slim-lora-868

LoRaWAN temperature and humidity node. Arduino Pro Mini 3.3V 8MHz + RFM95W (SX1276). Sends DHT11 data every 30 minutes. Sleep current ~5µA. Total cost ~€10.

Uses SlimLoRa — a minimal OTAA LoRaWAN library for ATmega that stores the session in EEPROM. The device joins TTN once and resumes without rejoining after a power cycle or reset.

Hardware

Component	Price
Arduino Pro Mini 3.3V 8MHz	~€3
RFM95W (SX1276) LoRa module	~€5
DHT11 sensor breakout (3-pin)	~€1.50
AMS1117-3.3 or similar LDO	~€0.20
18650 Li-ion cell	~€2

Remove the onboard power LED and voltage regulator from the Pro Mini. Power the board via the VCC pin at 3.3V, not RAW.

Wiring

Pro Mini	RFM95W
13 (SCK)	SCK
12 (MISO)	MISO
11 (MOSI)	MOSI
10 (SS)	NSS/CS
2 (INT0)	DIO0
3	DIO1
9	RESET (or tie RESET to VCC via 10kΩ)
3.3V	VCC
GND	GND

Pro Mini	DHT11
4	DATA
3.3V	VCC
GND	GND

Power

The RFM95W is always connected to 3.3V. SlimLoRa puts it into hardware sleep mode after each transmission. During sleep:

Component	Current
ATmega328P power-down	~0.4µA
RFM95W sleep mode	~0.2µA
DHT11 idle	~0.5µA
LDO quiescent	~1–3µA
Total	~5µA

At ~5µA average sleep and 30-minute intervals, a 3000mAh 18650 lasts several years.

Session Persistence

SlimLoRa stores the LoRaWAN session (DevAddr, NwkSKey, AppSKey, frame counters) in EEPROM after a successful join. On the next boot the session is restored and the device goes straight to transmitting — no rejoin needed, even after a battery swap.

To force a fresh join, clear EEPROM byte 0 (e.g. with the Arduino eeprom_write_test sketch or EEPROM.write(0, 0) at startup).

Credentials

cp secrets.h.example secrets.h

Fill in your TTN device keys — all MSB first, exactly as shown in the TTN console:

• DevEUI: 8 bytes MSB first
• JoinEUI (AppEUI): 8 bytes MSB first
• AppKey: 16 bytes MSB first

secrets.h is excluded from version control via .gitignore.

Library

Install SlimLoRa via the Arduino Library Manager. No other LoRaWAN or DHT library needed — DHT11 is bit-banged directly in the sketch.

Payload Format

2 bytes:

Byte	Value
0	Temperature (°C, integer) or 0xFF on read error
1	Humidity (%, integer) or 0xFF on read error

TTN payload decoder (JavaScript):

function decodeUplink(input) {
  return {
    data: {
      temperature: input.bytes[0] === 0xFF ? null : input.bytes[0],
      humidity:    input.bytes[1] === 0xFF ? null : input.bytes[1]
    }
  };
}

How It Works

1. Boot: SlimLoRa checks EEPROM for a saved session
2. No valid session → OTAA join (retries with random backoff until a gateway responds)
3. Session saved to EEPROM on successful join
4. loop(): read DHT11 → send unconfirmed uplink on port 1 → sleep 30 min → repeat
5. Sleep: ATmega in power-down mode, RFM95W in hardware sleep, SPI/ADC/timers disabled

setup() only runs the join logic once. loop() handles measurements forever.