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Topic: Leaf moisture sensor (Read 4867 times) previous topic - next topic

javiZgz

Hello Hamijos Mios !!

I have the leaf moisture sensor by HobbyBoards http://www.hobby-boards.com/catalog/product_info.php?cPath=21&products_id=1544, really useful to prevent diseases in plants. It's just a board with its 2 wires making a kind of grill. When there is fog, dew or rain, then the little drops of water connect the 2 wires and make the grill conduct some current. Amazingly simple, affordable and useful.

Its wires are covered in gold so the oxidation is minimum. ALL OTHER LEAF MOISTURE SENSORS ARE MADE LIKE THIS ONE, so I don't know how some people sell sensors like this one for 150 USD.  :o

In fact the sensor is just a resistance. When there is fog/dew/rain drops, the sensor is about 3 KOhms, and when the ambient is dry the we have 200 Kohms.

I have seen the schematic to adapt this leaf moisture sensor http://www.hobby-boards.com/catalog/product_info.php?products_id=70 (also by HobbyBoards). It's based in the famous 555 timer, so the output is a signal where you have to monitor its frequency by counting how many pulses within some milisec's

I wonder why not to do just a current divider, measuring in volts. Anyone has this sensor or any other similar sensor? What is the advantage of measuring in Hz?   :-/ :-/

Regards

dogsop

#1
Jul 06, 2009, 06:31 pm Last Edit: Jul 06, 2009, 06:40 pm by dogsop Reason: 1
In the case of a soil moisture sensor the reason why that circuit is used is so that an AC current is passed through the sensor. If you pass a DC current, which is what you are doing with a voltage divider, electrolysis will change the resistance of the circuit over time. I'm assuming that the same thing is true with the leaf moisture sensor.

I've seen other moisture sensors where people say that you can measure the DC resistance and get away with it as long as you don't do it 'too often' or for 'too long'. Exactly what that means is anyone's guess.

The 555 circuit they describe is easy to build and it is easy to measure the frequency so I'd stick with the frequency measurement.

Correction:
I wasn't looking at that circuit carefully. They aren't outputting frequency. The DS2760 in the circuit is measuring the current used by the 555 and providing that value over a one-wire interface. (Someone please correct me if I'm not reading this correctly.) The current used by the 555 is proportional to the frequency of the timer circuit so it works the same way.

I have no experience with one-wire interfaces so I just build a 555 circuit and hooked pin 7 up to one of the Arduino interrupt pins and counted pulses.

javiZgz

Hello Dogsop

You're absolutely right, in SOIL moisture sensors (like Watermark) we have to read from it with AC voltage, cos if we apply DC voltage we could damage the sensor. So 555 timer based circuitry is needed.

I don't know if it's worth measuring with DC voltages for super-short periods, as the sensor could be damaged.

In my case I am using now a LEAF moisture sensor http://www.hobby-boards.com/catalog/product_info.php?cPath=21&products_id=1544 . Please look at it, it's just a grid, there's no chemistry, and its behavior is like a resistance. I am 99% sure there is no risk in measuring w/ DC current, just with a simple current divider, but HobbyBoards sells a 555 timer based circuit to measure from the leaf moisture sensor, and you never know..

Also: I don't know if someone has some table of the leaf moisture sensor in order to make the readings meaningful, for example:
  - 200 KOmhs = very dry leaf
  - 20  KOmhs = slew fog
  - 5    KOhms = heavy rain

Doesn't anyone has this sensor??

dogsop

Quote
Please look at it, it's just a grid, there's no chemistry, and its behavior is like a resistance.


I don't believe this is true. I believe that the circuit is created when water bridges the gap between two of the copper traces. This means that the copper traces are subject to the same electrolysis as in a soil sensor.

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