Ahh ok, you don't mean WS2812s then, just plain LEDs. I don't really have any ideas for what to do with a photodiode, but I have some of those straight LEDs and

Ahh ok, you don't mean WS2812s then, just plain LEDs. I don't really have any ideas for what to do with a photodiode, but I have some of those straight LEDs and I really want to make something with those. What kind of thing would you want to make with LEDs as inputs?