Tags

,

In a previous post (link) I had quite the D’OH moment.  I had incorrectly designed the Wien Bridge filter.  That mistake must be fixed along with others.

The Goal

The goal of this post is to describe changes from the Healthy EC Dev-Rev1 PCB so that the accuracy of the EC measurements can be tested.  It is also expected the pump circuit will pass a test of turning a pump on and off.

Non Goal

The Healthy EC Shield is dependent on the Healthy pH to provide AGND power and access to a temperature reading.  While the temperature reading is extremely important to adjusting the EC measurement, I can determine the temperature of the bath through another method.  I will use my bench power supply instead of using a power source coming from the Healthy pH Shield.  By keeping the PCBs separate at this stage, the complexity of testing is decreased.  Certainly, I will need to do extensive testing with the two shields working together once they both evolve to a state where core functionality is stable.

Thanks to Those That Went Before

Thanks to Chris Gammell for his exceptional Contextual Electronics courses and his in depth sharing of his knowledge.  Thanks to Chris, I am confident I will be able to get the Shields working.  A year ago, I had no idea how to create a schematic, much less layout a PCB or solder SMTs…While each day is a new opportunity to astound me with how little I understand, unraveling the basics of electronics so that I can grasp key concepts and skills is something Chris has been instrumental in making happen.

Thanks to Ryan of Sparky’s Widgets for his open source minieC EC measurement breakout board, his documentation, and thoughtful answers/quick response to questions.  This effort stands on the shoulders of Ryan’s project.

The Design

Changes to the design for Dev-Rev2 of the Healthy EC Shield include:

  • fix the design of the Wien Bridge Oscillator.  I discussed what I need to fix in this post.
  • be able to test different resistor values for shrinking the AC Waveform generated by the Wien Bridge Oscillator.
  • be able to test different resistor values for the resistor that participates in the gain loop with the EC probe.

Shrink the AC Waveform

Shrinking an AC waveform is not the same as shrinking a family, but this image was the first thing that popped into my head. …

Honey I Shrunk The Kids

When I walked through the design of Ryan’s (Sparky’s Widgets) miniEC, one of the steps included shrinking the Vpp to ~ 200mA.  

ShrinkSignalWithPOT

 

Recall the EC probe reading contributes to a variable gain loop (discussed in this post) the Gain loop might take the Vpp beyond the limits of the power sources/rails of the op amp.  On top of that, VGND raises our relative GND to 2.5V.  This means the AC waveform exiting the op amp used to measure the EC cannot have a Vpp greater than 2.5V.  Just to be sure, the Vpp should probably be 2V or less.  If the AC waveform’s Vpp ~= 200mA, then a Gain of 10 is acceptable.  Any greater gain would cause the AC Waveform to be distorted.  This is a challenge for EC measurements that have a higher connectivity and thus lower resistance, such as a nutrient bath for tomatoes.  In plants that require a nutrient bath with a higher EC reading, the gain goes up to 15 – 20.

For Dev-Rev2, I use a scope to see how much distortion occurs given different EC values for nutrients.  If the range becomes too extreme to handle EC measurements for all varieties of vegetables, herbs, and fruits,  I will focus on growing really great lettuce.  The reasons:

  • when familiarizing myself with an experience such as getting “good enough” EC readings, minimizing variables is extremely helpful in focusing my head around the experience.  Herbs aren’t that much different than lettuce.  Tomatoes are tremendously different – in many ways – not just in EC but also lighting, size, temperature.  It is easier to grow lettuce hydroponically than tomatoes.  Although eventually I will climb that mountain.
  • Lettuce grows great hydroponically.  
  • Each kind of plant will require a different environmental setting, whether that be more light, different growth size, flowering, etc.  I want to evolve the Healthy Shields to be used within a hydroponic system that “just makes lettuce.”  This aligns with my general penchant for laziness.

For this Dev-Rev2, I will put a POT in the resistor responsible for shrinking the AC waveform.  I expect the resistance value to be somewhere less than 50K and more than 20K. Searching Digikey (link), I see this 50K POT should work  

50KPOT

 
 
This will allow me to experiment with different resistance values for the voltage divider with the goal of determining what is “the best” Vpp for the AC Waveform used as input into the non-inverting input of the op amp that measures the EC.

Adjust the Variable Gain

As noted in this post, the ideal EC range for lettuce is .8mS – 1.2mS.  Since R = 1/EC, R for the probe ranges from 1.25K to 830Ω

As shown in the image:

 

GetECMeasurementWithFixedRForVariableGain

the gain will vary based on the EC probe reading.  The gain can be adjusted by changing the resistance of the resistor inside the negative feedback loop.  I want to experiment what value for the resistance is “the best” for calculating the EC measurement by replacing the 3K resistor in the above image with a 5K POT (digikey link).

I’m off to layout the board.  I’m excited to see how much of what I learned about board layout finds its way into this iteration.  As I have said before, I find board layout to be…um..taxing…difficult…hard…

 

Thanks for reading this far.  Now – as always – please find many things to smile about.  

 

 

Advertisements