LoRa/TTN Payload

Overview

Since firmware X.X the MultiGeiger sends Geiger + environmental data in one combined uplink on Port 1 with a fixed length of 18 bytes.

Payload format

Port	Byte(s)	Hex	Description	Example
1	0/1/2/3	00000107	Number of counts	263
1	4/5/6	0249F0	Measurement time in ms (sample_time_ms)	150000
1	7/8	10C0	Software version (software_version)	1.12.0
1	9	16	Geiger tube ID (tube)	Si22G
1	10/11	0107	BMP/BME temperature x10 (temperature)	0x0107 = 263 -> 26.3 C
1	12	9A	BME humidity x2 (humidity)	0x9A = 154 -> 77.0% (0 for BMP280)
1	13/14	2794	BMP/BME pressure x10 (pressure)	0x2794 = 10132 -> 1013.2 hPa
1	15	01	THP sensor type	0=no sensor, 1=BMP280, 2=BME280, 3=BME680
1	16/17	0096	Gas resistance in kOhm (BME680 only)	0x0096 = 150 kOhm (0 for BMP280/BME280)

Notes

• If no THP sensor (BMP280/BME280/BME680) is present, bytes 10-17 are all 0x00 and byte 15 is 0x00.

• Software version encoding: top 4 bits are the major version (max 15), next 8 bits are minor, lowest 4 bits are patch.

• Implemented tube identifiers: SBM-19 (0x13), SBM-20 (0x14), Si22G (0x16).

• Supported THP sensors:

  • BMP280 (type 1): temperature + pressure only (humidity = 0, gas = 0)

  • BME280 (type 2): temperature + humidity + pressure (gas = 0)

  • BME680 (type 3): temperature + humidity + pressure + gas resistance

TTN decoder

To decode the payload in the TTN Console, add this JavaScript under Payload formatters -> Uplink:

function decodeUplink(input) {
  const bytes = input.bytes;
  const fPort = parseInt(input.fPort) || 0;

  if (fPort !== 1 || bytes.length !== 18) {
    return {
      data: {},
      warnings: [`fPort: ${fPort}, bytes: ${bytes.length} (expected: fPort 1, 18 bytes)`],
      errors: []
    };
  }

  // Geiger data (bytes 0-9)
  const COUNTS = bytes[0] * 0x1000000 + bytes[1] * 0x10000 + bytes[2] * 0x100 + bytes[3];
  const DT_MS = bytes[4] * 0x10000 + bytes[5] * 0x100 + bytes[6];
  const SW_VERSION = bytes[7] * 0x100 + bytes[8];
  const TUBE_NBR = bytes[9];

  // THP data (bytes 10-14)
  let TEMP_RAW = bytes[10] * 0x100 + bytes[11];
  if (TEMP_RAW & 0x8000) TEMP_RAW = TEMP_RAW - 0x10000; // signed
  const TEMP_C = TEMP_RAW !== 0 ? (TEMP_RAW / 10.0) : null;

  const HUMIDITY_RAW = bytes[12];
  const HUMIDITY_PERCENT = HUMIDITY_RAW > 0 ? (HUMIDITY_RAW / 2.0) : null;

  const PRESSURE_RAW = bytes[13] * 0x100 + bytes[14];
  const PRESSURE_HPA = PRESSURE_RAW > 0 ? (PRESSURE_RAW / 10.0) : null;

  // Sensor type (byte 15)
  const SENSOR_TYPE_RAW = bytes[15];
  const SENSOR_TYPES = ['none', 'BMP280', 'BME280', 'BME680'];
  const SENSOR_TYPE = SENSOR_TYPES[SENSOR_TYPE_RAW] || 'unknown';

  // Gas resistance (bytes 16-17, BME680 only)
  const GAS_RAW = bytes[16] * 0x100 + bytes[17];
  const GAS_KOHM = GAS_RAW > 0 ? GAS_RAW : null;

  // Calculate radiation values
  const CPS = DT_MS > 0 ? COUNTS / (DT_MS / 1000) : 0;
  const CPM = DT_MS > 0 ? Math.round(COUNTS * 60000 / DT_MS * 10) / 10 : 0;
  const USVH = CPS / 12.2792;  // Si22G factor

  const result = {
    counts: COUNTS,
    cpm: CPM,
    cps: Math.round(CPS * 100) / 100,
    uSvph: Number(USVH.toFixed(3)),
    sample_time_ms: DT_MS,
    tube_number: TUBE_NBR,
    sw_version: `V${(SW_VERSION >> 12) & 0x0F}.${(SW_VERSION >> 4) & 0xFF}.${SW_VERSION & 0x0F}`,
    sensor_type: SENSOR_TYPE
  };

  // Add THP data if sensor available (sensor_type > 0)
  if (SENSOR_TYPE_RAW > 0) {
    if (TEMP_C !== null) result.temperature_C = TEMP_C;
    if (HUMIDITY_PERCENT !== null) result.humidity_percent = HUMIDITY_PERCENT;
    if (PRESSURE_HPA !== null) result.pressure_hPa = PRESSURE_HPA;
    if (SENSOR_TYPE_RAW === 3 && GAS_KOHM !== null) {
      result.gas_resistance_kOhm = GAS_KOHM;
    }
  }

  return { data: result, warnings: [], errors: [] };
}

Decoded fields

• counts: GM tube counts in the interval

• cpm: counts per minute (from counts and sample_time_ms)

• cps: counts per second

• uSvph: dose rate in uSv/h (Si22G conversion factor)

• sample_time_ms: measurement interval in ms

• tube_number: tube ID (0x13=SBM-19, 0x14=SBM-20, 0x16=Si22G)

• sw_version: firmware version string

• sensor_type: none, BMP280, BME280, or BME680

• temperature_C: temperature (C, if sensor)

• humidity_percent: humidity (%, if sensor)

• pressure_hPa: pressure (hPa, if sensor)

• gas_resistance_kOhm: gas resistance (kOhm, BME680 only)

HTTP integration

HTTP integration to sensor.community is provided via the webhook service https://ttn2luft.citysensor.de. The LoRa payload is transformed server side into the sensor.community format.