Interfacing TDS/EC device

Projects General Guidance
1

I have been interested in getting an Arduino for a quite a while and recently I decided to purchase a board and "Arduino Cookbook". I have done some of the projects in the "Arduino Cookbook" but I am still very much a beginner to this stuff so please be gentle with me :).

Right now I am trying to interface a TDS meter (for measuring salinity) to the Arduino but I am having little luck doing so and I wanted to see if someone who is a little more experienced than me could give me some advice here. I have read through the forums and found similar posts but haven't found anything that helps me with what I am doing.

The meter I am working with is a cheapo ebay meter- Search "TDS meter" on ebay and look at the least expensive meters, which are blue with two buttons. I figured I could take the meter apart (see attached pics) and find a connection that fluctuates when the probes are exposed to different salinity levels. I took a look at this website- PPM Meter - ProjectWiki - which shows a schematic for a TDS meter and I assume a similar circuit is used in my meter. It looks like an AC wave is created, transmitted across the water via the probes, then the voltage from the reading probe is translated into a value and displayed on screen. My thought was to locate where the voltage was being read on the circuit board and then use that voltage to basically tap into the raw data from the device.

I've taken my multimeter, put the black probe on the negative battery terminal and probed around the circuit board while taking the device's probes in and out of the water (is this the right approach?). When the device is out of the water, the screen reads 000PPM and when the device is in the (tap) water, the device reads around 280PPM. I found two contacts on the board that fluctuate in voltage when the TDS level fluctuates. I think the two contacts are actually connected together on the PCB board (they read exactly the same and have almost no resistance between them) so I'm just gonna assume they are one in the same.

Here are the results when I have the probe out and in the water, respectively:
OUT -- IN
PPM: 000 -- 280
DCV: .25v -- .38v(unstable ~.36v-.40v)
ACV: .3v -- .6v.

I think I am on the right track here? I assume I just need to take that voltage (AC I believe) from that contact and figure out how to convert that to DC and route that into the Arduino? Any help/feedback would be really helpful.

Thanks,

Jason

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Here are the results when I have the probe out and in the water, respectively:
OUT -- IN
PPM: 000 -- 280
DCV: .25v -- .38v(unstable ~.36v-.40v)
ACV: .3v -- .6v.

I think I am on the right track here?

You need a couple more data points - probe in mildly salty water and probe in very salty water. If the values you see change in a linear fashion (or even non-linear), you are on the right track.

3

Hi
I am working on a similar project. The only difference is i am using inline ec monitor verses handheld. This is what i have learned...

1.Arduino analog input is very sensitive to ac noise..make sure your voltage out from meter does not have ac.As you know EC meters use ac signal across the tips of the electrode then it gets rectified to dc.
2.Ardunio accepts 0-5vdc analog input... .i dont know what full scale do u use for your measurment, it better be little higher then readings you get.
I like to know what range you want to use and what kind of application. If interested i can provide you conductivity kit board that works
with Arduino ...range 0-5000 0-5vdc respectivly only cost me $20.
you can read about it at
http://webpages.charter.net/tdsmeter/conductivity.html

4

I'm interested in buying a few interface cards. I just need 0-5000 ppm indications from a 0-5vdc range, analog.

Cheers!

5

Check these sensors and let me know what you think

http://webpages.charter.net/tdsmeter/conductivity.html