Building A PH Transmitter, need some help

[vdc24]

First I'll layout the basics of what I'm trying to do.

I have a siemens s7 216 plc with an anlog input module that I want to read a signal from a PH probe, they can't be directly connected because the ph probe provides a high impedence signal that the plc cannot read. There needs to be a middle man, which would be the transmitter that provides either a -5v to +5v signal or 0v to 10v signal at 4-20ma.

I've found a few schematics that seem to look like they'd work for this setup, here they are. I'd like to have to ability to calibrate the probe when its put in a test solution of a ph 7. But thats not really what I'm concered with at this point. This is for a diy project I'm working on, its not for profit and I'm trying to save $250 by building my own, a circuit like this premade goes for roughly that much which I think is redicolous for the average person not business.

Here are the schematics I've found that look promising. I'd like as much input as possible. I deal with mainly electrical aspects of stuff, I'm an electrician by occupation so my electronics background is limited to I know what components are and what they do, but its hard for me to sit there and peice together a circuit by hand on my own.

Schematics

https://www.cyberreefguru.com/electronics/phmeter3.gif

https://www.cyberreefguru.com/electronics/phmeter2.gif[/url]

 

[Russlk]

Your links don't work (forbidden). Please post a link to the data sheet for the PH probe.

 

[vdc24]

This is the circuit I'm mainly concered with, heres the schematic, its out of National Semi-Conductors spec sheet for the LMC6001.

**broken link removed**
**broken link removed**

Here are my main questions I have all the components except R1, R3, and R4.

I've had no luck locating R1 at all.

As for R3, and R4 they look like 5k resistor and 100k resistor but what is the 8 infront of r3 for and the 9 in front of r4 for?

-Roy

 

[Russlk]

There is a comma between the 3 and 8 indicating that R3 and R8 are 5K also R4 and R9 are 100K. I would use a stable resistor for R1 and compensate for temperature drift elsewhere.

 

[vdc24]

There is a comma between the 3 and 8 indicating that R3 and R8 are 5K also R4 and R9 are 100K. I would use a stable resistor for R1 and compensate for temperature drift elsewhere.

I feel so stupid, I missed the commas, I also didn't see that there were r8 and r9 and they weren't on the resistor list.

So a regular 100k resistor in place of r1 would not change the circuit? I could do temperature compensation in the programming in my PLC.

Now for my next question that I'll feel like an idiot after its answered.

I'm guessing these sections I have circle in red are where the four wires from the ph probe connect? Or does the probe just connect where it says ph probe?

**broken link removed**

 

[Nigel Goodwin]

I'm guessing these sections I have circle in red are where the four wires from the ph probe connect? Or does the probe just connect where it says ph probe?

The circled sections are ground symbols, the probe connects to where it's labelled (just two connections).

 

[FRIED]

Here is a chart that may help

 

[vdc24]

I'm guessing these sections I have circle in red are where the four wires from the ph probe connect? Or does the probe just connect where it says ph probe?

The circled sections are ground symbols, the probe connects to where it's labelled (just two connections).

Where is the second connection, the only thing I see is the postive out of the op amp going into the probe?

Also what type of capacitor should I be using, I know thats the symbol for non polarized, but what working voltage (the same as the circuit 5vdc?).

Thanks fried that will help a ton, any clue if theres a relationship between whats on the chart at each given degree that forms a slope/curve?

Edit: Ok I think I answerd the question regarding the two probe connections myself. One side of the probe is grounded to the circuit (the common side), and one side inputs into the opamp, when it feeds through the circuit to the output it can be read in a mV value by using the ground as the circuit and the output from the opamp?

Bear with me I'm learning as I do this, I'm use to light switches electro-mech relays, motors...etc

 

[vdc24]

Another question, what are the lines next to r3 and r8. And in the instructions for building the circuit it states r3 and r8 can be adjusted to calibrate the circuit, is this a tapped resistor, or PTO?

 

[Nigel Goodwin]

Another question, what are the lines next to r3 and r8. And in the instructions for building the circuit it states r3 and r8 can be adjusted to calibrate the circuit, is this a tapped resistor, or PTO?

R3 and R8 are variable resistors, it's probably best to use ones adjustable with a small screwdriver - commonly called 'preset resistors'.

 

[SNap]

Ph Transmitter

I am also going to build a PLC controlled Ph controller. I was wondring how it worked for you. I work in the water treatment industry and can answer any water quality questions you have.
David

 

[instruite]

Hi,
Some suggestions from my side:
1) In normal industrial standard to connect sensor module to plc it is recommended to use 1V-5V DC or 4-20mA, and the distance between sensor and PLC is long then its always best to use 4-20mA signal
2) I dont about the power supply at your end but in industry I think 24V DC power is available on Power line
So if you use the opamp which you have shown in schematic you will have to generate two power supplies +5V and -5V but if you use the above signals (1-5VDC, 4-20mA) then this can be done by a single positive power supply itself
3) As per the schematic you can check the below link
it consists of a 4-20mA transmiiter ckt on page 28 and they have given quite a good explaination (only read once, I have not gone into details yet)
if you have problem understanding it than you can ask me for more details
https://www.intersil.com/data/Xicor/an177.pdf
in above circuit you will only need is a circuit to convert you PH output into 0-2.5V voltage
which can be done by a simple addres/amplifier circuit (sorry I dont have time to draw it now, you try finding it on net if you are not able to find then I will try to make one and send you

Found one more link
**broken link removed**

Best of luck

 

[foppa]

I am also going to build a PLC controlled Ph controller. I was wondring how it worked for you. I work in the water treatment industry and can answer any water quality questions you have.
David

Did you build the build PH Transmitter.
If you build the PH Transmitter can I ask for the drawing.
I allso want to controll ph in my aquarium .
I use plc and termina "mac 90" https://www.saltvattensguiden.se/forumet/showthread.php?t=7430

john