I purchased a set of these sensors for $10+ on Amazon:

https://www.amazon.com/XLX-Photoresistor-Optoresistor-Assortment-Light-dependen/dp/B01M3TO7RA/ref=sr_1_9

The GL5528 seems to work best, just plugged in between pin 7 and ground. Then I set pin 7 as an input with pull up and was able to sense light and dark, just passing my hand over the sensor. 

TODO:

- Add another channel for the clock

- Write some Arduino code to show the state of those pins on a graph and try different transmission speeds to see if the signal gets through at "fast enough" rates.

- Try the wire library to see if I can receive I2C data

My time is stretched to the breaking point right now so if someone wants to help out by duplicating this and doing those steps and sharing the code / results I would be VERY appreciative.