Latch mosfet boite lettres

Mailbox LoRa Tracker

A weight-sensing mailbox notifier built around a Heltec WiFi LoRa 32 (V2) and an HX711 + 4 load cells. A door-triggered latching relay wakes the board with zero standby current, it weighs whatever was just dropped in, sends the result over LoRa to a receiver elsewhere in the house, waits for an ACK, then cuts its own power.

Built for a shared/collective mailbox (multiple households behind one postal-carrier door), where a simple door contact alone can't tell "someone delivered to me" from "a neighbor's delivery, same door."

How it works

Door opens (mechanical contact, no power needed)
  -> latching relay powers the board
  -> tare the HX711 (empty platform reference)
  -> wait for the door to close, or a safety timeout
  -> weigh again, with a settling/stability loop
     (absorbs shocks from a dropped/thrown package)
  -> if weight > threshold: send "COLIS:<grams>" over LoRa,
     wait for an ACK (bounded retries)
  -> cut its own power (transistor + relay), whatever happened above

Every branch converges on the same final power-cutoff step, so the board always ends up switching itself off in bounded time, regardless of whether a package was actually detected or an ACK came back.

Hardware

• Heltec WiFi LoRa 32 V2 (ESP32 + SX127x), one per side (sender/receiver)
• HX711 + 4x 50kg half-bridge load cell sensors (bathroom-scale style)
• 2-relay latching power circuit (door contact triggers wake, transistor cuts power at end of cycle)
• 2nd door contact (independent GPIO) to detect when the door closes

GPIO map (sender)

GPIO 34/35 are hardwired on the Heltec module to the SX127x's DIO2/DIO1 and must never be reused for anything external.

Repo contents

• src/bin/courrier_emetteur.rs — sender firmware (in the mailbox)
• src/bin/courrier_recepteur.rs — receiver firmware (indoors, mains-powered)
• src/sx127x.rs — minimal SX127x LoRa driver (SPI, ping/ack style)
• KiCAD/Latch colis/ — schematic + PCB for the power-latch/relay board, plus etching files (single-sided, hand-etched)

The schematic uses a couple of custom symbols/footprints (HX711 module, Heltec pinout) from a personal KiCad library that isn't included here yet — expect missing-footprint warnings if you open the project directly.

Status

Weighing is calibrated and validated on the bench (13g to 2.25kg, <1% error against a reference scale). Mechanical assembly (load cell mounting, door contacts on the shared mailbox door) is still in progress.

License

MIT — see LICENSE.