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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Hi,
Just for others, @Andrey is talking about the schematic on http://www.espruino.com/EspruinoBoard
The arrangement is because you want to be able to swap from battery power to USB power. While I could use a diode between the battery and voltage regulator (like is done for USB), that would introduce (at minimum) a 0.3v voltage drop - which isn't good for running off of 3.7v Lithium batteries.
Instead the FET is used so that when USB is unplugged it turns on and there is no voltage drop.
On the new Pico board I've also connected that transistor to a pin on the Espruino, allowing you to use it as a powered output (or to force the battery to be charged from USB!).