I'm probably way of beam here but when I read the data sheet for the 684 and then read about the 'Schmitt Trigger' it seemed to me that this worked on a pre defined voltage level (the voltage can vary up to the set value). Maybe this could be the way to ensure that any data sent only gets sent at a 'set' light / voltage level.

Am I reading this right?

I know I could also just use the potential divider set to a voltage / light level and even add in a time delay routine (say an hour) to make sure the data only gets sent once and when it is dark.

Interesting though.

Dave