SIM900A GSM/GPRS Module

The SIM900A is a quad-band GSM/GPRS module (900/1800 MHz, often 850/1900 MHz depending on region).

It’s basically a tiny cellular phone for your microcontroller:

Send and receive SMS messages

Make and receive voice calls

Connect to the internet via GPRS (TCP/IP, HTTP, FTP)

Works with Arduino, Raspberry Pi, PIC, STM32

Most modules come as a board with antenna connector, power regulation, and UART interface for easy integration.

✨ Key Features

✅ Quad-band GSM: 850/900/1800/1900 MHz
✅ GPRS class 10 (up to 85.6 kbps)
✅ UART interface for microcontroller connection
✅ Supports AT commands (industry standard)
✅ Voice calls, SMS, and data transfer
✅ Low power consumption in sleep mode
✅ External antenna support
✅ Operating voltage: 3.2 V – 4.8 V

📊 Electrical Specifications
Parameter Value
Operating voltage 3.2 V – 4.8 V (typ 4 V)
Peak current ~2 A during transmission bursts
Standby current ~20 mA
GPRS data rate Up to 85.6 kbps (class 10)
Frequency bands Quad-band GSM: 850/900/1800/1900 MHz
SIM card Standard 1.8–3 V SIM
Interface UART (TX/RX)
Operating temp -40 °C – 85 °C
Antenna connector SMA or IPEX (depending on module)
🔌 Pin Configuration (Common SIM900A breakout)
Pin Function
VCC Power supply 4 V
GND Ground
TXD UART transmit (to MCU RX)
RXD UART receive (to MCU TX)
RST Module reset (active low)
DTR Sleep/wake control
NETLIGHT Status LED output (network registration)
ANT External antenna connection

💡 Note: Some modules also include audio pins (MIC/SPK) for voice functionality.

⚙️ How It Works

Microcontroller sends AT commands over UART to control the module.

Module communicates with cellular network (registers, SMS, calls, GPRS).

Data/voice/SMS is sent or received.

Module requires stable 4 V supply and can draw up to 2 A during transmission, so proper power supply is critical.

🛠️ Common Applications

IoT devices with SMS or internet reporting

Remote sensors and telemetry

Home automation via SMS

GPS + GSM tracking devices

Automated SMS alert systems

Arduino / Raspberry Pi GSM projects

💡 Design Tips
✅ Power supply

Must handle 2 A peaks → 4 V regulated supply recommended.

✅ Antenna

Use a proper GSM antenna, 50 Ω impedance, for stable network registration.

✅ UART voltage

Module logic is 3.3 V–5 V compatible, but check level shifting if needed.

✅ AT Commands

All control is via standard AT commands, e.g.:

AT → test connection

AT+CMGF=1 → set SMS text mode

AT+CMGS="number" → send SMS

ATD<number>; → make a call

✅ Network

Check local GSM frequencies — SIM900A works best on 900/1800 MHz in Asia, 850/1900 MHz in the Americas.

✅ Quick Summary

SIM900A = GSM/GPRS module for microcontrollers

Perfect when you want:
✔ SMS alerts and notifications
✔ Voice calling from Arduino/MCU
✔ Internet via GPRS (IoT applications)
✔ Remote control and telemetry