Positive feedback system can be found in everywhere, existed widely in creature contained from Lac operon of prokaryotes to human beings, and it has been used to construct various attractive circuits such bistable state, oscillators and other inconceivable systems. In our project this year, the significant performance of sensitivity, which researchers has not concerned to, will be developed in positive feedback loop established by LuxR/LuxI quorum sensing system. But this loop will be open because of its background expression. The AiiA, which can hydrolyze the AHL, will be control the background expression, together with the Lac operon being the system main switch. Furthermore, the input defined by user will have a negative correlation to the opening time. Hence, there need a model to describe this relation and even realize quantitative determination.