A Simple Guide To SIM800L: Control Your IoT Projects From Anywhere

I think Sim800l is one of most valuable modules in IOT because in conditions that you don't have internet or WIFI, you can use it only with a sim card. Also its so cheap and it is so easy to run and use. In my experiments i found it so easy to start working and it only needs a good power source. based on data sheet it needs between 3.4 to 4.2 volt, but i found around 4.1 volt is good and it will has a stable connection without any problems. When you power up Sim800l, if you see led on module blinks every 1 seconds it mean connection has problem and i found in most of cases its because of power supply. when it blinks every 3 seconds, its ok and connected. I use a 18650 battery which has around 4.2 volt when it is fully charged. You can also use Li-Po batteries.
One of other benefits of Sim800l is its power usage which is very low and make it a good choice for portable devices. In sleep mode it only use 1 mAh and when it is under load for receiving SMS, it uses around 50 mAh. I calculated for a normal usage with a 5000 mAh battery, it last around 2 weeks.
In this blog at first we just power up Sim800l and show received SMS in terminal, next we add a feature to send back a confirmation SMS as a SMS arrived and in last step we control a simple led with a command that we will send it through our SMS. This project is a starting point for using Sim800l and in future i am going to make other projects with it and replace this simple led with other things like GPS module to make a Tracker Device.

Project Overview

This repository contains a step-by-step implementation of SMS-based control using the SIM800L GSM module. The project is divided into three main parts:

1. Basic SMS Reception - Setting up the SIM800L to receive and display SMS messages in the serial monitor
2. LED Control via SMS - Extending the basic setup to control an LED by sending "ON" or "OFF" text messages
3. SMS Reception and Confirmation - Like first part but it will sends back a confirmation SMS that your SMS delivered.

Perfect for IoT enthusiasts looking to add remote control capabilities to their projects without relying on WiFi or Bluetooth.

Hardware Requirements

• Arduino UNO or compatible board
• SIM800L GSM/GPRS module
• SIM card with SMS capability (and some credit)
• LED
• 220Ω resistor
• Breadboard and jumper wires
• Power supply (The SIM800L typically needs 3.7-4.2V)

SIM800l Pinout

Wiring Connections

SIM800L Pin	Arduino Pin
RX	D2
TX	D3
VCC	3.7-4.2V external supply
GND	GND

For the LED control project:

• Connect LED anode (longer leg) to Arduino pin D8
• Connect LED cathode (shorter leg) to a 220Ω resistor
• Connect the other end of the resistor to GND

Wiring Connections Diagram Without LED

Wiring Connections Diagram With LED

My Wiring Connection (Sorry if it's not so nice)

Software Setup

1. Install the Arduino IDE if you haven't already
2. No additional libraries are required for this project
3. Upload the appropriate sketch to your Arduino:
   • withoutLED.ino for basic SMS reception
   • WithLED.ino for LED control via SMS
   • WithoutLED&SendBackConfirm.ino for SMS reception and confirmation.

Project Progression

Part 1: Basic SMS Reception

The first sketch demonstrates how to:

• Initialize the SIM800L module
• Configure it to receive SMS messages
• Display incoming messages in the serial monitor

This provides a foundation for understanding how to communicate with the SIM800L module.
Sketch:

#include <SoftwareSerial.h>

#define SIM800_RX_PIN 2
#define SIM800_TX_PIN 3
#define SIM800_RST_PIN 4  // Optional, can be used to reset the module

SoftwareSerial sim800(SIM800_TX_PIN, SIM800_RX_PIN); // RX, TX for Arduino

void setup() {
  Serial.begin(9600);
  sim800.begin(9600);
  delay(1000);

  Serial.println("Initializing SIM800L...");
  sim800.println("AT");
  delay(1000);
  sim800.println("AT+CMGF=1"); // Set SMS text mode
  delay(1000);
  sim800.println("AT+CNMI=1,2,0,0,0"); // Auto show SMS
  delay(1000);
}

void loop() {
  if (sim800.available()) {
    Serial.write(sim800.read());
  }
  if (Serial.available()) {
    sim800.write(Serial.read());
  }
}

Part 2: Adding SMS Confirmation

Building on the first part, this sketch adds:

• Module sends back a confirmation that our SMS has been successfully received. Sketch:

#include <SoftwareSerial.h>

#define SIM800_RX_PIN 2
#define SIM800_TX_PIN 3
#define SIM800_RST_PIN 4  // Optional, can be used to reset the module

SoftwareSerial sim800(SIM800_TX_PIN, SIM800_RX_PIN); // RX, TX for Arduino

String incomingData = "";
bool messageReceived = false;
String senderNumber = "";

void setup() {
  Serial.begin(9600);
  sim800.begin(9600);
  delay(1000);

  Serial.println("Initializing SIM800L...");

