[barake]

Could use an optical sensor like the Timex Datalink watches from the 90s[1]

Could set the earrings on a phone screen and transfer the patterns over. Sounds like a fun project.

1: https://en.m.wikipedia.org/wiki/Timex_Datalink

[zokier]

LEDs are actually sensitive to light themselves so with some wizardy you might not even need separate sensor

[mattegan]

I love simple light-based data transmission stuff. I've seen it included in things like guitar pedals that have just a few config bits that someone might want to change infrequently. An app to change some settings can be as simple as just a little webpage! So simple!

Here's a (tiny) demo of this for my PCB business card project from years ago [1]. If IIRC this proof of concept was as simple as using a phototransitor on a GPIO connected to the UART peripheral with a very low baud rate.

[1]: https://www.matt.egan.me/entry/electronic-business-card-pt3#...