As I mentioned in my blog post and the video:
https://opensprinkler.com/raspberry-pi-zero-works-with-opensprinkler-pi/
the pre-configured SD card image was made using an older Raspbian image (obviously long before zero came out) which does not boot on RPi zero. We will need to update the SD card image soon. But in the meantime, it’s much easer to just install the firmware yourself: it’s just three lines of command:
https://opensprinkler.freshdesk.com/solution/articles/5000631599-installing-and-updating-the-unified-firmware#install