SIM900A GSM/GPRS Module
Dual-band cellular module for SMS and voice call projects.
An earlier dual-band GSM/GPRS module still popular for SMS-based alert systems and simple cellular call projects. Built around SIMCom's SIM900A chipset, it lets a microcontroller send and receive SMS messages, make and answer voice calls, and access basic GPRS data over a standard 2G cellular network, entirely through simple AT command sequences sent over UART — making it a long-standing favorite for remote alert systems, SMS-controlled switches, and IoT projects in areas without reliable Wi-Fi.
Specifications
| Chipset | SIMCom SIM900A dual-band GSM/GPRS module |
| Frequency bands | Dual-band 900/1800MHz (region-specific — check compatibility with local carriers before buying) |
| Operating voltage | 3.4V–4.5V DC (a stable ~4V-5V regulated supply capable of ~2A peak is required for reliable operation) |
| Operating current | ~0.5-1A average, spikes up to ~2A during transmit bursts |
| SIM card | Standard-size 2G SIM card required |
| Functions | SMS send/receive, voice calls, basic GPRS data (2G only — no 3G/4G support) |
| Interface | UART, controlled via standard AT command set |
| Audio | Onboard microphone and speaker connectors for voice call projects |
Pinout
| Pin | Name | Description |
|---|---|---|
| 1 | VCC | Power, requires a stable regulated supply (module-specific, commonly 5V input to an onboard regulator) |
| 2 | GND | Ground |
| 3 | TX | UART transmit — connect to microcontroller RX |
| 4 | RX | UART receive — connect to microcontroller TX |
| 5 | SIM slot | Standard 2G SIM card holder |
| 6 | Antenna connector | External GSM antenna required — reception is very poor without one |
The SIM900A needs a genuinely stable power supply capable of sourcing short current spikes near 2A during network registration and transmit bursts — insufficient power is the most common cause of random resets or failure to register on the network. Because this is a 2G-only module, confirm that your local carrier still supports 2G service before building around it, as many regions have begun sunsetting 2G networks.
Variants
If starting a new project today, the SIM800L is generally the better choice — smaller, quad-band, and more power-efficient than the older SIM900A. Given that 2G networks are being phased out in many countries, verify local 2G coverage before committing to either, and consider a SIM7600-series 4G module for future-proofing.
| Variant | Temp range | Hum range | Accuracy | Protocol | Price |
|---|---|---|---|---|---|
| SIM900A (dual-band) | ~$5-10 | ||||
| SIM800L | ~$3-6 | ||||
| SIM7600 series | ~$15-30 |