That is simple enough that it could actually keep working through the isekai teleport, and it also probably would comport with the arithmetic knowledge of the smartest person in the village, and might not get you immediately killed as a witch or warlock.
Lots of small tech, but if we want it to stay useful to them longer than one battery charge, I’d say a crank-powered flashlight or lantern.
I suppose anyone doing a lot of math, like a moneychanger, would also lose their shit over a solar powered calculator.
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A solar powered calculator, that one I like.
That is simple enough that it could actually keep working through the isekai teleport, and it also probably would comport with the arithmetic knowledge of the smartest person in the village, and might not get you immediately killed as a witch or warlock.