  // Reset module if needed
  // pinMode(SIM800_RST_PIN, OUTPUT);
  // digitalWrite(SIM800_RST_PIN, LOW);
  // delay(1000);
  // digitalWrite(SIM800_RST_PIN, HIGH);
  // delay(3000);

  // Initialize module
  sendATCommand("AT", 1000);
  sendATCommand("AT+CMGF=1", 1000); // Set SMS text mode
  sendATCommand("AT+CNMI=1,2,0,0,0", 1000); // Configure SMS notification

  Serial.println("SIM800L Ready");
}

void loop() {
  // Check for incoming data from SIM800L
  while (sim800.available()) {
    char c = sim800.read();
    incomingData += c;
    Serial.write(c); // Echo to serial monitor
    delay(10);
  }

  // Process complete messages
  if (incomingData.length() > 0) {
    // Check if it's an SMS message notification
    if (incomingData.indexOf("+CMT:") != -1) {
      // Extract sender phone number
      int phoneStartIndex = incomingData.indexOf("+CMT: \"") + 7;
      int phoneEndIndex = incomingData.indexOf("\"", phoneStartIndex);

      if (phoneStartIndex > 7 && phoneEndIndex != -1) {
        senderNumber = incomingData.substring(phoneStartIndex, phoneEndIndex);
        Serial.print("Sender number: ");
        Serial.println(senderNumber);

        // Send reply SMS
        sendSMS(senderNumber, "Your SMS has been received. Thank you!");
      }
    }

    // Clear the buffer after processing
    incomingData = "";
  }

  // Handle commands from Serial monitor
  if (Serial.available()) {
    sim800.write(Serial.read());
  }
}

// Function to send AT commands
void sendATCommand(String command, int timeout) {
  sim800.println(command);
  delay(timeout);
  while (sim800.available()) {
    Serial.write(sim800.read());
  }
}

// Function to send SMS
void sendSMS(String number, String message) {
  Serial.println("Sending SMS reply...");

  sim800.println("AT+CMGF=1"); // Set SMS text mode
  delay(500);

  sim800.print("AT+CMGS=\"");
  sim800.print(number);
  sim800.println("\"");
  delay(500);

  sim800.print(message);
  delay(500);
  sim800.write(26); // ASCII code for CTRL+Z to send message

  delay(3000); // Wait for message to be sent

  Serial.println("SMS sent!");
}

Part 3: LED Control via SMS

Building on the first part, this sketch adds:

• LED control functionality
• SMS command parsing
• Feedback through the serial monitor

Send "ON" or "OFF" in an SMS to the SIM card number to control the LED remotely.
Sketch:

#include <SoftwareSerial.h>

#define SIM800_RX_PIN 2
#define SIM800_TX_PIN 3
#define SIM800_RST_PIN 4  // Optional, not used here
#define LED_PIN 8        // Connect an LED to this pin

SoftwareSerial sim800(SIM800_TX_PIN, SIM800_RX_PIN); // RX, TX for Arduino

String incomingData = ""; // To hold incoming data from SIM800

void setup() {
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW); // Turn off LED initially

  Serial.begin(9600);
  sim800.begin(9600);
  delay(1000);

  Serial.println("Initializing SIM800L...");
  sim800.println("AT");
  delay(1000);
  sim800.println("AT+CMGF=1"); // Set SMS text mode
  delay(1000);
  sim800.println("AT+CNMI=1,2,0,0,0"); // Show SMS directly
  delay(1000);
}

void loop() {
  while (sim800.available()) {
    char c = sim800.read();
    Serial.write(c);
    incomingData += c;

    // Check for end of SMS
    if (c == '\n') {
      incomingData.trim(); // Remove extra spaces/newlines

      if (incomingData.indexOf("ON") != -1) {
        digitalWrite(LED_PIN, HIGH);
        Serial.println("LED turned ON");
      }
      else if (incomingData.indexOf("OFF") != -1) {
        digitalWrite(LED_PIN, LOW);
        Serial.println("LED turned OFF");
      }

      incomingData = ""; // Clear buffer for next message
    }
  }

  // Optional: forward Serial input to SIM800 for debugging
  while (Serial.available()) {
    sim800.write(Serial.read());
  }
}

Troubleshooting Tips

• Power Issues: The SIM800L is sensitive to power fluctuations. Use a dedicated power supply capable of handling current spikes.
• Network Registration: Make sure the SIM card is activated and has good signal strength.
• AT Commands: If you're having trouble, try sending basic AT commands through the serial monitor to test communication.
• Baud Rate: Ensure both the Arduino and SIM800L are communicating at the same baud rate (9600 in this project).

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👉 You can find the full source code here:
GitHub Repo: payamhsn/Iot-Sim800l-Starter

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At the end, i think this module is so valuable, because with this module and a relay which is a low cost setup, you can connect to any device with any voltage, from a Lamp to an Air condition systems and control them from anywhere that you are.
Thanks a lot for reading my blog. I wrote everything by myself and a little help of AI in making tables and some content. Love & Respect