The goal of this project is to use hardware/software to automatically read the pH and EC values of a nutrient bath.  Periodically, the probes reading the values will be automatically calibrated.

Probes and Solutions


The probes include:

  • a pH probe.  I will use the Extech 601500 Standard pH Electrode.  While I’ve tried a few probes over the past couple of years, I don’t have a feel if one is “better” than the other.  I chose this probe because I have been happy with other Extech tools and it seemed reasonably priced at just south of $40.
  • an EC probe.  I will use the probe I have used in the past.  This post serves as a good reference to the probe I will be using.


The solutions needed include:

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The container holding nutes will be used by both probes.  The container holding cleaner, pH 4, and pH 7 solution will be used by the pH probe.  The container holding the 1413µS/cm solution will be used to calibrate the EC probe.   The containers don’t have to be test tubes.  I’ll figure out what these are as the design becomes better known.

Probe Positioning

Probes need to move to a container and then insert themselves into the solution.  This requires back-and-forth movement along the x-axis along with up-and-down movement along the z-axis.

X-Axis Movement

X-Axis movement will be achieved using a similar method to the used by Adafruit for the sliding camera mount:



I’ll be using one stepper motor for X-axis movement.

Z-Axis Movement

The probes must move down into the solution with a container and then move up out of the container.  Since there is no Y-axis movement, the bottom of the probe must be high enough to avoid colliding with a container when it moves along the X-axis to align itself before moving down into a container.

Up to this point, I’ve decided on the L12-R Actuonix linear actuator.  My thought is the 50mm stroke should be enough.  The actual stroke required will become more clear when the containers are designed.

Filling and Emptying the Containers

Each of the five containers need to be filled before the probe is inserted and then emptied when the probe is removed.  


Except for the container holding the nutrient bath, containers will have a bottle filled with the appropriate solution positioned above with a feeder that puts the solution into the container.  Gravity will allow the flow of the solution into the container.  The flow of the solution is managed by a pinch valve similar to the one shown in this video:


The bottom of each container will be attached to a solenoid valve.  The solenoid valve for the container that has a probe inserted will open when the probe is removed.  The valve will remain open for enough time to empty the contents.

Tomorrow I plan to start prototyping.