Moisture Meter Background

Hi Eric,
Yep, I'm back again. I've been poring over the Moisture Meter schematic and the DS2760 data sheet. It looks to me as if the 555/sensor circuit is treated as an external sense resistor for the DS2760. If that's the case, then I'd read the Current Register for the moisture measurement. Have I got that right, or am I completely out to lunch?

If you have the time to bash something out, I wouldn't mind seeing a Theory of Operation for the Moisture Meter. Your 555/sensor circuit isn't one of the more common cookbook circuits, and I'd like to understand what you had in mind.

Cheers,
Tim Cole
Ottawa ON

Tim,

Let me see if I can give you a quick explaination.

When the attached sensor gets wet its resistance changes which changes the RC time constant of the 555 and it starts to oscillate faster. As the 555 oscillates faster it uses more current, which causes more voltage to be dropped across R2 since it is in the ground path for the 555. R2 is also across SNS and VSS of the DS2760 which are the pins that measure current by measuring the voltage drop across R2.

Hopefully that explaination helped.

Eric

Thanks, Eric. That's a clever circuit. One final question (or two interrelated ones): what's the range of resistances you'd expect for your moisture sensors, and what's your nominal time constant?

I ask because I'm hoping to put together a telescope dew sensor based on the moisture meter. The leaf moisture sensor seems like a good starting point, but I may need a larger sensor with a fairly large surface area.

Cheers,
Tim Cole
Ottawa ON

I'm using the Moisture Meter MM2-R1-A together with the Watermark Soil Moisture Sensor WGS1-P and I'm reading the current register of DS2760 to be -1.43437 via 1-wire and owfs.

Is this reading in volts? How do I convert -1.43437 to a useful soil moisture unit? Maybe kPa or something else. I would like to make a graph from the moisture readings and I'm looking for something meaningful to put on the Y-axis.

Thanks,
Thomas

Wagner said:

I'm using the Moisture Meter MM2-R1-A together with the Watermark Soil Moisture Sensor WGS1-P and I'm reading the current register of DS2760 to be -1.43437 via 1-wire and owfs.

Is this reading in volts? How do I convert -1.43437 to a useful soil moisture unit? Maybe kPa or something else. I would like to make a graph from the moisture readings and I'm looking for something meaningful to put on the Y-axis.

Thanks,
Thomas

Click to expand...

Thomas,

The Moisture Meter is not calibrated to any standard, it is a relative scale. You will need to get a reading when the sensor is dry and when it is completely wet. That way you will know the 0% and 100% reading, then you can interpolate for reading between.

Eric

Has anyone had any luck with there - I keep being told they are a waste of time!

The Moisture Meter is not calibrated to any standard, it is a relative scale. You will need to get a reading when the sensor is dry and when it is completely wet. That way you will know the 0% and 100% reading, then you can interpolate for reading between.

Eric

Click to expand...

Thanks Eric. I dropped the sensor in a bucket of water this morning, so hopefully this will give me the 100% limit tomorrow. Then I'll bring out my fan heater and dry it afterwards to get the 0% limit ;-)
I'm writing about the project on http://happyfarming.com.

comet said:

Has anyone had any luck with there - I keep being told they are a waste of time!

Click to expand...

Are you referring to the soil moisture sensor, or...?

Yes the moisture probe.

comet said:

Has anyone had any luck with there - I keep being told they are a waste of time!

Click to expand...

Do you have any links to people that have tried to make them work?

Apparently I get the same reading from the Current Register whether I put the probe in a bucket of water, or put it out in the open air. About -1.34. It seems to be highly temperature dependent too, but I'm not sure.

Then your sensor is probably not connected properly. -1.34 is the current reading when the board gets infinite resistance. If you want to check out raw readings from my sensor: http://neo.dyn-o-saur.com/index2.php or the graph: http://neo.dyn-o-saur.com/w1retap-rain.php

HTH, --Hans

Wagner said:

comet said:

Has anyone had any luck with there - I keep being told they are a waste of time!

Click to expand...

Do you have any links to people that have tried to make them work?

Apparently I get the same reading from the Current Register whether I put the probe in a bucket of water, or put it out in the open air. About -1.34. It seems to be highly temperature dependent too, but I'm not sure.

Click to expand...

HansF said:

Then your sensor is probably not connected properly. -1.34 is the current reading when the board gets infinite resistance. If you want to check out raw readings from my sensor: http://neo.dyn-o-saur.com/index2.php or the graph: http://neo.dyn-o-saur.com/w1retap-rain.php

HTH, --Hans

Click to expand...

We had a quite windy day around here, so at 13:30 PM I put the probe out in the wind, and now it seems like it's connected and really getting dry:



So now the calibration limits are -1.4 wet and -0.5 dry.

HansF said:

If you want to check out raw readings from my sensor: http://neo.dyn-o-saur.com/index2.php or the graph: http://neo.dyn-o-saur.com/w1retap-rain.php

Click to expand...

Very impressive page you've got there :-D

Rereading my post I notice that I made a mistake: the current I get with infinite resistance is around -0.13, not -1.3. So when you got a current of around -1.3 you definitely had a working sensor. Now that it's getting dry you will probably also end around -0.13 mA. Sorry for the mistake. Confused by the numbers. --Hans

P.S. My weather/1-wire monitoring interest was also triggered by my veg garden.

ericvic said:

The Moisture Meter is not calibrated to any standard, it is a relative scale. You will need to get a reading when the sensor is dry and when it is completely wet. That way you will know the 0% and 100% reading, then you can interpolate for reading between.

Eric

Click to expand...

I'm done calibrating for now. Dry reads as -0.2368 and wet reads as -1.400. The range is then |-1.400|-|-0.2368|=1.163 and 1% is 1.163/100%=0.01163.

I've been struggling with one of the Hobby Boards soil moisture sensors for about two days now and can't get a working reading from it. At the moment, I am getting the same reading from this thing whether it is soaked overnight in water or bone dry. I'm not sure at this point if I have bad code, a bad board or a bad Watermark Soil Sensor.

Is there any way to test out the HobbyBoards board with an external resistor to eliminate that as the problem? Barring that, is there any way to test out the watermark soil sensor itself to see if I received a dud?

Thanks if advance. This thing is frustrating the living daylights out of me right now.