Complete SIM900 GSM/GPRS Shield with Arduino Guide

1 SIM Card Preparation

Before using your SIM900 GSM/GPRS Shield, you need to properly prepare your SIM card:

• Use a Prepaid SIM Card: For testing, use a prepaid plan with unlimited SMS to avoid unexpected charges
• Disable PIN Lock: Insert the SIM in a smartphone and disable the PIN requirement in security settings
• Check Network Coverage: Ensure you have GSM coverage (2G network) in your area
• SIM Size: The shield uses standard SIM size (not micro or nano - adapters may be needed)

2 Power Supply Requirements

The SIM900 GSM/GPRS Shield has specific power requirements for reliable operation:

• Recommended: 5V power supply capable of providing 2A current
• Alternative Options: 9V at 1A or 12V at 1A
• Toggle Switch: Move to external power position when using dedicated power supply
• Power Jack: Standard DC power jack for external power connection

3 Getting Started Steps

1. Insert SIM Card: Place your prepared SIM card into the holder on the back of the shield
2. Connect Antenna: Attach the provided antenna to the U.FL connector for optimal signal
3. Configure Serial Port: Set jumpers for software serial communication (typically pins 7 and 8)
4. Power Up: Connect your 5V, 2A power supply and flip toggle to external power position
5. Turn On Shield: Press and hold power button for 2 seconds until Status LED lights up
6. Network Connection: NetLight LED blinks every 800ms while searching, then every 3 seconds when connected

4 Initial Testing Procedure

Before integrating with Arduino, test the shield independently using an FTDI programmer:

1. Connect FTDI Programmer: TX to shield RX, RX to shield TX, ground to ground
2. Power Separately: Use external 5V, 2A power supply (don't use FTDI for power)
3. Open Arduino IDE Serial Monitor: Select correct COM port, set to 19200 baud, Carriage return
4. Send Test Command: Type "AT" and press Enter - should receive "OK" response

5 Complete SMS Sending Code

Here's the complete code to send an SMS using your Arduino and SIM900 shield:

#include 

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

void setup() {
  // Arduino communicates with SIM900 at 19200 baud
  SIM900.begin(19200);
  // Give time for network registration
  delay(20000);   
  // Send the SMS
  sendSMS();
}

void loop() { 
  // Empty loop for this example
}

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

  // REPLACE WITH RECIPIENT'S MOBILE NUMBER
  // USE INTERNATIONAL FORMAT
  SIM900.println("AT+CMGS=\"+12345678901\""); 
  delay(100);

  // REPLACE WITH YOUR MESSAGE CONTENT
  SIM900.println("Hello from Arduino with SIM900!"); 
  delay(100);

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

Code Explanation: This example uses SoftwareSerial on pins 7 and 8 to communicate with the SIM900 shield. The sendSMS() function configures the module for text messages, specifies the recipient number, sends the message content, and terminates with Ctrl+Z (ASCII 26).

6 SMS Reception Code Example

To read incoming SMS messages, upload this code to your Arduino:

#include 

SoftwareSerial SIM900(7, 8);
char incoming_char = 0;

void setup() {
  SIM900.begin(19200);
  Serial.begin(19200); // For serial monitor
  delay(20000); // Network registration time

  // Set SIM900 to SMS mode
  SIM900.print("AT+CMGF=1\r"); 
  delay(100);
  // Configure new message indications
  SIM900.print("AT+CNMI=2,2,0,0,0\r");
  delay(100);
}

void loop() {
  // Display incoming SMS on serial monitor
  if(SIM900.available() > 0) {
    incoming_char = SIM900.read(); 
    Serial.print(incoming_char); 
  }
}

How It Works: This code configures the SIM900 module to forward received SMS directly to the serial output. When a message arrives, it's displayed character-by-character in the Arduino Serial Monitor. You can extend this code to parse specific information or trigger actions based on message content.

7 Phone Call Functionality

The SIM900 GSM/GPRS Shield supports full voice call functionality. For phone calls to work properly:

• Audio Connections: Connect a microphone to the MIC jack and headphones/speaker to the SPK jack
• Call Initiation: Use ATD+PHONE_NUMBER; command to dial a number
• Call Answering: Use ATA command to answer incoming calls
• Call Termination: Use ATH command to hang up active calls

8 Software-Controlled Power Management

Instead of manually pressing the power button, you can enable software-controlled power management:

1. Hardware Modification: Solder a bridge across the R13 resistor pads on the shield
2. Connect to Arduino: Run a wire from shield D9 to Arduino digital pin 9
3. Add to Setup: Include this code in your setup() function:

// Automatic power-on sequence
pinMode(9, OUTPUT);
digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);

This sequence simulates pressing the physical power button for one second, eliminating manual intervention and enabling completely autonomous systems.