Defining control scenarios in a smart home is a difficult task for end users. In particular, one concern is that user-defined scenarios could lead to unsafe or undesired state of the system. To help them explore scenario specifications, we propose in this paper a system that enables specification of constraints restricting the control commands that can be used inside user-defined scenarios. The system is based on timed automata model checking abstracted by event condition action rules. A prototype was implemented, including a user interface to interact with the user. The usability of the system and interface was evaluated in a user study which results are reported here.