Control a 12V Lamp via SMS with Arduino

In this tutorial we’re going to show you how you can control a 12V lamp via SMS using an Arduino UNO, a relay module, and the SIM900 GSM shield.

Before proceeding with this tutorial we recommend the following resources:

First, watch the video demonstration

Project overview

This project uses the SIM900 GSM shield to receive and send SMS with the Arduino. This projects aims to:

  • turn a 12V lamp on when you send an SMS to the Arduino with the text “ON”
  • turn a 12V lamp off when you send an SMS to the Arduino with the text “OFF”
  • you can request the lamp state by sending an SMS to the Arduino with the text “STATE”, the Arduino should reply back with the text “lamp is on” or “lamp is off”

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Parts required

In this project you need to connect the Arduino to the GSM shield and to a relay module connected to a 12V lamp. Here’s a complete list of the parts required for this project (click the links below to find the best price at Maker Advisor):

You can use the preceding links or go directly to MakerAdvisor.com/tools to find all the parts for your projects at the best price!

Preparing the GSM Shield

To use the SIM900 GSM shield:

1) You need an active SIM card. We recommend using a SIM card with a prepaid plan for testing purposes.

2) You have to turn off the SIM card pin lock. Insert the SIM card in your smartphone and turn off the pin lock in the phone security settings.

3) The GSM shield should be powered using an external 5V power supply that can provide 2A, or 9V 1A, or 12V 1A.

4) We’re going to use software serial to communicate with the shield. So, on the serial port select, make sure the jumper cap is connected as shown in figure below to use software serial.

5) You can turn on/off the shield by pressing the power key, or you can turn it on automatically with the Arduino via the D9 pin. To automatically turn the shield on and off, you need to solder the R13 connections as shown in the figure below.

6) If this is your first time dealing with the GSM shield, we recommend you reading the Preliminary Steps in our GSM shield guide here.

7) Insert the SIM card into the SIM card holder.

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8) Finally, wire the SIM900 GSM shield to the Arduino by following the schematic below.

9) You should power up the SIM900 GSM shield using the external 5V power supply. Make sure you select the external power source with the toggle switch next to the DC jack.

 

Note: You can test it the shield is working properly by sending AT comments from the Arduino IDE with an FTDI programmer – check Guide to SIM900 GSM GPRS Shield with Arduino

Wiring the Relay Module

In this project we control a 12V lamp. However, this project can be easily adapted to control mains voltage or other electronics appliances. Check our Guide for Relay Module with Arduino to learn how to control mains voltage with a relay.

We are using the following 12V lamp and its corresponding lamp holder.

Add the relay module and the lamp holder to the previous circuit.

For safety purposes, we’re placing our relay inside a plastic box enclosure.

Code

Below you can find the code for this project. Make sure you edit the code with the phone number the Arduino should reply to. We recommend you reading the code explanation before uploading the code.

/*
 * Complete Project Details https://randomnerdtutorials.com
 */

// Include Software Serial library to communicate with GSM
#include <SoftwareSerial.h>

// Configure software serial port
SoftwareSerial SIM900(7, 8);

// Variable to store text message
String textMessage;

// Create a variable to store Lamp state
String lampState = "HIGH";

// Relay connected to pin 12
const int relay = 12;

void setup() {
  // Automatically turn on the shield
  digitalWrite(9, HIGH);
  delay(1000);
  digitalWrite(9, LOW);
  delay(5000);
  
  // Set relay as OUTPUT
  pinMode(relay, OUTPUT);

  // By default the relay is off
  digitalWrite(relay, HIGH);
  
  // Initializing serial commmunication
  Serial.begin(19200); 
  SIM900.begin(19200);

  // Give time to your GSM shield log on to network
  delay(20000);
  Serial.print("SIM900 ready...");

