Introduction

The Adafruit  Feather 32u4 RFM95 LoRa Radio is a tiny little development Board. It already has a SX1276 onboard and an integrated 100mA lipoly charger. This makes it great for little handy nodes. 

Preperation of the node

Before you start you have to connect connect PIN6 from the Adafruit with IO1 of the RFM95 module. Just solder in a little wire. Next you have to connect an Antenna to the ANT Port. I used a 86mm wire Antenna.

Preperation of Arduino IDE

The Arduino IDE has to know the Board, so please follow Adafruits tutorial.

Programming the Adafruit Feather

Before we start we have you enable the LMIC library. I used the Library from Matthijs Kooijman, because it need less space on the Adafruit Feather.

Next you should create an TTN application with a device and change the device activation method to ABP. Copy the devices network session key and app session key in msb format an also copy the device address.

Now go to your libraries folder an open the ttn-apb example in arduino-lmic-master/examples/ttn-abp.

Edit the keys:

// LoRaWAN NwkSKey, network session key

// This is the default Semtech key, which is used by the early prototype TTN

// network.

  static const PROGMEM u1_t NWKSKEY[16] = { 0x2B, 0x7E, 0x15, 0x16, 0x28, 0xAE, 0xD2, 0xA6, 0xAB, 0xF7, 0x15, 0x88, 0x09, 0xCF, 0x4F, 0x3C };

  // LoRaWAN AppSKey, application session key

  // This is the default Semtech key, which is used by the early prototype TTN

  // network.

  static const u1_t PROGMEM APPSKEY[16] = { 0x2B, 0x7E, 0x15, 0x16, 0x28, 0xAE, 0xD2, 0xA6, 0xAB, 0xF7, 0x15, 0x88, 0x09, 0xCF, 0x4F, 0x3C };

  // LoRaWAN end-device address (DevAddr)

  static const u4_t DEVADDR = 0x03FF0001 ; // <-- Change this address for every node!

Edit the pin mapping:

// Pin mapping

const lmic_pinmap lmic_pins = {

    .nss = 8,

    .rxtx = LMIC_UNUSED_PIN,

    .rst = 4,

    .dio = {7, 6, LMIC_UNUSED_PIN},

};

Edit the do_send function:

void do_send(osjob_t* j){

    // Check if there is not a current TX/RX job running

    if (LMIC.opmode & OP_TXRXPEND) {

        Serial.println(F("OP_TXRXPEND, not sending"));

    } else {

        // Prepare upstream data transmission at the next possible time.

        #define VBATPIN A9

        float measuredvbat = analogRead(VBATPIN);

        measuredvbat *= 2;    // we divided by 2, so multiply back

        measuredvbat *= 3.3;  // Multiply by 3.3V, our reference voltage

         measuredvbat /= 1024; // convert to voltage

        byte buffer[8];

        dtostrf(measuredvbat, 1, 2, buffer);

        String res = buffer;

        res.getBytes(buffer, res.length() + 1);

        Serial.print("VBat: " ); Serial.println(measuredvbat);

        LMIC_setTxData2(1, (uint8_t*) buffer, res.length(), 0);

        Serial.println(F("Packet queued"));

    }

    // Next TX is scheduled after TX_COMPLETE event.

}

Ok, we are ready for uploading the program.

Testing

Open your serial monitor ans you should the something like this:

VBat: 4.03

Packet queued

-295622020: EV_TXCOMPLETE (includes waiting for RX windows)

Perfect, your node transmits his voltage to the TTN. Take a look into your TTN console and you should also see incoming data from your device.

With Node-RED, InfluxDB and Grafana you can now build a graph like this:

Conclusion

The Adafruit Feather 32u4 RFM95 LoRa makes a very tiny TTN node. With a 900mA LiPo battery the node was able to send his messages for about 84 hours.

The only disadvantage is the memory of 32KB, but for higher demands you can choose the Feather M0 LoRa with 256KB memory installed.