pH sensor detection circuit design

Projects General Guidance
1

Hi all,

I'm trying to understand how pH detection circuits work, and (hopefully) how to build & integrate them myself. I'm also trying to understand why sensor boards are so expensive, the cheapest (the "Logo_PHsensor v1.1" which is discussed here before) still cost me about USD 12 equivalent when ordering directly from China. I was also surprised by the complexity of the circuit on that board, compared to much simpler circuits I find online.

Two things I'd like to discuss.
First of all, the schematics itself. pH- is the shield of the coax, pH+ is the core, the pH sensor is on the other end of the coax cable.

Logo_PHsensor_pH_circuit.jpg
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The schematic of the detection side, measuring the voltage on the pH sensor. This is beautifully simple: an OpAmp wired as basic -2 gain amplifier. Nothing about it, really. Voltage to detect in, analog signal out ready to be read by a microprocessor or external ADC.

Then the bias/offset. This is needed to lift the output voltage of the sensor so pH=7, 0 V on the sensor, is in the middle and negative voltages (pH > 7) can be measured as well. This starts with a TL431 precision reference diode, wired to simply provide a 2.5V reference voltage. A voltage divider (10k pot RV1 with 10k fixed resistor R1) then provide that voltage to the OpAmp U1B wired as unity gain OpAmp.

The pH board also includes a remarkably complex circuit for temperature measurement (three OpAmps plus resistors/capacitors where I am used to just a single pull-up resistor?!) - see attachment for complete circuit if interested. This doesn't seem to be connected at all to the pH circuit.

Back to the pH sensor.

Is it necessary to use an OpAmp for the bias? Is a voltage divider (based on the stable reference TL431) not enough for this? After all the probe takes near-zero current.

The bias circuit uses an expensive, high-impedance OpAmp. I don't see the need of it, it seems to me that they just use the part that's available, the TLC4502 is a dual OpAmp package and probably not much more expensive than the single OpAmp 4501 while saving board space.

Second, separately, I'd like comments on the OpAmp to use.

This board uses the TLC4502 OpAmp, a high-impedance CMOS OpAmp with a 1 pA bias current. Should be good enough for a pH sensor. the input impedance is given as 1012Ohm, compared to typical 107-109Ohm for the sensor.

The discussion linked to above suggests the MAX407 (post #33), an OpAmp with bias current of <0.1 pA. An order of magnitude better, but the OpAmp also seems to be almost double the price of the TLC4502 even though it's a quite old component.

Then there's the LMP7721 OpAmp, with a bias current of only 3 fA (0.003 pA)! Cost about the same as the MAX407, but better specifications (it's a much newer part as well).

Note: I didn't research pricing in depth, just a quick look at Taobao for the typical prices for those components. Good enough for this stage.

Logo_PHsensor.jpg
Logo_PHsensor.jpg2338×1653 336 KB

2

Hi,
I think you will find with the use of such components the high cost is due to.

READING STABILITY AND ACCURACY AND PRECISION AND CONSISTENCY AND THE R&D TO GET THERE.
READING STABILITY AND ACCURACY AND PRECISION AND CONSISTENCY AND THE R&D TO GET THERE.
READING STABILITY AND ACCURACY AND PRECISION AND CONSISTENCY AND THE R&D TO GET THERE.
READING STABILITY AND ACCURACY AND PRECISION AND CONSISTENCY AND THE R&D TO GET THERE.

To Industrial/Laboratory Standard.

Tom..... :slight_smile:

3

I was hoping for more serious reactions & comments. Also I'm not talking about the probe (a decent one costs a bit more than that), I'm talking about made-in-China amplifier boards. Not much if any R&D involved there. Take schematic from Google, select suitable components, build some prototypes to test it out, and it's ready for production.

The most expensive component, the TLC4502 OpAmp, is priced at around USD 3 in volume of a couple thousand units on Mouser. Based on what I gleaned from the schematics, that's the primary component of the circuit. Add some resistors, capacitors and a shunt (about USD 0.10) and a coax connector and you're there. Assembled it will be just over USD 4 altogether. Expecting to see prices of no more than USD 6-8 equivalent on Taobao or Aliexpress for that, likely less. Margins are that thin indeed, but those shops have little overhead and provide no support whatsoever (so they can sell Arduino Pro Micro clones for no more than about USD 1.50 - including the ATmega328 which by itself is listed at higher prices on Mouser for largest volumes).

More from Mouser: the MAX407 is going for just over USD 6 a piece (with hardly any volume discount).

The super-high impedance LMP7721 is selling on Mouser at about USD 3 a piece for a couple thousand, but it's a single OpAmp. That's all one needs for a single pH probe of course.

4

pls help.
i have this module ph-4502c, and need corect it to use opr probe, where voltage between -2000/+2000mV.
i thing .. lowgrade Vref and lowgrade gain ??

thaks

5

It's biased at +2.5V so you should get an output between +0.5 and +4.5V.

6

Sounds good. Walmart sells a pH pen that is all that, plus the combination electrode and an LCD display, battery holder, waterproof case, etc. for $8.50*.

Why are you making thousands of them?

*nvm, Walmart mislabeled a TDS meter as a pH meter. Amazon has this pH meter for thirteen dollars.

7

Pleease specifically

Now is Uref = 1,25V ? and I need Uref change to 2V ?
Now must change Enhanced OPAMP to out to 1.0 ?

Which resistor change an its value? R2 ?

R8 R9 - jumper ?

Sory - My electronic experience is low.

8

The pH sensor board you linked to is biased at +2.5V with unity gain on the OpAmp. It should work directly with your probe.

The circuit I designed last year uses different voltages, as it's designed around standard pH probes (so +/-700 mV), trying to make the most of the ADC range. The circuit works, I built it on a piece of protoboard but never continued working on it. I'm quite sure it'll work a lot better on a properly designed board as I had a really hard time soldering the PCB connector to it. The OpAmp comes in in SSOP8 package, which I soldered on a breakout board, that plugged into a DIP socket - also not exactly the most ideal way of handling that.

If you do not understand that circuit, there's little chance you can get it to work well, let alone make the modifications needed for your +/-2000 mV probe. You have to adjust the bias to 2.495V (so no voltage divider there), and change the output OpAmp circuit to unity gain.

For better sensing, I've played around (but never built) with redesigning the circuit to produce a 0-1V output, as this can be read using the more stable built-in reference. Another idea I had is to connect the 2.495V reference to AREF of the processor, and then bring the output voltage into that range. Both ways you can greatly stabilise the reading you get in the Arduino. Or have an ATtiny read the voltage and interface with that over I2C. That's my ultimate pH sensor, if only because that'll work with both 3.3V and 5V power.

Then there's just that issue of ground loops left to deal with...

9

thank you, but sorry one more

I'm thinking, that gain on the OpAmp (U1A) is 2.0, (divired on feedback 10k/10k) is too much -
(2,5V + 2000mV) x 2 - over 5V

I hope must change gain - removing divider R7/R6 -> direct connect pin 1-2 opamp.

4502C is original used with pH probe (+/- 500mV ?)
I want use orp probe (+/- 2000mV)

10

Indeed - unity gain.

11

thans a lot of