
As the LED inside the optocoupler has a forward voltage of 1.2 volts at 10mA, the 390 ohm resistor is required as our Arduino digital out is 5 volts. Dout is connected to the particular digital out pin from the Arduino board. Pins 4 and 5 on the optocoupler are connected to each side of the button contact on our remote control.
The next consideration is the power supply. The remote control theoretically needs 12 volts, however the included battery only measured just over nine. However for the optimum range, the full 12 should be supplied. To save worrying about the battery, our example will provide 12V to the remote control. Furthermore, we also need to supply 5 volts at a higher current rating that can be supplied by our Arduino. In the previous GSM chapters, I have emphasised that the GSM shield can possibly draw up to two amps in current. So once again, please ensure your power supply can deliver the required amount of current. From experience in my location, I know that the GSM shield draws around 400~600 milliamps of current – which makes things smaller and less complex.
The project will be supplied 12 volts via a small TO-92 style 78L12 regulator, and 5 volts via a standard TO-220 style 7805 regulator. You could always use a 7812, the 78L12 was used as the current demand is lower and the casing is smaller. The power for the whole project will come from a 15V DC 1.5A power supply. So our project’s power supply schematic will be as follows:
Now to mount the optocouplers and the power circuitry on the blank protoshield. Like most things in life it helps to make a plan before moving forward. I like to use graph paper, each square representing a hole on the protoshield, to plan the component layout. For example:
It isn’t much, but it can really help. Don’t use mine – create your own, doing so is good practice. After checking the plan over, it is a simple task to get the shield together. Here is my prototype example:
It isn’t neat, but it works. The header pins are used to make connecting the wires a little easier, and the pins on the right hand side are used to import the 15V and export 12V for the remote.
While the soldering iron is hot, the wires need to be soldered to the remote control. Due to the unfortunate size of the PCB tracks, there wasn’t much space to work with:
But with time and patience, the wiring was attached:
Again, as this is a prototype the aesthetics of the modification are not that relevant. Be careful when handling the remote, as any force on the wiring can force the soldered wire up and break the PCB track. After soldering each pair of wires to the button pads, use the continuity function of a multimeter to check for shorts and adjust your work if necessary.
At this stage the AC remote control shield prototype is complete. It can be tested with a simple sketch to turn on and off the related digital outputs. For example, the following sketch will turn on and off each outlet in sequence:
void setup()
{
pinMode(9, OUTPUT); // 1 off
pinMode(8, OUTPUT); // 1 on
pinMode(5, OUTPUT); // 2 off
pinMode(4, OUTPUT); // 2 on
}
void loop()
{
// outlets on channel 1 on
digitalWrite(8, HIGH);
delay(1000);
digitalWrite(8, LOW);
delay(5000);
// outlets on channel 1 off
digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);
delay(5000);
// outlets on channel 2 on
digitalWrite(4, HIGH);
delay(1000);
digitalWrite(4, LOW);
delay(5000);
// outlets on channel 2 off
digitalWrite(5, HIGH);
delay(1000);
digitalWrite(5, LOW);
delay(5000);
}
Now to get connected with our GSM shield. It is a simple task to insert the remote shield over the GSM shield combination, and to connect the appropriate power supply and (for example) GSM aerial. The control sketch is a slight modification of example 27.2, and is shown below:
#include <SoftwareSerial.h> char inchar; SoftwareSerial cell(2,3);
int aon = 8; int aoff = 9; int bon = 4; int boff = 5; int pressdelay = 1000; // duration to activate a button on remote
void setup()
{
// prepare the digital output pins
pinMode(aon, OUTPUT);
pinMode(aoff, OUTPUT);
pinMode(bon, OUTPUT);
pinMode(boff, OUTPUT);
//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=='#')
{
delay(10);
inchar=cell.read();
if (inchar=='a')
{
delay(10);
inchar=cell.read();
if (inchar=='0')
{
digitalWrite(aoff, HIGH);
delay(pressdelay);
digitalWrite(aoff, LOW);
}
else if (inchar=='1')
{
digitalWrite(aon, HIGH);
delay(pressdelay);
digitalWrite(aon, LOW);
}
delay(10);
inchar=cell.read();
if (inchar=='b')
{
inchar=cell.read();
if (inchar=='0')
{
digitalWrite(boff, HIGH);
delay(pressdelay);
digitalWrite(boff, LOW);
}
else if (inchar=='1')
{
digitalWrite(bon, HIGH);
delay(pressdelay);
digitalWrite(bon, LOW);
}
delay(10);
inchar=cell.read();
cell.println("AT+CMGD=1,4"); // delete all SMS
}
}
}
}
}
The variable pressdelay stores the amount of time in milliseconds to ‘press’ a remote control button. To control our outlets, we send a text message using the following syntax:
#axbx
Where a/b are remote channels one and two, and x is replaced with 0 for off and 1 for on.
