okay, i told scruss i'd share this and here it is -- a micropython library for receiving data slowly, but reliably, over a photocell. it's how the halo clock gets configuration data without using wifi or bluetooth -- you just hold the clock up to a web browser, and a little circle flashes, sending a brief packet of data (e.g. wifi credentials). both the receiver (in micropython) and the sender (in javascript) are included.