So, how does one actually use it? I get how to wire it, it’s the software control, that is unclear…
As Dan said, the OSPi software has a relay plugin that shows how to turn on/off relay.
The microcontroller-based OpenSprinkler has started supporting the relay from firmware 2.0.9 — it’s implemented by allowing a station to activate relay (similar to the way Master station is handled). Also, you can define a relay pulse value, which defines if the relay should be pulsed open/close, or should remain open as long as the associated station is open. Similar support will likely be integrated into the OSPi software in the future.
Can you clarify: flash the OS image to BBB? Which OS image? Also, since this post is under OpenSprinkler Pi (OSPi), please make sure you are NOT trying to flash OSPi image onto BBB because that obviously won’t work. Please use the OSBo image, the link of which is in OSBo user manual.