Electronics reminder snippet: Pull-down an IO pin with a transistor

NPN – eg 2n3904, 10k on base, 1k on collector

Use when the IO-pin has to be up by default

PNP – eg 2n3906, 1k on collector

Use when the IO-pin has to be down by default, eg IO2 on ESP8266 for boot conditions

Make some notes about what happens, and some code for an ESP with a case study (LED USB controller)