Here are some initial concepts for testing soil moisture for the robot to determine if the plant is to be watered. Here is the full write up of the moisture computer!
As a starting point, a circular plant tray, numerous small pots and soil, and a plant moisture meter was purchased. This meter has four levels of moisture and a chart on the back for determining which level for which type of plants. Measurements are made by plunging this meter into the soil next to the plant and the analog meter reads in four zones. It works by generating a small amount of measureable current in a bi metal sensor at the bottom when in the conductive soil. So its like making a battery using two dissimilar metals in a cup of salt water - a small battery is made with a tiny amount of current. Now this is not the same as measuring the soil resisitance like I will be doing but is directly proportional. My goal is to pinpoint the four zones and find the equivelent resistance for measureing with a micro controller.
The proto type probe is an active system, which you apply 5 volts from the PCB which contains the processor. It generates a current through the resistor and converts to a voltage to be remotely measured. Output is 0 - 3.5 v depending on moisture conductivity. It is very sensitive and just by holding the probes by hand, you can get nearly a volt out of it.
Probes are stainless steel #6 screws, three inches long. This avoids electrolytic corrosion deposition common with such probes made from brass or common steel that can rob your sensor of its accuracy. The goal is to measure the soil, display with 4 LEDs the level equivelent to the meter probe and transmit via optical link to the robot which will be parked near by.
BACK TO ROBOT PAGES HOME
Schematic of my moisture probe. I may change the sensitivity by varying the NPN emmitter resistor, but this works pretty well so far.
Early test results: The soil was mixed with various amounts of water to yeild readings on the purchased moisture meter from 1 - 4. Then each time I took a both a resistance and circuit output measurement. The results were quite variable, the readings varied from 3M Ohm to 2.5 M Ohm. But there was little correlation to soil moisture! In other words, nearly dry soil was unreadable as expected, but moist soil compared to total muck was nearly the same values. More testing....
And now a complete write up on the plant moisture sensor can be found HERE.