  // AT command to set SIM900 to SMS mode
  SIM900.print("AT+CMGF=1\r"); 
  delay(100);
  // Set module to send SMS data to serial out upon receipt 
  SIM900.print("AT+CNMI=2,2,0,0,0\r");
  delay(100);
}

void loop(){
  if(SIM900.available()>0){
    textMessage = SIM900.readString();
    Serial.print(textMessage);    
    delay(10);
  } 
  if(textMessage.indexOf("ON")>=0){
    // Turn on relay and save current state
    digitalWrite(relay, LOW);
    lampState = "on";
    Serial.println("Relay set to ON");  
    textMessage = "";   
  }
  if(textMessage.indexOf("OFF")>=0){
    // Turn off relay and save current state
    digitalWrite(relay, HIGH);
    lampState = "off"; 
    Serial.println("Relay set to OFF");
    textMessage = ""; 
  }
  if(textMessage.indexOf("STATE")>=0){
    String message = "Lamp is " + lampState;
    sendSMS(message);
    Serial.println("Lamp state resquest");
    textMessage = "";
  }
}  

// Function that sends SMS
void sendSMS(String message){
  // AT command to set SIM900 to SMS mode
  SIM900.print("AT+CMGF=1\r"); 
  delay(100);

  // REPLACE THE X's WITH THE RECIPIENT'S MOBILE NUMBER
  // USE INTERNATIONAL FORMAT CODE FOR MOBILE NUMBERS
  SIM900.println("AT + CMGS = \"XXXXXXXXXX\""); 
  delay(100);
  // Send the SMS
  SIM900.println(message); 
  delay(100);

  // End AT command with a ^Z, ASCII code 26
  SIM900.println((char)26); 
  delay(100);
  SIM900.println();
  // Give module time to send SMS
  delay(5000);  
}

View raw code

The code is pretty straightforward to understand. Continue reading to understand what each section of code aims to do.

Import library

The Arduino communicates with the shield using software serial communication. So, you start by including the SoftwareSerial.h library.

// Include Software Serial library to communicate with GSM
#include <SoftwareSerial.h>

Software serial port

Then, you create a software serial port on pins 7 and 8 (pin 7 is set as RX and 8 as TX).

// Configure software serial port
SoftwareSerial SIM900(7, 8);

Creating variables

You create a textMessage variable that will hold the text message received by the Arduino; and another variable to store the current lamp state (relay state), called lampState. The lampState variable is set to HIGH by default, because we want the relay off by default. The relay works with inverted logic, so you need to set it to HIGH to turn it off.

// Variable to store text message
String textMessage;

// Create a variable to store Lamp state
String lampState = "HIGH";

You also assign the pin number the relay is connected to, pin 12.

// Relay connected to pin 12
const int relay = 12;

setup()

In the setup() function, you start by turning on the GSM shield. The following lines are the equivalent of pressing the shield “power” button.

digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);
delay(5000);

The relay is set as an output, and its state is off by default.

// Set relay as OUTPUT
pinMode(relay, OUTPUT);

// By default the relay is off
digitalWrite(relay, HIGH);

Then, you initialize serial communication with the shield, and give it some time to log on to the network.

// Initializing serial commmunication
Serial.begin(19200); 
SIM900.begin(19200);

// Give time to your GSM shield log on to network
delay(20000);
Serial.print("SIM900 ready...");

The shield is set to SMS mode with the following:

// AT command to set SIM900 to SMS mode
SIM900.print("AT+CMGF=1\r"); 
delay(100);
// Set module to send SMS data to serial out upon receipt 
SIM900.print("AT+CNMI=2,2,0,0,0\r");
delay(100);

loop()

In the loop(), you read the incoming messages and compare if those messages contain the text ON, OFF or STATE. Accordingly to the message content, the Arduino will perform different tasks.

So, you start by checking whether there are messages to read. If there are, the message will be saved on the textMessage variable. For debugging purposes, the message is displayed on the serial monitor.

if(SIM900.available() >0) {
 textMessage = SIM900.readString();
 Serial.print(textMessage); 
 delay(10);
}

Turn the lamp on

If the message received contains the text “ON”, then the relay will be turned on, turning on the lamp. Then, you save the current lamp state on the lampState variable. Finally, you clean the textMessage variable, so that it is empty to receive new SMS.

if (textMessage.indexOf("ON") >= 0){
// Turn on relay and save current state
digitalWrite(relay, LOW);
lampState = "on";
Serial.println("Relay set to ON"); 
textMessage = "";

Turn the lamp off

On  the other side, if the SMS received contains the text “OFF”, the relay will be turned off, the new state will be saved, and the textMessage is cleaned.

if (textMessage.indexOf("OFF") >= 0){
// Turn off relay and save current state
digitalWrite(relay, HIGH);
lampState = "off"; 
Serial.println("Relay set to OFF");
textMessage = "";

Returning the lamp state

Finally, if the incoming SMS contains the text “STATE”, the Arduino should send a message to a predefined phone number saying whether the lamp is currently on or off.

if (textMessage.indexOf("STATE") >= 0){
  String message = "Lamp is " + lampState;
  sendSMS(message);
  Serial.println("Lamp state resquest");
  textMessage = "";

The message is sent using the sendSMS() function defined at the bottom of the code.

