Use mobile to power external LED(bulb) via a Micro USB port?
If one wants to turn a phone/tablet into host mode, the (ID pin) must be grounded. It is done in USB cable, in the overmold. The device-kind of USB cable has this pin floating. Unfortunately, a normal cable does not have this pin exposed, so you need a special "OTG adapter" cable, which has the ID pin grounded. Then you might be able to turn your mobile device into HOST mode, and it will output 5V on VBUS pin (Red wire). You can safely count on having at least 100mA from it, which can lit-up a substantial LED chip.

But I wonder why do you need an external LED when any phone has a built-in one, as a photo flash? There are lots of apps that use it as a flashlight...