Powering automated watering project

Question

I am making my mother an automated watering system for her garden with my Raspberry Pi. I would like the system to be able to detect soil moisture, as well as predict temperature and humidity to determine when it is best to water.

There are several different locations in our yard where I could deploy the Pi, most of which present a challenge as far as powering the device. If I am going to have the Pi do the watering itself (instead of just notifying me when it's time to water), it needs to be connected to a relay which triggers a solenoid valve that allows water to flow for a certain amount of time.

If I want to run such a system in a part of my yard which is difficult to power (expect by running an extension cord the length of my lawn), are there feasible and relatively inexpensive solar panel solutions that would power a Pi Zero W performing such an application indefinitely? From my testing, there is a decent 2.4GHz wifi signal in all of the garden locations that I would like watered, so internet connection is not a problem.

Regardless of if solar power is feasible (I could limit my efforts to the locations for which wired power is doable), does using a relay to send power to a solenoid valve sound like the best way to do the actual watering?

Note: I currently have a Pi 4B which could be used as a central controller device, and I was imagining several Pi Zero W's, one for each area I would like to be watered.

Answer 1

I've done something similar & offer a few comments below. As far as your question is concerned, the only question I actually see in your post is:

Q: does using a relay to send power to a solenoid valve sound like the best way to do the actual watering?

A: "Best" depends on a number of things, but a relay is certainly a reasonable way to switch power to drive your solenoid valve(s). Most solenoid valves will operate from either 12 VDC or 24 VDC, so you'll need either a relay or a transistor. Following are some considerations:

1. A small (20W) solar panel and re-chargeable battery(ies) are feasible for a power source, but you they won't provide enough power to run your RPi 24x7. You'll need to either manually power down your system, or use a real-time-clock to turn it on and off.

2. Most relays use a fair amount of power. If run times are extensive, you should consider using a latching relay.

3. Solenoid valves vs motor-driven valves: Solenoid valves also use a fair amount of power. As an alternative, consider a motor-driven valve. There are some valves that are said to have clever design features and more efficient than traditional solenoid valves.

4. Detection of soil moisture: There are a variety of wireless soil moisture sensors. I've opted out of these in favor of a fixed watering schedule that I can adjust manually when abnormal weather patterns persist.

Hope this helps - and good luck!