For some reason I was under the impression that you needed to have a command bit for the UV sensor, that was not the case. I will keep posting all the steps it took to get this app running and will also keep working on the other sensors. But quickly here is the updated version of the relevant code.

  alias Circuits.I2C
  alias LTR390_UV.Config

  import Bitwise

  @enable_register 0x00
  @gain_register 0x05
  @control_register 0x04
  @als_data_register [0x0D, 0x0E, 0x0F]
  @uvs_data_register [0x10, 0x11, 0x12]

  def discover(possible_addresses \\ [0x53]) do
    I2C.discover_one!(possible_addresses)
  end

  def open(bus_name) do
    {:ok, i2c} = I2C.open(bus_name)
    i2c
  end

  def write_config(config, i2c, sensor) do
    enable_byte = Config.to_enable_byte(config)
    control_byte = Config.to_control_byte(config)
    gain_byte = Config.to_gain_byte(config)

    # Write ENABLE register (1 byte) with command bit set
    I2C.write(i2c, sensor, <<@enable_register, enable_byte>>)

    # Write CONTROL register (1 byte) with command bit set
    I2C.write(i2c, sensor, <<@control_register, control_byte>>)

    # Write the GAIN register
    I2C.write(i2c, sensor, <<@gain_register, gain_byte>>)
  end