The easiest way is to use a dummy zone that activates the master station. If you have OpenSprinkler 2.3 (AC/DC) or OSPi, and using the latest firmware (2.1.6), it supports up to 56 stations. You don’t actually need the expansion boards to enable all 56 — you can go to Edit Options and enable all 56 even if you don’t have expansion boards. It’s software defined. Of course if you don’t have the actual expansion boards, those extra stations won’t work physically, but you can still use them as dummy zones, or as virtual stations (such as RF and remote stations).