Plants instrumentation is a crucial issue due to the importance of sensors in allowing the observability and in increasing the redundancy and the reliability. However, the task of designing a sensor network becomes complicated when the complexity of the system increases. In this paper, a strategy is proposed to design a minimal sensor network ensuring the observability of complex systems. The strategy is based on the decomposition of complex systems into subsystems. This decomposition helps in treating each subsystem separately and in allowing the use of reduced order observers rather than a large scale observer for the whole system. An example is given to illustrate the proposed strategy.