June 2 2017 -

I've assembled the sensors and actuators into a bird's nest of wires... but, it works fine for a prototype. The attached video shows how the assembled prototype will work (I wanted to show the components before I added them into the composting soil).

A few notes:

1. I originally planned to use a solenoid and use gravity to water the compost, but ultimately decided on a pump.
2. I have a difficult time calculating % humidity using the moisture sensor (there isn't a great correlation between the voltage out of the sensor to the % wetness of the soil). I'm looking into using a better sensor that (a) outputs % moisture and (b) is more robust (this sensor would only last a few months) [I'll probably use this sensor - https://www.amazon.com/Adafruit-Accessories-Temperature-Moisture-Sensor/dp/B00XW2NXD4]
3. I am using an ultrasonic sensor for the prototype, but plant to switch to a "dumb" float sensor (I don't need to know levels - just dry or not dry]
4. I was originally using Bluetooth LE to send data from Arduino to a Raspberry Pi - but decided to remove that logic and go with LoRa instead. Devices are in the mail now... hope to show that interaction later this month.

Last note - here's a simple process flow that outlines the logic.