Soil moisture sensor

[ericbrian]

I want to create a watering system for potted Cacti. Basically, I have 2 cacti that I am either giving too much water or not enough. So, I want to create a system that can sense when it is time to give them some more water.

I figured I could do that with putting two rods in the ground and if there is current flowing, then the soil is moist and they don't need water at some distant apart. But if I do it this way, there is a lot of energy use.

So, does anybody have an idea on how to do this?

Eventually, I want to hook up and ethernet shield through which I will send an alert to my Android phone.

Thanks for any help and thoughts!

[adafruit_support_bill]

You might want to check out capacitive moisture sensing. Vegetronix is one source of assembled probes. I've seen a few DIY projects out there too.

[philba]

Yes, you can do that. A more stable (and cheap!) approach is to use plaster of paris to hold the two rods so they are a consistent distance apart. The gypsum absorbs water and dries out fairly evenly. It also isolates the effect of fertilizer on conductivity. You will get stable and repeatable readings. http://hackaday.com/2010/03/15/soil-moisture-sensing/ I saw a similar description but the guy used plastic screen material to wrap the PoP plugs to make them less fragile but couldn't find it on the web. Perforated shrink wrap would be really nice though not sure it exists. I would look at using stainless steel instead of galvanized nails and would try to figure out a convenient way to connect the sensor wires (screw terminals?).

I don't understand your energy comment. Even in water the resistance isn't going to be that low. If you are really concerned about it, you can a) use a series resistor to reduce the max current and b) use a second pin to energize the sensor once every hour or so (i.e. use an output pin to provide "Vcc").

[fat16lib]

I have used a variety of moisture sensors including gypsum and Vegetronix sensors.

I really like the Vegetronix sensor. It uses RF to detect moisture in a region around the sensor. This works well for plants. You can use the Vegetronix as a portable sensor since it doesn't need to be in the ground for a long time to soak up water like the gypsum.

Gypsum blocks are not bad but are only really cheap if make them yourself. I used stainless mesh to make the electrodes and made about 20. The problem is testing and calibrating them. They also have limited life.

You could buy them. The cheapest I know about are these http://www.amazon.com/Delmhorst-GB-1-Gy ... sum+Sensor but you need to buy ten.