DD3-LoRa-Bridge-MultiSender/README.md

DD3 LoRa Bridge (Multi-Sender)

Unified firmware for LilyGO T3 v1.6.1 (ESP32 + SX1276 + SSD1306) that runs as Sender or Receiver based on a GPIO jumper. Senders read DD3 smart meter values, compress JSON, and transmit over LoRa. The receiver validates packets, publishes to MQTT, provides a web UI, and shows per-sender status on the OLED.

Hardware

Board: LilyGO T3 LoRa32 v1.6.1 (ESP32 + SX1276 433 MHz + SSD1306 128x64 + LiPo)

Pin Mapping

• LoRa (SX1276)
  • SCK: GPIO5
  • MISO: GPIO19
  • MOSI: GPIO27
  • NSS/CS: GPIO18
  • RST: GPIO23
  • DIO0: GPIO26
• OLED (SSD1306)
  • SDA: GPIO21
  • SCL: GPIO22
  • RST: not used (SSD1306 init uses -1 reset pin)
  • I2C address: 0x3C
• Battery ADC: GPIO35 (via on-board divider)
• Role select: GPIO13 (INPUT_PULLDOWN)
  • LOW = Sender
  • HIGH = Receiver
• OLED control: GPIO14 (INPUT_PULLDOWN)
  • HIGH = force OLED on
  • LOW = allow auto-off after timeout
• Smart meter UART RX: GPIO34 (input-only, always connected)

Notes on GPIOs

• GPIO34/35/36/39 are input-only and have no internal pullups/pulldowns.
• Strap pins (GPIO0/2/4/5/12/15) can affect boot; avoid for role or control jumpers.

Firmware Roles

Sender (battery-powered)

• Reads DD3 smart meter via optical IR (UART 9600 7E1).
• Extracts OBIS values:
  • Energy total: 1-0:1.8.0*255
  • Total power: 1-0:16.7.0*255
  • Phase power: 36.7 / 56.7 / 76.7
  • Phase voltage: 32.7 / 52.7 / 72.7
• Reads battery voltage and estimates SoC.
• Builds JSON payload, compresses, wraps in LoRa packet, transmits.
• Deep sleeps between cycles.
• Listens for LoRa time sync packets to set UTC clock.
• OLED shows status + meter data pages.

Sender flow (pseudo-code):

void sender_cycle() {
  meter_read(data);         // SML/OBIS -> MeterData
  read_battery(data);       // VBAT + SoC
  data.ts_utc = time_get_utc_or_uptime();

  json = meterDataToJson(data);
  compressed = compressBuffer(json);
  lora_send(packet(MeterData, compressed));

  display_set_last_meter(data);
  display_set_last_read(ok);
  display_set_last_tx(ok);
  display_tick();

  lora_receive_time_sync(); // optional
  deep_sleep(SENDER_WAKE_INTERVAL_SEC);
}

Key sender functions:

bool meter_read(MeterData &data);        // parse SML frame, set OBIS fields
void read_battery(MeterData &data);      // ADC -> volts + percent
bool meterDataToJson(const MeterData&, String&);
bool compressBuffer(const uint8_t*, size_t, uint8_t*, size_t, size_t&);
bool lora_send(const LoraPacket &pkt);   // add header + CRC16 and transmit

Receiver (USB-powered)

• WiFi STA connect using stored config; if not available/fails, starts AP.
• NTP sync (UTC) and local display in Europe/Berlin.
• Receives LoRa packets, verifies CRC16, decompresses, parses JSON.
• Publishes meter JSON to MQTT.
• Web UI:
  • AP mode: status + WiFi/MQTT config.
  • STA mode: status + per-sender pages.
• OLED cycles through receiver status and per-sender pages.

Receiver loop (pseudo-code):

void receiver_loop() {
  if (lora_receive(pkt) && pkt.type == MeterData) {
    json = decompressBuffer(pkt.payload);
    if (jsonToMeterData(json, data)) {
      update_sender_status(data);
      mqtt_publish_state(data);
    }
  }

  if (time_to_send_timesync()) {
    time_send_timesync(self_short_id);
  }

  mqtt_loop();
  web_server_loop();
  display_set_receiver_status(...);
  display_tick();
}

Key receiver functions:

bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms);
bool jsonToMeterData(const String &json, MeterData &data);
bool mqtt_publish_state(const MeterData &data);
void web_server_loop();                   // AP or STA UI
void time_send_timesync(uint16_t self_id);

Test Mode (compile-time)

Enabled by -DENABLE_TEST_MODE (see platformio.ini test environment).

• Sender: sends 4-digit test code every ~30s in JSON.
• Receiver: shows last test code per sender and publishes to /test topic.
• Normal behavior is excluded from test builds.