Sending an SMS

The sendSMS() function created at the end of the code, accepts a string as an argument. That string should be the message to be sent.

The number the Arduino answers to is set at the following line:

SIM900.println("AT + CMGS = \"XXXXXXXXXXXX\"");

Replace the XXXXXXXXXXXX with the recipient’s phone number.

Note: you must add the number according to the international phone number format. For example, in Portugal the number is preceded by +351XXXXXXXXX.

After adding the phone number the Arduino should reply to, you can copy the full code to your Arduino IDE and upload it to your Arduino board.

Wrapping up

In this project we’ve shown you how you can use the SIM900 GSM shield to control a 12V lamp and to request data (the lamp state) to the Arduino.

The concepts learned in this project can be applied to control any electronics appliances you like. You can also use what you’ve learned here to request the Arduino about sensor data – for example, request the latest readings of your weather station.

You can also build a surveillance system using a PIR motion sensor and a GSM shield that sends you an SMS when unexpected motion is detected.

We hope you’ve found this project useful. If you like this project, make sure you check our Arduino course: Arduino Step-by-step Projects with 25 Projects.

Thanks for reading.


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61 thoughts on “Control a 12V Lamp via SMS with Arduino”

  1. Really great project. What would be really interesting would be to adapt it so that BOTH relays could be operated by texting ON1, OFF1 and ON2, OFF2. I’m thinking of controlling heating and hot water systems separately.

  2. RTFM!
    You don’t need “a power supply that outputs different voltage values”.
    Both arduino and GSM shield can be powered from 7..12V supply.
    So 12V will be enough for all parts.

  3. Hi rui
    I would not recommend this power supply for amateurs. The mains input terminals are exposed and would offer a lethal shock if inadvertently touched. It would need to be mounted within enclosure to isolate terminals and provide some protection.

    Regards

    • Hi Graham.
      You are right. When dealing with that kind of power supplies you really need to know what you are doing.
      Adding an enclosure is a good way to isolate the terminals and is truly recommended.
      Thank you for warning us about that issue.
      Regards
      Sara 🙂

  4. Hello

    Excellent, very clearly written tutorial, thank you!

    Few issues I’d like to point out, even I do understand the nature of an simplified tutorial:

    – The state command doesn’t really tell if the lamp is burning or not. It merely tells if latest command was ON or OFF. The state command happily returns “Lamp is on” even if the lamp has burned out, 12 Volt source is not working, the relay has melted down due overcurrent… or the lamp is not really burning due to any other malfunction.
    Therefore if the application is anyway critical, I’d add an photoresistor or something else to examine if the lamp is really on.

    – Every relay has a maximum current. The relay module used as an example has a limit of max. 10A – a current that is easy to exceed with a couple of car headlight bulbs. I’d add an short mention of the relay max current, which a user should always check. Even the “Guide for Relay Module with Arduino” doesn’t mention the max current!

    Otherwise keep up the good work 🙂

    • Hi.
      Thank you for taking the time to share your tips with us.
      Indeed, adding a photoresistor is a good solution to check out the real lamp state.
      We wanted to show our readers how they request something to the Arduino. We think that after following this tutorial they can easily add the photoresistor to check out the real lamp status.
      Thanks for your feedback.
      Regards
      Sara

  5. You should probably note that this project will not work with a SIM900 module in Australia. There are no 2G networks in operation. You will need a 3G module – available but more expensive. Also ensure you buy one that meets the frequency range of the network you intend to use.

  6. Very elegant! I have used GSM-shields in several solutions. Using SMS gives good reliability and is dramatically simpler than GRPS/IoT solutions. – Strings quickly eats up Uno memory, but is excellent for a few short messages.