So there you have it – controlling almost any AC powered device via text message from a cellular phone. Imagine trying to do that ten, or even five years ago. As always, now it is up to you and your imagination to find something to control or get up to other shenanigans.
Have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other – and we can all learn something.
Tutorial: Arduino and GSM Cellular – Part Two
Continue to learn about connecting your Arduino to the cellular network with the SM5100 GSM module shield. This is 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.
Updated 02/03/2013
Assumed understanding for this article is found in part one. If you have not already done so, please read and understand it. In this instalment we continue with bare projects which you can use as a framework for your own creations.
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.
#include <SoftwareSerial.h> char inchar; // Will hold the incoming character from the Serial Port. SoftwareSerial cell(2,3); //Create a 'fake' serial port. Pin 2 is the Rx pin, pin 3 is the Tx pin.
int numring=0; int comring=3; int onoff=0; // 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.
Control Digital I/O via SMS
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.
#include <SoftwareSerial.h>
char inchar; //Will hold the incoming character from the Serial Port.
SoftwareSerial 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=='#')
{
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.
Have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other – and we can all learn something.
Tutorial: Arduino and GSM Cellular – Part One
Connect your Arduino to the cellular network with the SM5100 GSM module shield. This is chapter twenty-six 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.
Updated 18/03/2013
Introduction
The purpose of this tutorial is to have your Arduino to communicate over a GSM mobile telephone network using the SM5100B GSM Cellular Shield:
This is the first of several articles related to the SM5100B GSM shield. My goal is to illustrate various methods of interaction between an Arduino and the GSM cellular network using the shield, with which you can then use your existing knowledge to build upon those methods. Doing so isn’t easy – but it isn’t that difficult.
Stop! Please read first:
- It is assumed that you have a solid understanding of how to program your Arduino. If not, start from chapter zero
- Read the “boring stuff” section on my About page
- Sending SMS messages and making phone calls cost real money, so it would be very wise to use a prepaid cellular account or one that allows a fair amount of calls/SMS
- The GSM shield only works with “2G” GSM mobile networks operating on the 850, 900 and PCS1800 MHz frequencies. If in doubt, ask your carrier first
- Australians – you can use any carrier’s SIM card, except for “Three”
- Canadians – this doesn’t work with Sasktel
- North Americans – check with your cellular carrier first if you can use third-party hardware (i.e. the shield)
- I cannot offer design advice for your project nor technical support for this article.
- If you are working on a college/university project and need specific help – talk to your tutors or academic staff. They get paid to help you.
- Please don’t make an auto-dialler…
As mentioned previously, we’re using the Sparkfun GSM shield with the SM5100B module. When you order the shield, don’t forget to order the stacking header pin set as they’re not included with the shield, and you’ll need to solder them on yourself. Power -the GSM shield can often require up to 2A of current in short bursts – especially when turned on, reset, or initiating a call. However your Arduino board can only supply just under 1A. It is highly recommended that you use an external regulated 5V power supply capable of delivering 2A of current – from an AC adaptor, large battery with power regulator, etc. Otherwise there is a very strong probability of damaging your shield and Arduino.
Ignore this at your own risk
When connecting this supply DO NOT use the DC socket on the Arduino. Instead, connect the 5V (positive) from the supply to the 5V pin on the GSM shield, and the negative to the GND pin.
If you’re looking for a more permanent or easy-to-wire solution, get yourself a DFRobot power shield:
This shield sits on top of your GSM shield (which sits on top of your Arduino). Before use you need to set it up:
- The only jumpers that should be on the power shield are as shown in the image above;
- Connect a power supply of between 9 and 35V DC to the blue terminal block at the bottom-left of the shield;
- Connect a voltmeter/multimeter to the other blue terminal block at the top-left and adjust the potentiometer (blue thing between the terminal blocks) until the voltage measured is 5 volts; ignore the LEDs on the shield as they’re not that accurate;
- Run a wire from the positive power output to the 5V pin on the shield, and run another one from the negative power output to a GND pin on the shield;
- If you have the USB cable connected to your project while operating the GSM shield, remove the USB cable before turning off external power to the project.
