I want to get going on the schematic of the Healthy EC Shield while I wait to get fabricated PCBs of the Healthy pH Shield from OshPark. The job of the Healthy EC Shield is to read the nutrients in the bath and determine if more nutrients are needed. If so, pump in the amount of nutrients needed. Schematics are located in this GitHub repository. CAUTION – THE DESIGN and LAYOUT ARE NOT FINISHED.
The goal is similar to the goal when I posted the design of the Dev-Rev 2 of the Healthy pH Shield (link). I will walk through the Healthy EC Shield’s circuits.
Thanks to Those That Go Before
I can’t help smiling when I think about how amazing it is to be able to learn something each day. What has been occurring recently has been like a zap to my head! Just when I thought I understand something, Chris Gammell or Ryan (SparkysWidgets) – maker of many very cool boards – like the minipH and miniEC) kindly nudge what I think I know into the right direction of how something (in this case an EC Circuit) actually works. Both Chris and Ryan have been instrumental in my understanding of an EC Circuit and electronics. I highly recommend the offerings from both!
I am calling this build of the Healthy EC Shield Dev-Rev1. The (kicad) files for the schematic are located at this GitHub repository.
The design shares design choices with the Healthy pH Shield. They both
- use the MCP3901 as the ADC. This way, the same firmware functions will be needed and I there will be twice as many needed which increases my chances of buying at the less expensive bulk rate.
- use the same design for pumps. There is only one pump circuit on the Healthy EC Shield to pump nutrients.
The Sub Circuits
As with the Healthy pH Shield, I use Kicad’s hierarchical schematic feature to logically represent the sub circuits. I’ll only detail the EC circuit since the others are pretty much the same as found on the Healthy pH Shield (link).
The key thing to know about the EC circuit is the EC probe acts as a variable resistor within the gain loop segment of the EC Circuit. I’ve walked through this circuit several times (and every time I do – just when I think I understand it, there is something I don’t understand! Fascinating) (link).
The EC circuit schematic explains what EC_GND and the AC power source as input into the non-inverting input pin of the op amp (link in GitHub). I’ll include the “important snippets” below. But it is perhaps best to look at the schematic to get a better feel for the circuit.
EC_GND is a virtual ground so that a single (5V) power supply can be used for the op amp rails:
The AC signal is generated by shrinking down an AC Signal created by a Wien Bridge Oscillator:
Calculating the EC Value
I went through how to calculate the EC value in this post. Looking back at the picture I’ll redraw the R’s and V’s as:
- Vout = EC_Signal
- R1 = 3K
- R2 = 1/S = 1/EC
That is, this is part of a variable gain loop in which the only unknown is the value for R2. Once that is figured out, conductivity (EC) is the inverse of resistance.
Like I said, the explanation and example calculation are in this post.