This paper presents the first step in the design of a hardware circuit implementing evolution inside membranes of a transition P system. The work presented here is part of a very ambitious project: to find and to implement a hardware system, as general as possible, able to simulate P systems evolution. Due to the complexity of this generic system, the process has been divided into several stages. Each one of these stages has concrete objectives. In particular, a circuit able to determine active rules in a determined configuration for the membrane is presented here. In order to proceed in an appropriate manner, it is needed to define a data structure containing information about the initial membrane state, that is, the initial multiset of objects, the set of evolution rules and the corresponding priority relation among them. The circuit takes these data entrances and then produce as output a set of evolution rules, active rules, which are able to produce the needed changes into the system in order to make evolve it to the next configuration.