Here’s what it looks like once assembled with the antenna:
Next – use an antenna! The wire hanging from the shield is not an antenna. YOU NEED THE ANTENNA! There are two choices. Either use the smaller one for areas where handheld mobile reception is acceptable, such as this one:
Or if you are in an area of weaker reception, use an external antenna such as that used on a motor vehicle. If you are using the larger vehicle-style aerial, you might find that the plug will not fit to the shield’s connector. For example, consider the following:
On the left is the end of the lead from the carphone aerial, the right is the lead from the GSM shield. Problem! The solution is in the centre: an FME male to SMA male adaptor. This one came from element-14, part number 1826209 (it is a Multicomp R23-014-00-002611000).
Furthermore, care needs to be taken with your GSM shield with regards to the aerial lead-module connection, it is very fragile:
And finally, download this document (.pdf). It contains all the AT and ERROR codes that will turn up when you least expect it. Please review it if you are presented with a code you are unsure about.
Wow – all those rules and warnings?
The sections above may sound a little authoritarian, however I want your project to be a success. With the previous iterations of the tutorial people just didn’t follow the instructions – so I hope you do 
Are you using an Arduino Mega or Leonardo board?
Things are a little different for you. Those boards don’t support SoftwareSerial on digital pins 2 and 3 thus rendering the GSM shield a little trickier to use. Instead, bend back the D2 and D3 pins on the GSM shield as such (click image to enlarge):
Then run jumpers from D2 on the attached shield to D10 and another from D3 to D11. If you’re using the aforementioned power shield it would be on top of the GSM shield however the jumper wires would be the same. Finally in all the sketches, change the line SoftwareSerial cell(2,3); to SoftwareSerial cell(10,11); . If you have a Leonardo, get a Uno.
Initial check – does it work?
This may sound like a silly question, but considering the cost of the shield and the variables involved, it is a good idea to check if your setup is functioning correctly before moving on. From a hardware perspective for this article, you will need your Arduino board, the GSM shield with activated SIM card and an aerial, and a range of previously used components. Make sure your SIM card is set to not require a PIN when the phone is turned on. You can check and turn this requirement off with your cellphone. For our initial test, upload the following sketch:
#include <SoftwareSerial.h>
char incoming_char=0; //Will hold the incoming character from the Serial Port.
SoftwareSerial cell(2,3); //Create a 'fake' serial port. Pin 2 is the Rx pin, pin 3 is the Tx pin.
void setup()
{
//Initialize serial ports for communication.
Serial.begin(9600);
cell.begin(9600);
Serial.println("Starting SM5100B Communication...");
}
void loop()
{
//If a character comes in from the cellular module...
if(cell.available() >0)
{
incoming_char=cell.read(); //Get the character from the cellular serial port.
Serial.print(incoming_char); //Print the incoming character to the terminal.
}
//If a character is coming from the terminal to the Arduino...
if(Serial.available() >0)
{
incoming_char=Serial.read(); //Get the character coming from the terminal
cell.print(incoming_char); //Send the character to the cellular module.
}
}
Then connect the GSM shield, aerial, insert the SIM card and apply power. Open the serial monitor box in the Arduino IDE and you should be presented with the following:
It will take around fifteen to thirty seconds for the text above to appear in full. What you are being presented with is a log of the GSM module’s actions. But what do they all mean?
