1. I don’t have a particular sensor in mind, as long as it measures visible light it will work. I’ve looked at a few different options something like this https://www.amazon.com/Adafruit-Digital-Light-Luminosity-sensor/dp/B00K9M6PZ8 . At this point I’m more concerned a reliable program and sensor. If you know of a better solution, run with it. The cost is not a huge factor.
2. For control I need to be able to easily adjust the trigger variable. The plants grow through the season and their demands change. I will base number based excess water running out of the trough.
3. At the hottest part of a summer, during the middle of the day it should not be running less than every 10 mins for 2 min irrigation events. So it would be alright to start counting again immediately after an irrigation event is triggered. However, in the event an event is triggered during an watering event, starting again would not be a problem.
BTW when we get a running prototype. I will be willing to send you something for your time. We are a small business, there is value in this running properly.