Sunday, August 30, 2009

pHduino 0.2 released

A new pHduino hardware version was released. The problem about sensor voltage offset compensantion was resolved. See the changelog for the changes.

Changelog - Hardware version 0.2
  • Fixed the problem of the offset signal amplification before the compensation.
  • Increase the value of the resistors to reduce the electric current consume (Dr. do Lago).
  • Changed the charge pump inverter capacitors from 1000uFx6.3V to 100uFx25V.
  • Removed zener diode and trimpot used to adjusts the voltage reference by a resistor of fixed value.
  • Added a ADC channel to reads the reference voltage.

Friday, August 21, before to get sleep, I had an idea to resolve the offset signal compensation problem with gain. On Saturday and on Sunday, I changed the PCB layout and did a clean-up on the firmware code.

On Monday, 24, I changed the electronic circuit of the first version to the new circuit. Dr. do Lago helped me selecting the right value of the resistors to reduce the current consume.

On Wednesday, 26, my friend Mr. Vidal, realized a comparative test between the pHduino and a pHmeter from a commercial brand using a titration of HCl with NaOH in a didactic laboratory. He said me the pHduino worked fine. The results I will show soon.

On Friday, 28, I made a PCB with the new layout, on Saturday, 29, I bought more electronic components and I mounted the circuit at my home toolshop.

Today, Monday, 31, I took some new shots of the circuit mounted.

The solution

The basic idea to resolved the main problem is apply an adjustable electric potential, using a trimpot, at the place of the voltage divider ground of the non-inverter amplifier (Stage1). Dr. do Lago deduced for me the equation for the Stage1.

I know that this point does not have a low impedance. I should put a follower voltage. However, the current that flows by the R1 and R2 is about 100 times lesser than the current that flow by the voltage divider (defined by R3, R4, and R5) that applies the voltage offset at R1. This circuit is suitable for this application.

pHduino Project


  1. hola amigo buen proyecto....esta placa es compatible con la arduino mega? y donde puedo conseguir el sensor ph? puede ser cualquier sonda de ph?

  2. Yes, this shield is compatible with Arduino Mega, Serial Single Side, Arduino Diecimila or Duemilanove. Read the last paragraph from Arduino Mega page.

    About pH sensor, there are different kinds of glass electrodes. It will depend of your application. To buy one, you need search for sellers from your country and select one brand.

    A glass electrode is not like a temperature sensor. You need take some special cares. I am writing some documents at the wiki page from the pHduino project (FAQ, HOWTO, etc).

  3. gracias amigo....aqui en venezuela no venden estos electrodos.. pero consegui estos en ebay

    me imagino q ue pueden servir!!!!

  4. There is a point: I don´t know if this pH electrode has a reference electrode, i.e. it is a combined glass electrode (glass electrode and reference electrode together). It is not clear in the technical description. Modern electrodes are combined. I thinks it is, but I am not sure.

  5. if some of these electrodes friend saw, come with uan reference solution to calibrate like this ...

    apologize for any inconvenience the project I think I want to adapt interezante my cichlid fish aquarium ...

    pd on this link is the result of electrode eBay Search


  6. When I said about reference electrode, I mean about a second electrode integrated with the pH electrode. It is common to say that this is a combined glass electrode. This second one is important because it is the electrochemical reference for the glass electrode.

    About the calibration, you need some solutions (two or three) with the pH very well defined to calibrate the response of your pH-meter with a glass electrode. These kind of solutions are called buffer solutions. You can buy them separated. You can buy the bottles of buffer solutions ready to use, small sachets with the solution inside (open it and put the electrode inside), or sachets with the powdered salt (open and dissolve it in a volume of water).

    Important! The buffer solution has a time of life and you need maintain it in a refrigerated area (don't mix with food!)

    I will try write some documents to put these topics in an easy way in the project wiki pages. I suggest you enter the group of discussion to store there all these important points, at the Google Groups.

  7. Hi, I am just interested in the ph metering (not the ucontroller). Correct me if I am wrong.

    To calibrate the circuit I need to
    i. put probe in ph7 solution and adjust R3(offset) so that Vsense is 0V
    ii. put probe in ph4 solution and adjust R2(gain) so that Vsense is 1V
    Then use some linear maths to get the ph of the tested solution.Is this correct?

    If I am to test using just a digital meter, all i need is stage1?

  8. This shield is compatible with board Arduino UNO?

    I'm a new programer Arduino


  9. Yes. The pHduino shield works with 5V.