ESP8266 Community Forum

The 8266 data sheet doesn't give much detail on the electrical characteristics of the GPIO pins. A fully-spec'd microcontroller would have, for example, graphs of output voltage vs. current for HIGH and LOW output states at various supply voltages.

I'm trying to design an interface to drive a bipolar transistor to switch 15V to a small fan. Assuming 3.3V supply, my best guess so far is a 330Ω series resistor to the base of the transistor, which is tied to ground with 1KΩ to prevent any unwanted turnon.

as you allready correctly stated:

The 8266 data sheet doesn't give much detail on the electrical characteristics

neither general nor on gpios
unfortunately even paid for personal doesn't get more information or else igrrr wouldn't have to resort on trial and error ...

RobLewis wrote:drive a bipolar transistor to switch 15V to a small fan.

Why using such old technology like bipolar transistor which is hard to saturate with improper base current ?
Simply use a MOSFET which doesn't work with current but with voltage, just make sure that you choose one that has a Vgs thresold low enough to be driven by a 3V GPIO, such as the AO3402, which has Vgs of 1.5V and can drive 30V at 4A.