Abstract: An adaptable controller for controlling variable numbers of switches is disclosed. Specifically the controller is useful in an irrigation system having a number of water sprinklers whose operation may be controlled by user-defined programs. The system has a processor unit, an interface unit and a number of add-on units. Switches for actuating electrically-operated devices such as sprinklers are located on the interface unit and the add-on units. The controller allows a user to enter operating parameters relating to the switches in the form of the user-defined programs. The processor is capable of ascertaining the number of switches on the interface unit and the add-on units and limiting the number of switches selected and controlled by the user-defined programs.