How to Use GSM SIM900A: Examples, Pinouts, and Specs

The GSM SIM900A is a compact and reliable GSM/GPRS module designed for communication over mobile networks. It supports dual-band GSM (900/1800 MHz) and provides functionalities such as sending and receiving SMS, making voice calls, and accessing the internet via GPRS. The module is widely used in IoT applications, remote monitoring systems, home automation, and vehicle tracking systems due to its versatility and ease of integration.

• Internet of Things (IoT) devices for remote communication
• SMS-based alert and notification systems
• Voice call-enabled embedded systems
• GPS tracking and vehicle monitoring
• Home automation and security systems
• Industrial automation and telemetry

The GSM SIM900A module is designed to operate efficiently in a variety of environments. Below are its key technical details:

The SIM900A module typically comes with a breakout board for easier integration. Below is the pin configuration:

The GSM SIM900A module can be easily integrated into a circuit for communication purposes. Below are the steps and best practices for using the module:

1. Power Supply: Connect the VCC pin to a stable 4.0V power source and GND to ground. Ensure the power supply can handle peak currents of up to 2A.
2. UART Communication: Connect the TXD and RXD pins to the UART pins of a microcontroller (e.g., Arduino UNO). Use a logic level converter if the microcontroller operates at 5V logic.
3. SIM Card: Insert a valid SIM card into the SIM card slot. Ensure the SIM card is activated and has sufficient balance for SMS, calls, or data usage.
4. Antenna: Attach a GSM antenna to the module for better signal reception.
5. Initialization: Use AT commands to initialize the module and configure its settings. For example, send AT to check if the module is responding.

• Use decoupling capacitors near the power supply pins to stabilize the voltage.
• Ensure the antenna is securely connected to avoid signal issues.
• Avoid placing the module near high-frequency components to reduce interference.
• Use proper heat dissipation techniques if the module operates in high-temperature environments.

Below is an example of how to send an SMS using the GSM SIM900A module with an Arduino UNO:

• Connect SIM900A TXD to Arduino RX (Pin 0).
• Connect SIM900A RXD to Arduino TX (Pin 1).
• Connect SIM900A VCC to a 4.0V power source and GND to ground.

#include <SoftwareSerial.h>

// Define RX and TX pins for SoftwareSerial
SoftwareSerial SIM900A(7, 8); // RX = Pin 7, TX = Pin 8

void setup() {
  // Initialize serial communication
  Serial.begin(9600); // For communication with the PC
  SIM900A.begin(9600); // For communication with the SIM900A module

  // Wait for the module to initialize
  delay(1000);
  Serial.println("Initializing SIM900A...");

  // Send AT command to check module response
  SIM900A.println("AT");
  delay(1000);
  while (SIM900A.available()) {
    Serial.write(SIM900A.read()); // Print module response to Serial Monitor
  }

  // Send SMS command
  SIM900A.println("AT+CMGF=1"); // Set SMS mode to text
  delay(1000);
  SIM900A.println("AT+CMGS=\"+1234567890\""); // Replace with recipient's phone number
  delay(1000);
  SIM900A.println("Hello, this is a test SMS from SIM900A!"); // SMS content
  delay(1000);
  SIM900A.write(26); // Send Ctrl+Z to send the SMS
  delay(1000);
  Serial.println("SMS sent!");
}

void loop() {
  // No actions in the loop
}

1. Module Not Responding to AT Commands

   • Ensure the module is powered correctly and the SIM card is inserted.
   • Check the UART connections and baud rate settings.

2. No Network Signal

   • Verify that the antenna is connected securely.
   • Check if the SIM card is activated and has network coverage.

3. SMS Not Sending

   • Ensure the SIM card has sufficient balance.
   • Verify the recipient's phone number format (e.g., include the country code).

4. Module Restarts Frequently

   • Check if the power supply can handle peak currents of up to 2A.
   • Use a capacitor (e.g., 1000µF) near the power pins to stabilize the voltage.

Q: Can the SIM900A module be used with 5V microcontrollers?
A: Yes, but you need a logic level converter to interface the 3.3V UART pins with 5V logic.

Q: Does the module support 4G networks?
A: No, the SIM900A only supports GSM (2G) networks.

Q: How can I check the signal strength?
A: Use the AT command AT+CSQ. The module will return a signal strength value.

Q: Can I use the module for internet access?
A: Yes, the SIM900A supports GPRS for basic internet connectivity. Use AT commands to configure GPRS settings.