changeLEDBrightness(level) {
		switch(level) {
			case 'bright':
				level = 0;
				break;
			case 'dim':
				level = 1;
				break;
			case 'off':
				level = 2;
				break;
			default:
				return Promise.reject(new Error('Invalid LED brigthness: ' + level));
		}
		return this.call('set_led_b', [ level ], { refresh: [ 'ledBrightness' ] })
			.then(() => null);
	}
Set the LED brightness to either `bright`, `dim` or `off`.