Micro USB MHL To HDMI Adapter HDTV - not working on my Galaxy S5 I have bought a Micro USB MHL To HDMI Adapter HDTV Cable For Samsung Galaxy S3 i9300 Note2 N7100 from eBay, and just assumed backward compability (yes I know...)

So know I have this thing, and a Galaxy S5, and it's not working. On my sisters   LG G3 it's also not working.

Anything I can do beside giving it away to the next person who has a S3?
The short answer is you're stuck.  I own the same cable for my S3, and, in researching it, I found that you must buy the exact cable for the specific phone you're using it with.  Samsung changed the pin configuration for the S3 (see here), so, you've got an incompatibility issue.