Since the Garden Squid is an open platform and designed to be tinkered with, I'm going to document some of the major design decisions. Hopefully this will help you understand the reasoning and allow you some insight when making your own changes.

Relays

Many of the automation projects involve relays. Relays are great. They allow you to switch a larger amount of current than you would do with a transistor, and do it at a more mechanical level. The one problem with relays is that they have a larger amount of current and voltage, which present extra design challenges from parts to PCB on up. Most devices are tested and certified by Underwriters Laboratories (UL) to be safe, especially when they plug into an AC outlet.

Automated gardens need to switch devices on and off, and many times these devices use AC wall voltages. The wattage for each device can be fairly low, for a small underwater pump (<50W), or it could be high (1000W) in the instances of a high wattage light.

People should not take lightly playing with AC current at any amount. When you take a large amount of devices, and wire them close together, you should be even more careful. Since water is involved in any garden, you need to be even more careful.

So let's recap the problems with relays:

1. Need to be aware of design constraints for AC power. This really raises the bar for who can build the project, and I would never want anyone to risk life or limb for it. Safety is key.

2. Need to put all the relays in a centralized, protected location. Centralizing a large amount of current is inherently dangerous, and possibly tricky given house wiring with 15A breakers.

X10

Now let's talk about X10. X10 is basically a wireless relay in a small box. You plug it into your wall outlet, and then plug your device into it. The relay inside is controlled by signals sent over the house wiring.

X10 modules are freely available and have been around for 20+ years. The protocol is well known and understood, and the modules can be picked up on the cheap if you're on the look out for deals.

X10 modules are tested by UL, and I have only had one fail on me in many years of use. This was because I was likely switching a larger current than it was rated for. X10 modules are well rated for current and wattage, and are even available in 240V models for high power lighting.

The wireless nature is great because you don't need to run extension cords from a central location, and can simply plug in the module where it makes sense.

The CM19A is a USB wireless X10 transciever. You can plug it into your raspberry pi and with the mochad server (https://sourceforge.net/projects/mochad/) you can easily shell script or automate turning on and off X10 devices.

For makers, this is great, because it reduces the risk of fire or electric shock because there is no AC current in the project, which means even a beginner can have some fun, do something amazing, and not risk a fire from bad soldering.