GSM / Cellular
Interfacing SIM900A GSM / Cellular with Arduino Uno
What is the SIM900A?
SIM900A practical reference
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.
Key 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 and connection notes
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.
- VCC: Power, requires a stable regulated supply (module-specific, commonly 5V input to an onboard regulator)
- GND: Ground
- TX: UART transmit — connect to microcontroller RX
- RX: UART receive — connect to microcontroller TX
- SIM slot: Standard 2G SIM card holder
- Antenna connector: External GSM antenna required — reception is very poor without one
Integration checklist
- Confirm the exact part number and breakout-board revision.
- Verify supply voltage, current demand, logic levels, and signal direction against the datasheet.
- Check controller pin assignments and bus address or interface mode against the example code.
- Power up with external loads disconnected, then test signals before full integration.
- If readings or communication fail, check power and ground first, then pin mapping, interface settings, and library initialization.
Troubleshooting and reproducibility
Record the board used, wiring, library/version, supply arrangement, and measured behavior. Use the selected board-specific wiring example as the source of truth. Never assume visually similar modules share identical voltage tolerance or pin order.
Setting Up SIM900A with Arduino Uno
Library
SoftwareSerial by Arduino core
Built into the Arduino core — no install needed, just #include <SoftwareSerial.h>.
Wiring
| Component pin | Arduino Uno pin | Note |
|---|---|---|
| VCC | — | DO NOT power from the Arduino 5V pin. Needs its own 5V, 2A-capable supply — transmit bursts draw well beyond what USB/Arduino power can deliver. |
| GND | GND | Common ground between Arduino and the SIM900A's own power supply is required. |
| TXD | D2 (RX) | Module TX → Arduino RX. |
| RXD | D3 (TX) | Most SIM900A breakout boards are 5V-logic-tolerant on RXD, unlike the SIM800L — check your specific board's datasheet if unsure. |
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- Same power rule as any GSM module: the Arduino 5V pin cannot supply the current spikes this draws during transmit — use a dedicated 2A-capable 5V supply, sharing only GND with the Arduino.
- The SIM900A breakout typically includes its own voltage regulation and is generally more forgiving on logic levels than the bare SIM800L module, but always check your specific board's markings before assuming 5V-safe.
- Like the SIM800L, this needs an active SIM with 2G/GSM network coverage — confirm your carrier still supports 2G in your area before troubleshooting wiring.
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