SX1278 LoRa (Ra-02) with Arduino Uno — 3.3 V supply and level shifting

LoRa sends small packets over kilometres on very little power, which makes it the radio for farm sensors, water-tank level reports from a rooftop, or a gate alert far from the house. The Ra-02 module with the Semtech SX1278 chip is the common 433 MHz version in India.

It is more work on a 5 V Uno than on an ESP32, for two reasons. The chip is 3.3 V-only and not 5 V tolerant, so the four lines the Uno drives (NSS, SCK, MOSI, RST) go through a level shifter, while MISO and DIO0 come back directly. And it draws over 100 mA while transmitting, more than the Uno’s 3.3V pin should supply, so it gets its own small AMS1117 3.3 V regulator fed from the Uno’s 5V.

The sketch is one file for both ends. The sender transmits a numbered “Hello” every 5 seconds; the receiver prints each packet with its signal strength (RSSI) and SNR.

Wiring

Board quirks

• The SX1278 is 3.3 V-only and not 5 V tolerant. Unlike the nRF24L01, its inputs can be damaged by the Uno’s 5 V SPI signals, so NSS, SCK, MOSI and RST go through the level shifter.
• Never transmit without an antenna: the reflected power can destroy the RF stage. Solder or screw the antenna on first.
• Ra-02 modules have 1.27 mm pads, not breadboard pins. Use an adapter board or solder short wires.
• Ra-02 is a 433 MHz module. India’s LoRaWAN band is 865–867 MHz (SX1276/RFM95 modules). For point-to-point hobby links on 433 MHz, keep the power low and check the current WPC rules for low-power devices.
• The library and SPI fit easily on the Uno (this sketch uses about 4 KB of the 32 KB flash), so there’s room for a sensor on the sender.

Code

// SX1278 (Ra-02, 433 MHz) LoRa module on an Arduino Uno R3, Sandeep Mistry's LoRa library.
// The SX1278 is 3.3 V-only. 3.3V from an AMS1117 3.3 V regulator (fed from 5V), GND → GND.
// Through a level shifter: NSS → D10, SCK → D13, MOSI → D11, RST → D9.
// Direct (3.3 V outputs into the Uno): MISO → D12, DIO0 → D2.
// Upload with SENDER = true on one Uno and false on the other. Never power it without an antenna.
#include <SPI.h>
#include <LoRa.h>

const bool SENDER = true;
const long FREQ = 433E6;

void setup() {
  Serial.begin(9600);
  LoRa.setPins(10, 9, 2);  // NSS, RST, DIO0
  if (!LoRa.begin(FREQ)) {
    Serial.println(F("LoRa init failed - check 3.3V, the level shifter and SPI wiring"));
    for (;;) {}
  }
  LoRa.setTxPower(14);           // dBm; keep within your local limits
  LoRa.setSpreadingFactor(9);    // higher = longer range, slower
  LoRa.enableCrc();
  Serial.println(SENDER ? F("LoRa sender ready") : F("LoRa receiver ready"));
}

void loop() {
  if (SENDER) {
    static unsigned int n = 0;
    LoRa.beginPacket();
    LoRa.print(F("Hello #"));
    LoRa.print(n);
    LoRa.endPacket();  // waits until sent (about 0.2 s at SF9)
    Serial.print(F("Sent #"));
    Serial.println(n++);
    delay(5000);
  } else {
    int size = LoRa.parsePacket();
    if (size) {
      Serial.print(F("Received '"));
      while (LoRa.available()) Serial.print((char)LoRa.read());
      Serial.print(F("'  RSSI "));
      Serial.print(LoRa.packetRssi());
      Serial.print(F(" dBm  SNR "));
      Serial.println(LoRa.packetSnr(), 1);
    }
  }
}

// SX1278 (Ra-02, 433 MHz) LoRa module on an Arduino Uno R3, Sandeep Mistry's LoRa library.
// The SX1278 is 3.3 V-only. 3.3V from an AMS1117 3.3 V regulator (fed from 5V), GND → GND.
// Through a level shifter: NSS → D10, SCK → D13, MOSI → D11, RST → D9.
// Direct (3.3 V outputs into the Uno): MISO → D12, DIO0 → D2.
// Upload with SENDER = true on one Uno and false on the other. Never power it without an antenna.
#include <SPI.h>
#include <LoRa.h>

const bool SENDER = true;
const long FREQ = 433E6;

void setup() {
  Serial.begin(9600);
  LoRa.setPins(10, 9, 2);  // NSS, RST, DIO0
  if (!LoRa.begin(FREQ)) {
    Serial.println(F("LoRa init failed - check 3.3V, the level shifter and SPI wiring"));
    for (;;) {}
  }
  LoRa.setTxPower(14);           // dBm; keep within your local limits
  LoRa.setSpreadingFactor(9);    // higher = longer range, slower
  LoRa.enableCrc();
  Serial.println(SENDER ? F("LoRa sender ready") : F("LoRa receiver ready"));
}

void loop() {
  if (SENDER) {
    static unsigned int n = 0;
    LoRa.beginPacket();
    LoRa.print(F("Hello #"));
    LoRa.print(n);
    LoRa.endPacket();  // waits until sent (about 0.2 s at SF9)
    Serial.print(F("Sent #"));
    Serial.println(n++);
    delay(5000);
  } else {
    int size = LoRa.parsePacket();
    if (size) {
      Serial.print(F("Received '"));
      while (LoRa.available()) Serial.print((char)LoRa.read());
      Serial.print(F("'  RSSI "));
      Serial.print(LoRa.packetRssi());
      Serial.print(F(" dBm  SNR "));
      Serial.println(LoRa.packetSnr(), 1);
    }
  }
}

Library: LoRa (Sandeep Mistry) — compiled with LoRa 0.8.0.

Expected serial output

LoRa receiver ready
Received 'Hello #0'  RSSI -47 dBm  SNR 9.8
Received 'Hello #1'  RSSI -48 dBm  SNR 9.5

LoRa receiver ready
Received 'Hello #0'  RSSI -47 dBm  SNR 9.8
Received 'Hello #1'  RSSI -48 dBm  SNR 9.5

Common errors