Good choice!
A Raspberry PI is kind of oversized just to run Opensprinkler.
There is also another branch where I used a normal NodeMCU Lua Lolin V3 and a 8Ch Relay board:
https://github.com/ThFischer/OpenSprinkler-for-ESP12F_Relay_X4/tree/NODEMCU_74HC595/NodeMCU-74HC595#readme