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)
Turn on switch.

        :return: None