One of the problems I'm currently facing is how I'm going to go about setting up the watering system and the heat exchanger using commercially available building supplies.

This means I have access to copper pipe, plumbing supplies and the like.

I have also ordered about 10 low voltage submersible mini-aquarium pumps. Here's a Link to the Aliexpress page I ordered them from.

The general idea is that I will be using several voltage relay's or MOSFETS triggered by the ESP8266 microcontroller to supply power to the pumps.

Now, the problem I have here is how to go about designing the heat exchanger. I can either take thin diameter copper tubing and run it through the existing plastic planter trays or run it in coils around the different soil sizes.

I'm thinking of doing for the heat exchanger what I'm doing for the moisture sensors. Once the soil moisture sensors arrive, I will be testing them against each other to finding out which design emerges victorious.

Therefore, I will likely setup two different systems. If anyone has better practical engineering suggestions, I would be happy to hear them.

The first approach involves setting up a copper tube in a grid horizontally along the proposed aluminum aircraft wire prototype. Essentially, the copper tube will run inside and in between the planting tray and the plastic bottom. This would hopefully provide the most direct passive heating or cooling for the soil from the existing room temperature.

I cannot understand how important this is, as my initial tests have revealed that the temperatures on my window sill reach upwards of 35 degrees celsius. (This kills the lettuce and presumably it will do the same with other leafy vegetables.)

The second approach involves circulating water openly into a horizontally hung container and placing the commercially available planter trays inside that. However, this has a downside in that the tray itself would be airgapped from the water which means it's effectiveness as a heat exchanger is dubious.

So, I'm thinking of setting up two small scale experiments to test this theory for each tray singularly.