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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
Ahh ok - basically in order to tell a PC when a device is connected you need a pull-up resistor. The transistor allows you to switch that on and off, so the board can disconnect itself. It's more useful when it goes to sleep, and then it can effectively tell the PC it's been unplugged.
Newer STM32 chips have it all built-in I think, but the F103 doesn't and needs it externally.