The LoRa Field Node.

What it does.

Wire what you have to it. It finds what is there and reports it: DS18B20 temperature probes on one wire (up to sixteen), an SHT-30 air temperature and humidity sensor (one or two), a photoresistor for light level, and up to eight capacitive soil-moisture probes. It also reports its own battery if one is fitted. Nothing is configured per sensor: a probe added today shows up at your endpoint on the next cycle under its own factory serial number.

On its cadence, three minutes by default, it reads everything, packs the readings into a frame of a few dozen bytes, listens for a moment to make sure nobody else is talking, and transmits. Then it waits. There is no Wi-Fi radio running, no internet, no account, no pairing, and no acknowledgement. If a frame is lost, the next one three minutes later carries fresh readings, and the gateway reports the gap.

It is the Collect layer for the places Wi-Fi cannot reach: the back field, the pump house, the greenhouse at the end of the row. The LoRa page explains why this radio carries a reading a kilometre on almost no power.

Flash it.

Plug your board in, find it below, and press its button. The LoRa radio is soldered onto these boards, so the image is built for the exact board, and the installer checks the chip matches before it writes anything.

Attach the antenna before you power the board. A LoRa radio transmitting into nothing can damage its own output stage. The little stub antenna in the box is enough for the bench; a real antenna is where range comes from.

What you need.

How it works.

1. On its cadence the node reads every sensor it found.
2. It packs the readings into one small frame: for each sensor, a one-byte tag, a measurement code, and a scaled value. Ten sensors fit in about sixty bytes.
3. It listens for a moment, and if the channel is quiet, transmits. A random pause of up to three seconds keeps a dozen nodes from ever falling into lock-step.
4. The gateway hears the frame, files each reading under the sensor's own serial number, averages the window, and pushes oat-ods to your endpoint.

Every fifth cycle, and at boot, the node also sends a roster: the map from each one-byte tag to the sensor's full hardware address. That is how a DS18B20's 64-bit factory serial rides the air as a single byte, and why a probe moved from a Wi-Fi node to a LoRa node keeps its history at the endpoint.

It is a private radio LAN with the manners of UDP. No handshake, no acknowledgement, no routing, no network server. The gateway is the only thing that needs an internet connection, and it needs it for the same reason every OAT gateway does: to push your readings to a place you keep.

Wiring.

The defaults below are for the pins each board leaves free once its radio and screen are spoken for. Every pin is a setting, so if your board or your sensor wants a different one, change it on the console rather than on the bench.

Power the sensors from the board's Vext pin, which the firmware switches on at boot, or from 3V3. A photoresistor needs a fixed resistor of about 10k to ground to make a divider. A capacitive soil probe outputs a voltage on its own; feed it 3V3, not 5 V, so its output stays inside what the chip can read. The DS18B20 and SHT-30 pages carry the pin-by-pin hookup and the reasons behind each choice.

Set it up.

This node has no Wi-Fi, so it has no setup web page. Setup is the USB console: plug it in, open a serial terminal at 115200 (the browser console on this site works), and type help.

1. Flash the board and leave it plugged in. It boots, finds its sensors, and prints what it found.
2. Type status. Every probe is listed with its serial number and its first reading. If a sensor is missing, check its wiring, then type scan.
3. Leave the radio plan alone unless you changed it on the gateway. The default plan on both sketches is the same: 915.0 MHz, 125 kHz, spreading factor 7, sync word 12.
4. Set the cadence if three minutes is not what you want: set cadence 120. Sixty seconds is the floor.
5. Type tx to send a frame right now, then look at the gateway's setup page. Your node appears under Field nodes heard with its signal strength.

Settings survive reboots. A fresh flash wipes them, on purpose, so a re-used board never carries the last owner's radio plan. Calibrating the soil probes: set soildry to the millivolts a probe reads in dry air and set soilwet to what it reads in a glass of water; status shows both the raw millivolts and the percentage, and the raw value is sent too so you can recalibrate at the endpoint later without visiting the node.

What it sends.

Over the air, not oat-ods. A LoRa frame carries about 200 bytes and one oat-ods reading is 330 bytes of JSON, so the node sends a compact binary frame instead and the gateway expands it. Your endpoint sees ordinary oat-ods, tagged oat-lora-gateway, with each field sensor as its own stream and the node's signal strength, noise margin and battery as gauges beside it.

The whole over-air contract is one header file, oat_lora_frame.h, shared by both sketches. Adding a measurement is one line in its code table, on both sides. Codes are never renumbered.

Range, airtime and power.

The default plan, spreading factor 7 at 125 kHz, is the campus setting: a 60-byte frame costs about a tenth of a second of airtime, which is why a hundred nodes on a three-minute cadence share one channel with room to spare. Line of sight with the gateway antenna up high, it reaches hundreds of metres to a kilometre or two. Metal-framed greenhouses eat signal; a node whose antenna pokes outside the frame hears and is heard far better than one on the bench inside.

For a node genuinely far away, step the spreading factor up on both sides: 9 reaches a few kilometres at four times the airtime. The US 400 ms dwell rule makes 10 the practical ceiling at 125 kHz. The LoRa page walks the trade-off.

Power. This node stays awake. It does not sleep between readings, on purpose: an awake node can listen before it talks, take a reading on demand, and keep its console live. The cost is about half a watt. On mains, that is nothing. Off grid, size for December rather than June: a 20 W panel and a 12 V 10 Ah LiFePO4 battery carry it through a week of overcast. The solar page has the sizing. Oversize the panel and skip the firmware complexity; that is the appropriate trade here.

For makers & trainers.