jwbernin:
Thanks, that answers my next question as well! (Can I drive all 15 sense points from one pin set?)My last question is on the circuit design. Should the resistor be on the high side or the low side of the sense pin when taking the reading? It seems to me it should be on the low side, but the how-to I looked at (and thus the example) has it on the high side (d1 high d2 low for reading, d2 high d1 low for reversal).
Well electrically it won't matter, but your readings are not going to be symmetrical (not equal) when taken D1 HIGH D2 LOW Vs D2 HIGH D1 LOW at the same sensor location in either case. A simple 'moisture sensor' as you are using has one very big difficulty and that is 'calibrating' the actual voltage read Vs the actual moisture of the soil. You will have to 'characterize' your readings to determine what readings are equal to 'just right', 'too high', or 'too low'. Plus by driving the sensor current in opposite directions the voltage values will be the inversion of each other, if that makes sense? So you will have to come up with two sets of 'moisture to voltage' values to use in your sketch depending on which direction your sensor current is being driven by your output pins. So you have selected a design trade-off, inexpensive easy and to make moisture sensors that will require enough software tricks to come up with repeatable and reliable control decision points for your application. As I've never worked with trying to measure moisture I can only help with the electrical side, not the calibration or the 'fit for service' question(s).
Lefty