adding USB-C charging to the original Nokia 3310

while it's almost useless these days, the 3310 has significant nostalgia value for many of us and I wanted one as an emergency burner for use when travelling. the original 3.7v barrel jack PSU is not common now though. USB-C time.

the 5v from a USB PSU is not 3.7v, but some reading suggested to me that Nokia phones from this era are quite flexible about input voltage. I found some examples of people doing this very thing, so I soldered the USB V+ and GND directly to the mainboard where the original charger connector went, and crossed my fingers. strictly speaking this is not a very good idea, but I was able to measure a steady 3.6v at the battery connector both with the original charger connected, and with the USB mod in place. this implies that the battery management is still working correctly which was my primary concern, but failure of other components not rated for 5v so close to the battery could still be quite bad. with the relative simplicity of the board and the relatively small (26%) difference in voltage, I'm comfortable continuing to use it (and continuing to cross my fingers), especially given how infrequently/briefly it will ever be plugged in.

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it works! as a bonus, I was able to leave the socket and pins intact for the headset connector. in practice I am almost certainly never going to use this, but managing to successfully work around it was satisfying. the type-C connector was held into the modified plastic I/O block at the bottom of the phone using the compression of the mid-frame, then glued with JB-Weld epoxy. once cured this allows the phone to be dismantled as before, without any danger of the connector coming loose.