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ESP32 with SIM900A GSM Module

15-07-2025

📡 What is SIM900?

The SIM900 is a GSM/GPRS module from SIMCom. It allows microcontrollers like ESP32 or Arduino to:

✅ Send/receive SMS
✅ Make/receive phone calls
✅ Connect to the Internet (GPRS)
✅ Send data to servers via HTTP, MQTT, TCP/IP

It communicates via UART (TX/RX) and uses AT commands.

📦 Features of SIM900

Feature	Description
Network	2G GSM (850/900/1800/1900 MHz)
Communication	UART (AT commands)
Baud Rate	Default 9600
SIM Card	Standard-size SIM (2G supported)
Power Supply	5V–12V, 2A recommended
Antenna	Required for GSM signal
Optional	Mic & Speaker (for calls)

🔌 SIM900 to ESP32 Connection

SIM900 Pin	ESP32 Pin
TX	GPIO 16 (ESP32 RX)
RX	GPIO 17 (ESP32 TX)
GND	GND
VCC	⚠️ External 5V 2A power supply

⚠️ DO NOT power SIM900 directly from ESP32 or USB. It needs high current, especially during network operations.

🧾 Arduino Code Example – Sending SMS with ESP32 + SIM900

🧰 Libraries Required:

You don't need extra libraries – just Serial and AT commands.

⚙️ Wiring

SIM900 TX → ESP32 RX (GPIO 16)
SIM900 RX → ESP32 TX (GPIO 17)
SIM900 GND ↔ ESP32 GND

SIM900 VCC → External 5V 2A

📱 Other Useful AT Commands:

Command	Function
`AT`	Check module
`AT+CSQ`	Signal strength
`AT+CCID`	SIM card number
`AT+CREG?`	Network registration
`ATD+91XXXXXXXXXX;`	Make a call
`ATH`	Hang up
`AT+CMGR=1`	Read SMS from slot 1
`AT+CMGD=1`	Delete SMS from slot 1

⚙️ Using Serial Monitor to Test

You can manually send AT commands via Serial Monitor using this setup:

Use Serial passthrough sketch to forward Serial to SIM900.

📡 Real-World Projects Using SIM900 + ESP32

Project	Description
🚨 GSM Alarm System	Send SMS when motion or fire is detected
📦 Asset Tracker	Send location data via SMS (with GPS)
🛜 IoT without WiFi	Use GPRS to send data to cloud
🔐 Smart Door Lock	Unlock with OTP via SMS
📈 Remote Sensor	Send sensor values (temperature, moisture, etc.) to phone

⚠️ Power Supply Tips

SIM900 can draw up to 2A peak. If underpowered, it may restart or fail to send SMS.
Use external power supply (e.g., 5V 2A adapter or battery pack).
Keep GND common between ESP32 and SIM900.

❓ Common Issues and Fixes

Problem	Solution
No SMS sent	Check SIM is active, balance, and signal
Module keeps restarting	Power supply issue – use 2A source
AT commands don’t respond	Check TX/RX pins or baud rate
"SIM not inserted"	Clean SIM contacts and reinsert

HOW TO OPERATE

// Define ESP32 hardware serial port for SIM900

#define SIM900_TX 17 // ESP32 TX connected to SIM900 RX

#define SIM900_RX 16 // ESP32 RX connected to SIM900 TX

#define PWRKEY 4 // SIM900 Power Key pin (adjust if needed)

#define GSM_BAUD 9600

// Initialize hardware serial for SIM900

HardwareSerial sim900(2);

void powerOnSIM900() {

pinMode(PWRKEY, OUTPUT);

digitalWrite(PWRKEY, LOW);

delay(1000); // Hold PWRKEY low for 1 second

digitalWrite(PWRKEY, HIGH);

delay(9000); // Wait for the module to initialize

}

void setup() {

Serial.begin(115200); // Serial Monitor

sim900.begin(GSM_BAUD, SERIAL_8N1, SIM900_RX, SIM900_TX); // SIM900 UART

powerOnSIM900();

Serial.println("Testing AT communication...");

// Test AT command

sendATCommand("AT");

// Set SMS text mode

sendATCommand("AT+CMGF=1");

// Send SMS

Serial.println("Sending SMS...");

sim900.println("AT+CMGS=\"+919305706040\""); // Replace with recipient's number

delay(1000);

sim900.print("Hello from ESP32 and SIM900");

sim900.write(26); // CTRL+Z to send

delay(5000);

printResponse();

}

void loop() {

// Add code to handle incoming messages or other functionalities

}

// Function to send an AT command and print response

void sendATCommand(const char *command) {

Serial.print("Sending: ");

Serial.println(command);

sim900.println(command);

delay(1000);

printResponse();

}

// Function to print response from SIM900

void printResponse() {

while (sim900.available()) {

Serial.write(sim900.read());

}

Serial.println("----------------------");

}

ESP32 with SIM900A GSM Module

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