Espruino Wifi very hot / broken

Argh - sorry to hear that. It's interesting about the power usage you mention - my bet would be that at some point 12v got applied directly to some of the chip's IO pins, and that might have physically damaged the chip at that point. While the rest of the chip would limp on, that part of the GPIO would be shorted and would be drawing loads of power.

The voltage regulator itself is thermally protected, so the good news is if you have access to a hot air rework station, probably replacing the STM32F411 chip (and reflashing it with Espruino) would bring everything back to life.

If you're new to electronics that might be a bit difficult - but you may know someone that would be willing to give it a try?

As @Robin says, if you're dealing with digital IO then OptoIsolators are a really neat, safe way of connecting systems together without risking any damage. There's not really any less soldering than if you used a voltage divider.

If you do need to use a voltage divider (eg for analog) then there are some things you can do to be nice and safe:

• Add a 1k resistor between your input and the signal pin of Espruino - that way even if you go connect 12v, there's a limit to the amount of current that can be put into Espruino - which will most likely save it. It works for output pins too.
  
• Put a capactor on the Espruino input pin - it'll reduce the frequency of the signal you can receive, but with the resistor (above) it really gets rid of any short pulses voltage. It also makes your analog signal readings more accurate :)
  
• Use two schottky diodes on the Espruino pin, one to GND and one to 3.3v. This will 'clamp' the voltage to the correct range (but is overkill really unless you actually expect the voltage to go out of range very often)
  
  

Hope that's some help!