If you have a DC-powered OpenSprinkler, it can drive 5VDC relays or 12VDC relays — for 5VDC relays the default power adapter should work fine, for 12VDC relays, you should change to a 12VDC power adapter (the controller itself can accept any input voltage from 5VDC to 12VDC). The impulse high voltage is generally fine for relays because it only lasts for a couple hundred milliseconds and won’t damage the relays. Also, you can turn off the boosting time (in Edit Options -> Advanced, change Boosting time to 0) to avoid the impulse voltage.
If you have an AC-powered OpenSprinkler, it can drive 24VAC relays.