I am working on a project in which i have to send commands using sms (GSM shield) to the arduino uno to control dc motor. What i want to know is that how can i define the instruction strings that i can send in form of sms and arduino uno reads it and executes it.

P.S. I am a total beginner at arduino coding so please help me out.

  • You can define them in any way you want - they are strings, they just contain text. It's up to you to decide what you want that text to say and how you want the Arduino to act when it gets that text.,
    – Majenko
    Sep 11, 2015 at 14:00

1 Answer 1


Your GSM shield will receive the SMS which your Arduino code can retrieve as a string of characters (that description hides a mountain of detail, most of which you won't need to know). It doesn't care what the content of the SMS message is.

Your Arduino code will have to translate from the message text to motor-control commands so you want some text commands that will be easy to parse, but not so trivial that any random message might wrongly be interpreted as legitimate commands.

The digit pair "15" might command motor 1 to speed 5. But you wouldn't want any message with a couple of digits in it to change a motor. (You didn't say you had multiple motors, and if not you can leave that part out).

Let's make the message distinctive enough that it's unlikely to appear in some other context, yet simple enough to distinguish as legitimate. It would be nice if it's simple to thumb-type, too!

"motor,1,5" could convey the same information as the "15" in our first example, doesn't limit you one-digit values, isn't likely to appear in a "Hey Dude, lunch at 12:15?" type of conversation, is easy to parse and isn't too long to type on a tiny keyboard. You could extend it later, so "motor,1,+2" could mean to increase motor 1's speed by two units, "motor,1,off" could mean power it down (not just zero speed), and "motor,all,off", well, that's probably obvious.

Look up the function sscanf() [oops: not sprintf()] for one way to parse the incoming strings and collect their data.

Update: sscanf() does the reverse of sprintf(): It reads a string, collects values from it, and assigns them to variables. Search sscanf on the web for a description of its use.
Here is a sample program that reads a few possible SMS messages, checks whether they are motor commands (according to my simple example SMS above), and if it is, assigns the contained values to some variables in your program. You can see by the return values of sscanf() whether and how many conversions it made from each message, thus your code can tell whether a received SMS looks like a motor command, as well as what the command values are:

 * sscanf example - Uses sscanf to test and collect data from a
 * a command string.

#include <WProgram.h>

void test_sscanf(const char *sms);

// Sample of SMS messages to test whether our sscanf() format string
// can recognize good ones and reject bad ones. Experiment by adding
// ones of your own:
const char *SMS[] =
   "Hello World,3,2",
   "motor 123 456",
   "moto, 7, 13",
   "motorized, 4, 11"

void setup() {

   delay(1000);     // delay for user to launch a terminal (if you're quick)

   // Test each SMS example in the SMS array
   for( uint8_t s = 0; s < sizeof(SMS)/sizeof(char *); ++s ){
      test_sscanf(SMS[s]);  // test one SMS string

void loop() {
   ;            // nothing to do here

// Tests a sample SMS, prints the number of variables it assigned, and what
// their values were. '0' means it didn't assign any, probably because the
// command-word failed to match.
void test_sscanf(const char *sms)
   int16_t assigned, values[2];

   // Print the SMS and convert it.
   Serial.print("\nSMS: \""); Serial.print(sms); Serial.println("\"");
   assigned = sscanf(sms, "motor,%d,%d", &values[0], &values[1]);

   // Display the results
   Serial.print(assigned);  Serial.println(" values assigned"); // How many values? A good command has 2.
   if( assigned > 0 ){
      for( uint8_t i = 0; i < assigned; ++i){
     Serial.print("value["); Serial.print(i+1); Serial.print("] = "); // print each assigned value
  • So basically i have to define a variable with datatype 'char' then in the if-while-else loop we match the sms with the predefined value of the variable. Sep 11, 2015 at 19:56
  • You would first match "motor," (in my example) to decide whether this string is really a motor command. Then you would convert the two following numeric values and use them to control your motor.
    – JRobert
    Sep 12, 2015 at 1:45
  • how do i do that? Sep 12, 2015 at 6:36
  • and i forgot to mention i am using single 24v dc motor with driver ic ofcourse. Sep 12, 2015 at 6:39
  • I am unable to understand >sscanf() Sep 12, 2015 at 6:46

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