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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.
@ltj
I second @DrAzzy, though I would not recommend an old computer PSU (ATX). The low voltage supply, 5V, is not stable when no current is drawn from the 12V output. For stand-alone operation using mains, a phone charger would be the best option - it's 5V, commonly ~1A these days and compact/efficient due to a switching topology. The Espruino power setup is also documented here.
@ChrisB, using the bat_in or usb_vbus, vbat will be protected by a 1A fuse so no matter the input voltage you will be allowed to draw more than <1A on vbat. If you choose to power the Espruino directly on vbat be warned that you do not get reverse polarity protection nor a fuse to protect you peripherals and the board. You can still switch high currents safely with the Espruino using mosfets or relays though.
If you have peripherals connected that runs on higher voltages, yes, then you would need more than one power supply. Alternatively you can use one mains PSU that delivers 24V DC and then use voltage regulators to give you 12V and 5V as well from the same source. No matter what, all your devices must share a common ground to function properly.