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Topic: wireless soil moisture (Read 19611 times) previous topic - next topic

SSzretter

Just wondering if anyone knows of any soil moisture sensing projects , especially if it is wireless capable?

I was thinking I could use xbee's for the wireless and something like this for the sensor: http://www.vegetronix.com/Products/VG400/

But if there is someone who has already started a project, that would save me time!

dogsop

I've been playing with soil moisture sensors but I may not be much help.

I originally set out to use homemade gypsum block sensors such as these: http://www.webpages.uidaho.edu/~chuckm/irr/irrpg.htm The problem I had was that when I tested them the soil would get bone dry before the sensor reading would start to drop. The plaster just seems to retain moisture far too long to be useful.

I created a homemade version of this sensor instead: http://www.hobby-boards.com/catalog/product_info.php?cPath=22&products_id=1547

It seemed to work well in indoor tests I did last winter but since I put them out in my garden a month ago we have yet to go more than 5 days without rain so the soil really hasn't gotten dry enough for a real test.

I'm using a 555 timer circuit to measure conductivity through the sensor.



Here is a full description of the circuit.

http://www.emesystems.com/OL2mhos.htm

Rather than using the voltage across a resistor to measure the current to the timer circuit I just connected the discharge pin (pin 7) of the 555 to one of the Arduino interrupts and count the frequency which increases with moisture level.

I'm not doing anything with wireless. I just have a red and green LED. Green if the frequency is above a certain value, red if it is below.

As I said, if it ever stops raining I'll let you know if it works.  :(

Ran Talbott

So,  how did you make the home-made substitute for the Watermark sensor?

Inquiring cheapskate minds want to knw   :)

Ran

SSzretter

I opened up my 'rapitest' soil moisture probe and see that it looks like a simple analog resistance meter - just two wires hooked to an analog meter.   So it looks like it measures the resistance between the tip of the metal probe and the rest of the probe.

So, I guess my next question is, how might I connect this to an arduino (analog input?) so the arduino can record the readings?

SSzretter

Well, I found that the little meter I have does not hold it's reading, so it is probably not going to work anyway..

I did find a couple links to circuit diagrams for soil moisture sensors that use LEDs, should be easy to hook to the arduino?

http://www.electronics-lab.com/projects/science/018/index.html

http://www.electronics-lab.com/projects/science/008/index.html


chupit

SSzretter,

Why not use circuit that was mentioned by dogsop? It should provide frequency output depending on resistance of inputs. You can count frequency or you can use freq-to-volt converter to convert frequency to voltage that could be useful for analog input.

SSzretter

Because, correct me if I am wrong, but it looks like I would have to buy that watermark sensor, and some other parts...   cost at least $36 for the sensor...  
I am looking for something lower cost, but I may not be interpreting the schematic properly..

Grumpy_Mike

Quote
Well, I found that the little meter I have does not hold it's reading


This is because as you apply a DC voltage across two points you get electrolysis of the water. You get a film of tiny bubbles of gas over the electrodes, this reduces the surface area and so the resistance goes up. The trick is to use AC to measure the resistance not DC. Also don't measure continuously, once an hour is more than enough. So only apply the bias voltage when you want to measure.

SSzretter

I also found this, would this even require a special circuit?  Just provide it voltage and get back voltage...?
http://www.vegetronix.com/Soil-Moisture-Probe.phtml

Grumpy_Mike

Looks good to me, simply connect it up to the analogue input.

chupit

Hi Grumpy_Mike,

Unfortunately my cheapmind does not allow me to purchase such an expensive sensors (with delivery price to EU ~$50), especially if I would like to have around 5 moisture measurement points.

What do you think, can we use circuit mentioned by dogsop to take measurement from probes created from just metal wire instead of Watermark sensor?

In the description of this circuit it is said that freq is changed proportionally to resistance, so in my opinion it should also work fine with just two probes made of wires, but I'm not sure.

Anyway, I'm going to assemble this circuit this weekend and to see how frequency will change compared with resistance and will I be able to use it for moisture measurement.

Thanks.

SSzretter

Yes I have at least 3 points currently, probably 5 by next year myself, and very little budget.    I did not get the impression that dogsop's circuit was low cost - it looks like it requires a $36 sensor, and a $20 chip, and ?
Did I miss a link?

chupit

SSzretter,

I still think that sensor is not 100% necessary for this circuit. Sensor will help make measurement more precise, but we can avoid using sensor. The circuit itself if you can assemble it from the parts, should not cost more than 5$ or something similar to that. The core of that circuit is LM555 which cost around 0.15 Euro cents + some capacitors and resistors.  
But, unfortunately, I'm not electronics expert at all, so there is only one way to be sure for me, it is to test... which I'm going to do in near future :)

SSzretter

What about the TK1530 in the schematic?

chupit

As I understood, that's only for power regulation, to get 3V from source that can be 3.5 volts and as high as 15 volts. You can get 3V3 directly from Arduino and use resistors to make it exactly 3V.

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