Well that was (NOT 🙂 ) exciting.  Why was I seeing this:

ACWaveForm on VGND NO probe attached

when the scope probe was in TP 18 relative to AGND?  Yuk…..

The Goal

The goal of this post is to become familiar with the characteristics of the all important VGND – used as the GND for the pH probe readings.  With a VGND, the op amp can use a single power source since all voltage readings will be positive.

Thanks to Those That Went Before

I just can’t thank Chris Gammell enough.  His Contextual Electronics courses and mentorship have given me the ability to rapidly grow my abilities in designing and building PCBs.  Something I find I have quite a passion for when applied to growing healthy food (through hydroponics).  I look back to last year…I am amazed and exceptionally grateful for all I have learned.  This would not have been possible without Chris.

Given my thirst for learning from those that went before, Ryan – of Sparky’s Widgets – is another whom I am hugely thankful for.  Ryan has made some wonderful breakout boards, like the minipH and minieC that I highly recommend.  The circuit design of the Healthy Shields are an evolution of the open source schematics and advice from Ryan.  He truly knows his stuff.  I am very grateful that Ryan open sourced the minipH and minieC hardware.  In addition, Ryan has been extremely thoughtful in response to what must be naive questions from me given his level of expertise!

Observations of VGND

As I noted earlier, I was seeing a strange sine-like waveform when the scope probe was in TP 18 relative to VGND.  Not good…

What was happening – and this took me awhile to figure out and get right (although I am not sure why!) – the probe’s GND was loosely connected AND I had too many external electrical doo-dads and wires running around, under, and above my Healthy pH Shield Dev-Rev2 I was testing.  I let out a sigh of relief after removing the excess electronics and wires, revisiting how the probe’s GND was connected to AGND and saw:

ACWaveForm on VGND  probe attached better

in this case, the Healthy pH Shield is inserted into the Arduino.

Healthy pH Shield With Probes

When the Healthy pH Shield is not plugged into the Arduino, there is (relatively much) less noise:

ACWaveForm on VGND  detached from Arduino


More noise occurs when the pH probe is connected through the BNC:



Lessons Learned

  • Observations are important but are only as good as the method used to measure.  In this case, I was not seating the scope probe correctly.  AND…I repeatedly was not doing this.  Certainly, slowing down, triple checking is very important when using tools and observing.  Sounds obvious, but something I need to constantly remind myself to do.
  • How well the BNC/probe is attached to the Healthy pH Shield will make a significant difference in the noise picked up on VGND.
  • A capacitor should be added close to the voltage dividers (R3 and R4) to smooth out the noise – especially since the Arduino adds additional noise – even when not plugged in.


Well – that’s that!


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