If you are like me, you appreciate smart things. Smart lights, smart thermostats, smart speakers, etc. One problem that can really use a smart solution is home sprinkler systems.

Traditionally, home sprinklers were programmed to come on at the same time and turn off at the same time. Maybe you could set different schedules for each day of the week. Well, that’s great. Sometimes you have a string of cool weather and your soil will stay moist and need less water. Sometimes you will have a lot of sun and heat and require more. Sometimes it rains and you don’t need to water at all.

There is one company in particular that makes an excellent off-the-shelf smart sprinkler, Rachio. There are some advantages to buying a product (like it’s a lot easier.) But building your own will be cheaper and a lot more customizable.

How do sprinkler controllers work?

Your sprinkler controller at home is a very simple electro-mechanical device. It relies on a power supply, relays, and solenoids.

Red represents power, blue represents water.

Nearly all sprinkler systems will work off of a 24vAC power supply. Some might go as high as 30v, but the important point to notice here is that this is AC. Almost everything else in your life with a transformer plug these days (some kind of box between the outlet and your device) is converting your wall power (110vAC in the USA) to a lower voltage DC. I don’t want to explain the distinction between the types of power, but let us just say that they are very different.

That power supply goes to a box which accepts input for scheduling and contains a series of relays (1 for each zone in your system.)

Each relay takes a signal wire and a power wire. The type of current and voltage you can supply vary depending on the relay board. For this project, you just need to get one that can handle 24vAC.

Watch this video on how relays work.

Relay board wired up.

The right-most terminal on each relay is for power. These are chained together from one power supply source.