Moisture sensor.(Just another DIY)
Headphone plug turned moisture sensor.
For a little while now I've been looking at how to do a diy moisture sensor that I could use for my garden project. There are a few designs, but there are drawbacks to each. For instance, one design uses galvanized nails or wire that you stick into the soil. Problem with these is that they are coated in a chemical, and of course will eventually corrode. Another design uses uses two wires encased in gypsum(plaster of paris). As the gypsum absorbs moisture the resistance drops. This is great but even at full saturation this may not vary by much. See this guys site for more info. http://gardenbot.org/howTo/soilMoisture/ So in comes my design. Since the sensor requires 2 metal probes or surfaces, I decided to try a headphone jack as I acquired several that a friend was getting rid of. Since I just started the testing of this little sensor I'm not sure how fast the probe may corrode, but I am able to get readings on fully saturated soil, partial saturation and of course completely desiccated. The circuit is based on this sensor with some slight resistor modification to cut down the current used. http://seeedstudio.com/wiki/Grove_-_Moisture_Sensor
Thanks to Patrick for pointing out the sensor above.
Voltage meausrements currently go from 0 to 1.65v when fully saturated(soil). Direct contact outputs around 1.85v and using fingers over the contacts gives around 0.4-0.5v and when giving it a good squeeze, around 0.8v. The resistors I used were R1 = 8.5k and R2 1k. This drops the curent from around 14ma to around 1ma. I'll set this up to do constant reading and send that data via xbee to a base station setup. I'm still considering how best to use this setup. A simple approach would be to just use a quad comparator, and when the level goes below a certain point, it'll trigger a signal to the proc that it needs water or something. I could also just keep constant measurements throughout the day....not sure just yet.