the esp8266 mcu is widely used in iot projects. however, most gpio pins on this mcu have special / multiple functionality and are not safe to use in all circumstances.
these “features” are still not well documented and not explained in a friendly manner for newbies. i see lots of cases when the “issues” are simply caused by not knowing which pins to use in which case.
here is a basic table for startup. i’ve collected the info from various sources on the internet, there could be erroneous or missing informations, so, please comment if something is off…
edit: i’ve updated the google sheet, so in the far right it now shows the sonoff th10/16 and 4ch pin usage