Now smartphones have built-in flash/light that attach to smartphone body, you can not adjust light angle or distance to make new photography experience. However, now you can make external fill light for your smartphone without paying huge money.
All you need is a smartphone that supports OTG USB and USB OTG adapter, USB led reading light, and USB female to female or USB hubs.
You can see the above image that all parts are easy to find on EeBay,Aliexpress and they are low cost.
Beware, LED light should not consume over 3 watts if you use higher power LED your smartphone power supply may overheat and damage. The LED light I use consume only 1.2 watts, so the smartphone I tested seem to be ok with it.
I use USB hubs because I can use it as stand lighting. USB female to female adapter can be a light stand so you must hold it while you taking a photo, that 's not a good idea.
USB OTG adapter I used is a very low cost (1$) and work well, you can find it on eBay or Ali express too.
Now, let's see my video below if you want to see how good it's work.
Enjoy your new smartphone photo technic.