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Moving right along, it is time to test the pH circuit.  The pH circuit is in the pH.sch kicad file (GitHub repository).  

Virtual Ground

The first thing I tested was the voltage divider used to provide a .45 VGND to the op amp so that a single source power source can be used.

VGNDinpHSchematic

Easy-peasy.  Soldered on the two resistors…measured on TP18.  Read .45V.  YIPPEE…

pH Analog Signal

Now on to the heart of this circuit.  Getting a reading from a pH probe.  Besides the op amp, I soldered on the  8 position terminal block (data sheet).  As shown on the HealthypH kicad schematic, the pH probe uses the first two pins of the terminal block

 

TBForpHProbe

Minor Change – Enlarge Terminal Block Pins

I’m going to change the footprint of the terminal blocks such that the through holes allow the terminal blocks to be smoothly placed versus the snug fit which made me need to apply a bit of force to get the terminal block flush to the PCB.  It should take much – it just needs a little wiggle room.

Reading the pH

I simulated a probe by using the AWG feature of the Gabotronics XMiniLab.  While I use a Rigol scope as my main scope, I am very glad to have the XMinilab.  It is not only great for on-the-go but the AWG feature has come in spectacularly handy.  I fed in a sine way of roughly +/-.42V:

AWG XMini Lab

 

 

The test points I looked at and then took readings at test points 17 and 1 relative to TP 18 (VGND).  Here is an image of the test points on the pH schematic:

pHCircuitSchematicOpAmp

And..YIPPEE! The scope saw a sine wave with a Vpp of 848mV.  This is very close to the AWG sine wave I use as input into pins 1 and 2 of the terminal block.

 

pH Test pins 17 and 18

  Probe in Test Pin 17 Relative to Test Pin 18

pH TestPins 1 and 18  Probe in Test Pin 1 Relative to Test Pin 18

THAT was TOTALLY exciting. Wow, it seems to be working.  I’ll come back to this and look at it more closely.  For now, I’m going to journey on to circuits not yet explored…

I was surprised with how quickly I was able to get this part of the Healthy pH Shield working.  On the one hand, the circuit is simple.  On the other hand, I might be missing something.

What’s Next

Time to enter the place where analog and digital collide or cross – depending on your point of view.  On to digital access through the MCP3901 ADC and the Arduino.

 

 

 

Thank you for reading this far.  Please find many things to smile about.

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