- +SIND: 1 means the SIM card has been inserted;
- the +SIND: 10 line shows the status of the in-module phone book. Nothing to worry about there for us at the moment;
- +SIND: 11 means the module has registered with the cellular network
- +SIND: 3 means the module is partially ready to communicate
- and +SIND: 4 means the module is registered on the network, and ready to communicate
If your terminal returned a +SIND 8 instead of 4, that is OK, we’ll sort that out in a moment. From this point on, we will need to use a different terminal program, as the Arduino IDE’s serial monitor box isn’t made for full two-way communications. You will need a terminal program that can offer full two-way com port/serial communication. For those running MS Windows, an excellent option is available here. It’s free, however consider donating for the use of it. For other operating systems, people say this works well. So now let’s try it out with the terminal software. Close your Arduino IDE serial monitor box if still open, then run your terminal, set it to look at the same serial port as the Arduino IDE was. Ensure the settings are 9600, 8, N, 1. Then reset your Arduino and the following should appear:
The next step is to tell the GSM module which network frequency(ies) to use. Please download this document (.pdf), and view page 127. There is a range of frequency choices that our module can use. If you don’t know which one to use, contact the telephone company that your SIM card came from. Australia – use option 4. Choose your option, then enter
AT+SBAND=x
(where X is the value matching your required frequency) into the terminal software and click SEND. Then press reset on the Arduino and watch the terminal display. You should hopefully be presented with the same text as above, ending with +SIND: 4. If your module returns +SIND: 4, we’re ready to move forward.
Our next test is to call our shield. So, pick up a phone and call it. Your shield will return data to the terminal window, for example:
As you can see, the module returns what is happening. I let the originating phone “ring” twice, and the module received the caller ID data (sorry, blacked it out). Some telephone subscribers’ accounts don’t send caller ID data, so if you don’t see your number, no problem. “NO CARRIER” occurred when I ended the call. +SIND: 6,1 means the call ended and the SIM is ready.
Have your Arduino “call you”
The document (.pdf) we downloaded earlier contains a list of AT commands – consider this a guide to the language with which we instruct the GSM module to do things. Let’s try out some more commands before completing our initial test. The first one is:
ATDxxxxxx
which dials a telephone number xxxxxx. For example, to call (212)-8675309 use
ATD2128675309
The next one is
ATH
which “hangs up” or ends the call. So, let’s reach out and touch someone. In the terminal software, enter your ATDxxxxxxxx command, then hit send. Let your phone ring. Then enter ATH to end the call. If you are experimenting and want to hang up in a hurry, you can also hit reset on the Arduino and it will end the call as well as resetting the system. So by now you should realise the GSM module is controlled by these AT commands.
To use an AT command in a sketch, we use the function
cell.println()
for example, to dial a phone number, we would use
cell.println("ATD2128675309");
To demonstrate this in a sketch, consider:
A simple sketch to dial a telephone number, wait, then hang up. Replace xxxxxxxx with the number you wish to call.
#include <SoftwareSerial.h>
SoftwareSerial cell(2,3);
void setup()
{
cell.begin(9600);
delay(25000); // give the GSM module time to initialise, locate network etc.
// this delay time varies. Use example 26.1 sketch to measure the amount
// of time from board reset to SIND: 4, then add five seconds just in case
}
void loop()
{
cell.println("ATDxxxxxxxxx"); // dial the phone number xxxxxxxxx
// change xxxxxxx to your desired phone number (with area code)
delay(20000); // wait 20 seconds.
cell.println("ATH"); // end call
do // remove this loop at your peril
{
delay(1);
}
while (1>0);
}
The sketch in example 26.2 assumes that all is well with regards to the GSM module, that is the SIM card is ok, there is reception, etc. The delay function in void setup() is used to allow time for the module to wake up and get connected to the network. Later on we will read the messages from the GSM module to allow our sketches to deal with errors and so on. However, you can see how we can simply dial a telephone. You could now have a home alarm system that can call you upon an event happening, etc.
Send an SMS from your Arduino
Another popular function is the SMS or short message service, or text messaging. Before moving forward, download and install Meir Michanie’s SerialGSM Arduino library from here.
Sending a text message is incredibly simple – consider the following sketch:
#include <SerialGSM.h>
#include <SoftwareSerial.h>
SerialGSM cell(2,3);
void setup()
{
delay(30000); // wait for GSM module
cell.begin(9600);
}
void sendSMS()
{
cell.Verbose(true); // used for debugging
cell.Boot();
cell.FwdSMS2Serial();
cell.Rcpt("+xxxxxxxxx"); // replace xxxxxxxxx with the recipient's cellular number
cell.Message("Contents of your text message");
cell.SendSMS();
}
void loop()
{
sendSMS();
do // remove this loop at your peril
{
delay(1);
}
while (1>0);
}
It’s super-simple – just change the phone number to send the text message, and of course the message you want to send. The phone numbers must be in international format, e.g. Australia 0418 123456 is +61418123456 or USA (609) 8675309 is +16098675309.
Well that is all we have time for in this instalment. Now you can move onto GSM and Arduino – part two.
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