  7. hi
    thanks

    I am using different type of gsm module in my country but same sim900

    there is no jumper settings, R13 connections, etc., but have 4 pins 5v, gnd, tx and rx…

    what are the changes should i do?

    • Hi.
      I don’t know what kind of module you’re using.
      So, I don’t have enough info to fully answer your question.
      But if you have TX and RX, you should connect them to the Arduino pins we shown on the schematic diagram.
      You have to search what pin you should connect on your module to automatically turn on the shield.
      However, you can always manually turn on the shield, and in that case, you just need the TX and RX connections, GND, and VCC.
      I hope this helps.

  8. i am using some different gsm module and i can send and receive sms but output pin12 continuously giving 5v without change,., need solution…

  9. Excellent tutorials.
    I followed the previous ones, and they all work great.
    Is there a way to use this code to activate and deactivate a PIR motion sensor instead of a relay and make a call when motion is detected.
    Have tried modifying the code but it ether does nothing or makes a call on powering up without motion.
    Thanks
    kind regards
    when motion is detected,

    • Hi Richard.
      I haven’t tested that particular project.
      But I think you need to:
      – create an Interrupt on the PIR motion sensor pin on RISING mode.
      – when motion is detected the code should call a function that sends the SMS.
      It may also help taking a look at interrupts with Arduino: arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/
      I hope this helps 🙂

  10. That was a great tutorial, but my challenge is that after sending the first command say “ON” and it worked, the second command refused to arrive so nothing happens. What could be the problem? Thanks in anticipation

      • Ok, thanks for your prompt reply. The second could be any, either you want to turn on a second device or even want to turn the device you turned ON, ie. First -“ON” goes, but sending another ON or OFF does not do anything again. Thanks in anticipation

        • Hi Tammy. Are you using our code without any changes (just changing the phone number?).
          If it receives the first message, but not the second one, maybe the first message is not being “closed” or the code is stuck in some place.
          If you’re using our exact code, I don’t know what can be happening. This code worked just fine for us.
          I hope this helps.

    • Hello suleiman,
      With some changes in the code and more if statements, you should be able to control as many relays as your Arduino has digital pins, but I only have this example with one relay…

  11. I forgot to tell you that I have the answer like this ” ⸮ ” in the serial monitor with send ON or OFF

    • Make sure you change the baud rate in your Arduino IDE serial monitor to 19200, so you can see the information in the Serial monitor (the baud rate option is located on the bottom right corner of the Serial Monitor window).

      Regards,
      Rui

  12. Hello how to connect pin number on arduino Mega please? I’m connect Pin 18 and 19 for RX/TX and pin 9 sim900 to pin9 arduino mega but is not operational??
    Please help me?

  13. Would it be possible to turn the lamp on and off with a ring or always with a ring to keep it on for x seconds?

  14. Hey this project can use gsm sim900A. not sim900 but sim 900A. can? if can, how the circuit diagram to connect gsm sim900A to arduino uno?

  15. Hi..
    It’s really nice project…
    I tried it all works good.. except it’s not return state… only shows in serial monitor…
    Can you help..
    I’m using sim900a

    • Hello, I’m not sure what you mean… What exactly do you want to do? You can create an if statement that triggers an action based on the received message… Thanks for asking!

  16. Awesome tutorial. I am using a SIM800L module and had to fiddle with the code a bit. The line – SIM900.println(“AT + CMGS = \”XXXXXXXXXXXX\””) had spaces in the AT Code which I needed to delete before it worked.
    Thanks, much appreciated!!!!

    • Hi Bart, I am planning on using the SIM800L as well. I was wondering if you had ti change anything else to the code? (except for changing SIM900 to SIM800L of course :))

      Thanks in advance!

    • Hi Izari.
      I haven’t experimented with the SIM900A.
      But it seems that both boards use the same AT commands: pantechsolutions.net/blog/basic-at-commands-for-sim900a-gsmgprs-module/ so, the code should be compatible with both boards.
      Regards,
      Sara

    • Hi.
      The relay is controlled via SMS.
      So, it can be controlled from anywhere as long as you have GSM.
      Regards,
      Sara

  17. Hello

    Great project. Everything is working BUT when I try to check STATE I do not receive anything … there is no SMS back? What is wrong?

    Regards,
    Nesa

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