docs: def turn_on(self, channel, callback=None):
          """
          Turn on switch.
  
          :return: None
          """
          if callback is None:
              def callb():
                  """No-op"""
                  pass
              callback = callb
          message = velbus.SwitchRelayOnMessage(self._address)
          message.relay_channels = [channel]
          self._controller.send(message, callback)