Test sender (pseudo-code):

void test_sender_loop() {
  code = random_4_digits();
  json = {id, role:"sender", test_code: code, ts};
  lora_send(packet(TestCode, compress(json)));
  display_set_test_code(code);
}

Test receiver (pseudo-code):

void test_receiver_loop() {
  if (pkt.type == TestCode) {
    json = decompress(pkt.payload);
    update_sender_test_code(json);
    mqtt_publish_test(id, json);
  }
}

LoRa Protocol

Packet layout:

[0]    protocol_version (1)
[1]    role (0=sender, 1=receiver)
[2..3] device_id_short (uint16)
[4]    payload_type (0=meter, 1=test, 2=time_sync)
[5..N-3] compressed payload
[N-2..N-1] CRC16 (bytes 0..N-3)

LoRa radio settings:

• 433 MHz, SF12, BW 125 kHz, CR 4/5, CRC on, Sync Word 0x34

Data Format

JSON payload (sender + MQTT):

{
  "id": "dd3-01",
  "ts": 1737200000,
  "energy_kwh": 1234.567,
  "p_total_w": 950.0,
  "p1_w": 500.0,
  "p2_w": 450.0,
  "p3_w": 0.0,
  "v1_v": 230.1,
  "v2_v": 229.8,
  "v3_v": 231.0,
  "bat_v": 3.92,
  "bat_pct": 78
}

Device IDs

• Derived from WiFi STA MAC.
• short_id = (MAC[4] << 8) | MAC[5]
• device_id = dd3-%04X

Receiver expects known senders in include/config.h via:

constexpr uint8_t NUM_SENDERS = 1;
inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };

OLED Behavior

• After reset, OLED stays ON for 10 minutes regardless of switch.
• After that:
  • GPIO14 HIGH: OLED forced ON.
  • GPIO14 LOW: start 10-minute auto-off timer.
• Pages rotate every 10s.

Power & Battery

• Sender disables WiFi/BLE, reads VBAT via ADC, uses linear SoC map:
  • 3.0 V = 0%
  • 4.2 V = 100%
• Uses deep sleep between cycles (SENDER_WAKE_INTERVAL_SEC).

Web UI

• AP SSID: DD3-Bridge-<short_id>
• AP password: changeme123
• Endpoints:
  • /: status overview
  • /wifi: WiFi/MQTT/NTP config (AP and STA)
  • /sender/<device_id>: per-sender details

MQTT

• Topic: smartmeter/<deviceId>/state
• QoS 0
• Test mode: smartmeter/<deviceId>/test
• Client ID: dd3-bridge-<device_id> (stable, derived from MAC)

NTP

• NTP servers are configurable in the web UI (/wifi).
• Defaults: pool.ntp.org and time.nist.gov.

Build Environments

• lilygo-t3-v1-6-1: production build
• lilygo-t3-v1-6-1-test: test build with ENABLE_TEST_MODE
• lilygo-t3-v1-6-1-868: production build for 868 MHz modules
• lilygo-t3-v1-6-1-868-test: test build for 868 MHz modules

Limits & Known Constraints

• Compression: uses lightweight RLE (good for JSON but not optimal).
• OBIS parsing: heuristic SML parser; may need tuning for some DD3 meters.
• Payload size: JSON < 256 bytes (enforced by ArduinoJson static doc).
• Battery ADC: uses simple linear calibration constant in power_manager.cpp.
• OLED: no hardware reset line is used (matches working reference).

Files & Modules

• include/config.h, src/config.cpp: pins, radio settings, sender IDs
• include/data_model.h, src/data_model.cpp: MeterData + ID init
• include/json_codec.h, src/json_codec.cpp: JSON encode/decode
• include/compressor.h, src/compressor.cpp: RLE compression
• include/lora_transport.h, src/lora_transport.cpp: LoRa packet + CRC
• include/meter_driver.h, src/meter_driver.cpp: SML/OBIS parse
• include/power_manager.h, src/power_manager.cpp: ADC + sleep
• include/time_manager.h, src/time_manager.cpp: NTP + time sync
• include/wifi_manager.h, src/wifi_manager.cpp: NVS config + WiFi
• include/mqtt_client.h, src/mqtt_client.cpp: MQTT publish
• include/web_server.h, src/web_server.cpp: AP/STA web pages
• include/display_ui.h, src/display_ui.cpp: OLED pages + control
• include/test_mode.h, src/test_mode.cpp: test sender/receiver
• src/main.cpp: role detection and main loop

Quick Start

1. Set role jumper on GPIO13:
   • LOW: sender
   • HIGH: receiver
2. OLED control on GPIO14:
   • HIGH: always on
   • LOW: auto-off after 10 minutes
3. Build and upload:

pio run -e lilygo-t3-v1-6-1 -t upload --upload-port COMx

Test mode:

pio run -e lilygo-t3-v1-6-1-test -t upload --upload-port COMx