Third Eye Kit, Node D: Location Telemetry (Belt/Pack)
=========================================================

MICROCONTROLLER: Arduino Mini Pro (5V)

GPS MODULE WIRING (NEO-6M):
──────────────────────────────
  Module Pin | Arduino Pin | Function
  ─────────────────────────────────────
  RX         | D0          | Hardware Serial [see NOTE below]
  TX         | D1          | Hardware Serial [see NOTE below]
  VCC        | +3.3V       | Power
  GND        | GND         | Ground

GSM MODULE WIRING (SIM800-class):
────────────────────────────────────
  Module Pin | Arduino Pin | Function
  ─────────────────────────────────────
  RX         | D2          | SoftwareSerial RX
  TX         | D3          | SoftwareSerial TX
  VCC        | +4.2V (regulated) | Power
  GND        | GND         | Ground
  KEY        | D8          | Power control (module on/off)

POWER:
──────
  Lithium cell (per node_d.ino header comment)
  GSM module requires a separately regulated +4.2V supply (per BUILD.md);
  do not power SIM800-class modules directly from an unregulated cell.

FIRMWARE CONFIGURATION (node_d.ino):
────────────────────────────────────
  #define GPS_BAUD     9600   // GPS (NEO-6M) baud rate, hardware Serial
  #define GSM_RX_PIN   2      // GSM SoftwareSerial RX
  #define GSM_TX_PIN   3      // GSM SoftwareSerial TX
  #define GSM_BAUD     9600   // GSM (SIM800) baud rate
  #define GSM_KEY_PIN  8      // GSM module power-key control

PROVISIONING NOTES:
────────────────────
- Authorized phone numbers are configured in firmware, not wiring, see
  BUILD.md Step 5.2 (AUTHORIZED[] allowlist) before field deployment
- Verify 2G coverage in the deployment area before relying on this node
  (BUILD.md Step 5.1 / 5.3)

TESTING:
────────
1. Upload firmware to Arduino Mini
2. Open Serial Monitor (9600 baud)
3. Power on Node D with SIM inserted
4. Watch for GPS fix and "GSM: registered on 2G" status
5. Trigger a location request by calling/SMSing from an authorized number
6. Expect a reply within 30 seconds with GPS coordinates

TROUBLESHOOTING:
─────────────────
- No GSM response: check GSM_KEY_PIN (D8) toggling and module power supply (+4.2V)
- No GPS fix: check VCC (+3.3V)/GND wiring and antenna placement outdoors
- SMS not sent: verify AUTHORIZED[] contains the requesting number

NOTE:
─────
- Both node_d.ino's inline comment ("GPS Module (NEO-6M), RX = pin 0
  (hardware Serial), TX = pin 1") and BUILD.md describe the GPS module on
  D0/D1 (the AVR's single hardware UART). node_d.ino also calls
  Serial.begin(9600) on that same hardware UART for USB debug logging.
  Neither BUILD.md nor node_d.ino documents how GPS traffic and USB debug
  logging share that one hardware serial port without colliding, this is
  a genuine gap in the existing docs/source, not resolved here.