lora-sensor

A long-range water-pressure telemetry rig for outdoor irrigation: a battery-powered LoRa sensor node reads a TE M32JM transducer and transmits to a WiFi-connected receiver that pushes metrics to Grafana Cloud.

[M32JM transducer] -- I²C --> [RAK4631 sensor node] --LoRa P2P--> [Heltec V4 receiver] --WiFi/OTLP--> [Grafana Cloud]
                              (battery-powered,                    (mains-powered,
                               solar enclosure)                     OLED display)

Hardware

Sensor node

Part	Notes
RAK4631 WisBlock Core	nRF52840 MCU + SX1262 LoRa radio in one module
RAK19007 WisBlock Base	Carrier board for the RAK4631; sensor leads land on the header pins
RAK Solar Unify Enclosure	Outdoor IP65 housing with integrated solar panel + LiPo + battery management
TE M32JM-000105-100PG	0–100 PSI gauge pressure transducer, M8-4 connector, I²C interface
M8-4 to flying-leads cable	4-conductor shielded; red=V+, black=GND, white=SCL, green=SDA
FEBCO 805 DCVA test-cock fitting	The transducer threads into the test cock farthest from the water meter on a FEBCO 805 Double Check Valve Assembly (downstream of both check valves, on the irrigation-side outlet), tapped open via the cock's small ball valve. No tee or pipe modification required — just PTFE tape on the sensor's NPT threads.

Wiring (sensor → RAK19007 header pins):

M32JM	RAK pin	Notes
Red (V+)	WB_IO1	Gated 3.3 V supply. WB_IO1 is configured in H0H1 high-drive mode so it can source the sensor's 3.3 mA cleanly
Black (GND)	GND
White (SCL)	WB_I2C1_SCL
Green (SDA)	WB_I2C1_SDA

The shield braid is left floating at the sensor end and tied to GND at the board end.

Sensor-node quirks worth knowing:

• The RAK4631's stock Adafruit BSP wires the default SPI to IO_SLOT pins (P0.03/29/30), not the SX1262 pins (P1.10–15). lora_radio.cpp calls SPI.setPins(PIN_LORA_MISO, PIN_LORA_SCK, PIN_LORA_MOSI) before SPI.begin() to redirect.
• PIN_LORA_ANT_PWR (P1.05 / pin 37) must be driven HIGH to enable the SX1262's antenna switch. Forgetting this gives a clean RADIOLIB_ERR_CHIP_NOT_FOUND at init.
• The M32JM's I²C interface forbids repeated-START (datasheet §1.8); Adafruit's Wire/TWIM emits Sr in subtle ways that lock the sensor up returning STALE+zeros. The driver in m3200.cpp is a verbatim port of the datasheet's bit-banged sample code.
• M3200_ZERO_OFFSET_CPSI is hardcoded per-sensor (currently -25, i.e. +0.25 PSI of zero correction). Re-measure if you swap the transducer.

Receiver node

Part	Notes
Heltec WiFi LoRa 32 V4	ESP32-S3 + SX1262 + 0.96″ SSD1306 OLED on one board. Pinout matches V3, so PlatformIO uses board = heltec_wifi_lora_32_V3.
868/915 MHz LoRa whip antenna	The bundled antenna; range is fine indoors with the sensor 30 m away through a wall + 15 m of yard.
USB-C power	Mains-powered; sits indoors on WiFi. A 90° USB-C cable helps if the board is wedged into a tight spot.

Power

• Sensor: 3.7 V LiPo (charged by the Solar Unify panel) → RAK4631's onboard regulator. Battery readback through WB_A0 and the onboard 1.5 MΩ / 1.0 MΩ divider; calibrated against a known supply (see power.cpp).
• Receiver: USB-C, no battery.

Component summary

Role	Board	Radio	Sensor / Display
Sensor	RAK4631 (nRF52840) in Solar Unify	SX1262 @ 22 dBm	TE M32JM-000105-100PG (0–100 PSI, I²C)
Receiver	Heltec WiFi LoRa 32 V4 (ESP32-S3)	SX1262 @ RX-only	0.96″ SSD1306 OLED

LoRa link

• 917 MHz · BW 125 kHz · SF11 · CR 4/8 · sync 0x12 · preamble 12
• 917 MHz sits between Meshtastic US ch28/29 and off the smart-meter cluster — tuned for clean reception at long range
• TX 22 dBm (SX1262 max), ~32 dBm link budget improvement vs. SF7
• Custom binary packet (shared/packet.h): 16 bytes, magic + version + CRC16, carries pressure (cpsi), temp (cdegC), battery (mV), seq, and flags

Sensor-node behavior

• Samples every 15 s; transmits on (a) ≥0.5 PSI delta, (b) 5-min heartbeat, or (c) at boot
• Minimum 10 s between transmissions
• Bit-banged I²C — the M32JM datasheet §1.8 forbids repeated-START, which Adafruit's TWIM driver emits
• Battery protection: hourly heartbeat-only below 3.3 V, long-sleep below 3.0 V
• Flags: HEARTBEAT, SENSOR_FAULT, STALE_DATA, LOW_BATT, EVENT

Receiver behavior

• Continuous LoRa RX, validates CRC + magic + version, displays last reading on OLED
• Pushes OTLP HTTP/JSON to Grafana Cloud Prometheus every 30 s
• Metrics: pressure, temperature, battery, RSSI, SNR, last-RX age, per-flag packet counters, last-packet error gauge
• Diagnostic WiFi failure modes: SSID-not-visible vs. auth-failed vs. timeout, with nearby-AP scan dump on failure

Build & flash

Both nodes use PlatformIO.

# Sensor node — RAK4631 over USB DFU
cd sensor-node && pio run -t upload

# Receiver node — ESP32-S3 over USB serial
cd receiver-node && pio run -t upload && pio device monitor

Configuration

Copy the receiver's secrets template and fill in your own values:

cp receiver-node/include/secrets.h.example receiver-node/include/secrets.h
# edit receiver-node/include/secrets.h

You'll need:

• A WiFi SSID + password
• A Grafana Cloud Access Policy token with metrics:write scope
• Your Prometheus instance ID (visible at https://grafana.com/orgs/<org>/stacks)
• The OTLP endpoint URL for your region

secrets.h is gitignored.

Operational tuning (sample interval, heartbeat cadence, delta threshold, LoRa params) lives in each node's platformio.ini build_flags so it can be overridden per environment without editing source.

Repository layout

shared/             packet.h — binary wire format used by both nodes
sensor-node/        RAK4631 firmware (sampling, TX, battery management)
  docs/             vendor datasheets (M3200/M32JM transducer)
receiver-node/      Heltec V4 firmware (RX, OLED, Grafana push)

The m3200.cpp driver implements the bit-banged I²C protocol described in §1.8 of the M3200 datasheet, which forbids repeated-START.