Building a pH meter circuit - is it feasible?

Question

Let's say I have a budget of $15-20 to build a pH meter. I will then read the analog values from a microcontroller or such, but I want to first focus on somehow getting a reliable pH value on a shoestring budget.

First of all, is this even possible for an electronics hobbyist? I see existing pH meters out there but they are quite expensive.

There are commercial options running into the hundreds of dollars to cheap ones like these: http://www.sparkyswidgets.com/product/leophi which still require an external probe.

Ideally I want to build a pH meter circuit that has electrodes that simply dip into water to measure its pH.

I also found this: http://overskill.alexshu.com/cheap-ph-meter-hack-for-arduino/ but it's based on hacking an existing pH meter. I'd first like to hear opinions on whether a complete DIY sensor circuit is possible to do.

Answer 1

This is definitely possible. You can start with a cheap pH probe like this (available for $6 on AliExpress): You'll need a female BNC connector to attach the probe to, such as the Molex 0731000105 (available for $1.48 from DigiKey). Finally, you'll need a very-low-input-bias-current op-amp, with the LMC662CN from TI an excellent candidate (available for $1.56 from DigiKey), plus a few passives.

Assuming that the microcontroller board you plan on using has a 5V supply available, you can use it to power the op-amp and generate a reference voltage, which will have to be about 1.2 V because of the common-mode range of the op-amp. The final setup will look something like this:

^{simulate this circuit – Schematic created using CircuitLab}

The resistor divider generates the reference voltage, and the op-amp acts as a unity gain buffer. This means that your output will only be the reference plus or minus about 0.4 volts over the whole pH range, so you'll need to either add some gain or use an ADC with decent resolution.

Answer 2

To measure the pH value, a probe with glass isolation is used. Even that cheap hacked pH pen meter has that. That probe is not something you can make yourself. There are however plant testers (often called 3-in-1 plant Ph soil meters) that use two different metals to measure the pH value. That is not very reliable.

Do you have solutions with different pH values ? Then you could try such a 3-in-1 plant soil meter to see if it is useful for you. The resulting measured pH value depends on the nutrients and salts in the water.

You did not tell what kind of water you want to test. When it is for an aquarium, you might need a galvanic isolated pH module with a good pH probe.

That hack of a cheap yellow pH pen is very nice, but not easy. There are now even cheaper pens with metal pins instead of the glass isolated probe. I don't know if they are any good. As far as I know there is at the moment not a cheap module with a glass isolated probe and a interface for a microcontroller. If you want a reliable pH value, you have to pay for it.

Measuring, for example, temperature and baromic pressure can be done with very reliable and cheap sensors. Maybe you have to wait a few years for a good and cheap pH sensor module.

Answer 3

I did almost exactly what is described here for my swimming pool, same sensor, similar O/P amp (LMC6482) and same approach of referencing the probe to +1.250v. It does not work very well. The pH reading changes when the pump starts. And I suspect that the system ground is probably in contact with the water somewhere. I would recommend grounding the probe and powering the O/P amp with +5v and -5v generated by an LM2662. If you use the dual O/P amp LMC6482, you can use the second O/P amp to level shift and amplify.