I'm going to need a valve to control water flow. I assume the plants will be watered once or at most twice a day if they're thirsty. I've looked around a bit and found quite some different valve implementations in the hobbyist community.
Here are my requirements:
- Must work in low-pressure, gravity-feed system (elevated bucket)
- Must be cheap, uhm, affordable
- Must not use a lot of power
- Must not require mains voltage
- Should default to closed as a fail-safe
The nice folks at 43oh had some good suggestions and I found some more on instructables. Let's get started:
Hose twist & pinch
Grab a garden hose and either twist or pinch it very hard and the water will stop flowing. There are valves based on this category: just throw in a servo and you're good.
Here's a nice twist valve:
The downside is that it will require constant effort, i.e. power consumption, by the servo. I'm looking for a normally-closed system or at least for a valve which holds its state without power consumption.
This pinch valve is a bit better suited for my needs as stays open or closed without power consumption. It was even featured on hackaday!
I'm not quite sure why there are stop switches in there, but perhaps the servo is not accurate enough. In any case, this is very cool, but looks a bit too complicated for my tasted. And by complicated, I mean I can put less work in and get similar results ;). A common theme with those twist or pinch valves is the mechanical stress on the hose. Some hoses will become stiff and brittle very quickly. A silicone hose is probably a good idea. Commercial pinch valves are available (and pricey), so someone probably solved that problem.
Magnetic valves are often found in washing machines. I could pull one from a washing machine, but these require mains voltage and often require some water pressure to operate correctly. There are variants on eBay which use 12V and do not require water pressure to operate. They're also very affordable at around 5-10€ (1, 2). The main drawback here is that they use a lot of power. The eBay sellers do not list power consumption, but I've read somewhere else that they draw up to 300mA at 12V. That is a lot! If I run the valve for 10 minutes per day, my eneloops would be empty in 40 days. Of course, I could run the irrigation system with more throughput and shorten the valve open time. But it just doesn't seem right.
A big advantage of magnetic valves, however, is their normally-closed state. If the power fails, the valve reverts to a closed state. You can also buy normally-open valves, but that's not terribly useful for me.
In summary, magnetic valves are quite good, but the high current draw just puts me off.
Build Your Own Valve
I'm including "The Plant Buddy" more for the novelty factor. Of course, it works well, but it's not terribly practical for me. I'd like to hide the valve and electronics away in an enclosure somewhere. Still, a great idea. Interesting content starts at around 0:45!
Servo-Controlled Ball Valve
Take a ball valve and connect a servo. These are available commercially, but very pricey! This video shows a nice DIY variant:
Sadly, these are not normally-closed. But the servo only uses power to turn the valve, which shouldn't take long at all. I found conflicting information on power consumption for servos, so I will just have to myself. I imagine the servo also draws power when idle, so I will hook it up to a relay or a mosfet to turn it off when not in use. Another plus: the servo runs on 6V, so I can just use four Eneloop AA cells.
If I use a small valve and a small servo, then the power savings should be even greater. I assume that I will not need a massive 3/4" valve to water three plants. I did not find many suitable valves online. I will have to go to the hardware and find the smallest valve they have. Then I can select a servo based on that.