IoT ESP32 MQTT SMS Gateway

A feature-rich ESP32-based IoT gateway that combines WiFi, MQTT, GSM connectivity, and SMS/voice call capabilities. Built for the LilyGo T-Call A7670E board with the A7670 cellular modem.

Features

• WiFi Connectivity: Easy setup via WiFiManager captive portal
• MQTT Communication: Bidirectional MQTT messaging with auto-reconnect
• GSM/LTE Modem: Full A7670 modem support via TinyGSM
• SMS Gateway: Send and receive SMS messages via MQTT commands
• Voice Calls: Make, receive, and manage voice calls remotely
• USSD Support: Query network information and balance
• Event Publishing: Real-time event notifications (calls, SMS) to MQTT
• NTP Time Sync: Automatic time synchronization
• mDNS Support: Device discovery on local network

Hardware

Tested on: LilyGo T-Call A7670E

Pin Configuration

• Modem UART: TX=27, RX=26
• Modem Control: PWR=4, DTR=32, RESET=5, RING=33
• LED: GPIO 13

Requirements

Software

• PlatformIO (with Espressif32 platform v6.12.0)
• Python package intelhex (for esptool):

  ~/.platformio/penv/bin/pip install --upgrade intelhex

Libraries

• TinyGSM (included in lib/)
• StreamDebugger v1.0.1
• WiFiManager v2.0.17
• ArduinoJson v7.3.0

Configuration

Edit src/constants.h to configure:

#define NETWORK_APN     "your-apn"           // GSM APN
#define MQTT_SERVER     "mqtt://your-server" // MQTT broker URL
#define MQTT_USER       "username"
#define MQTT_PASSWORD   "password"
#define MQTT_COMMAND_TOPIC "esp32/commands"
#define MQTT_EVENT_TOPIC   "esp32/events"

Building & Flashing

# Build
pio run

# Upload
pio run --target upload

# Monitor
pio device monitor

MQTT Interface

Command Topic (esp32/commands/<MAC>)

Send JSON commands to control the device:

Send SMS

{
  "action": "sms",
  "number": "+491234567890",
  "message": "Hello from IoT device!"
}

Make Call

{
  "action": "call",
  "number": "+491234567890"
}

Hang Up

{
  "action": "hangup"
}

Accept Call

{
  "action": "accept"
}

Send USSD

{
  "action": "ussd",
  "code": "*100#"
}

Event Topic (esp32/events/<MAC>)

The device publishes events with ISO8601 timestamps:

Incoming Call

{
  "timestamp": "2025-11-03T10:15:30Z",
  "mac": "AC1518B62E50",
  "event": "call",
  "number": "+491234567890"
}

Call Status Update

{
  "timestamp": "2025-11-03T10:15:35Z",
  "mac": "AC1518B62E50",
  "event": "call_update",
  "status": "ESTABLISHED"
}

SMS Received

{
  "timestamp": "2025-11-03T10:20:00Z",
  "mac": "AC1518B62E50",
  "event": "sms",
  "message": "SMS content here"
}

First Run

1. WiFi Setup: Device creates AP AutoConnectAP on first boot
2. Connect to AP and configure WiFi credentials
3. Device connects to WiFi and MQTT broker
4. Subscribe to command topic to control device

Known Issues

• MQTT Disconnect after SMS: Fixed by filtering URC messages (only read lines starting with +)
• esptool.py error: Requires intelhex Python package with Espressif32 v6.12.0+

Troubleshooting

MQTT Connection Issues

• Check broker URL, credentials, and network connectivity
• Monitor serial output for error messages (115200 baud)

Modem Not Responding

• Verify SIM card is inserted and unlocked
• Check APN configuration for your carrier
• Ensure antenna is connected

Build Errors

# Clean and rebuild
pio run -t clean
pio run

License

This project uses TinyGSM library (Apache License 2.0). See individual library licenses in lib/ directory.

Contributing

Issues and pull requests are welcome!

Author

Created for IoT gateway applications with ESP32 and cellular connectivity.