The Wireless Sensor Networks (WSN) have distributed and embedded nature and they are characterized by strong interactions between their hardware and software parts, therefore, their specification is a complex task. Formal methods need strong skills in mathematics in order to model and analyze WSN applications. However, semi-formal methods are simple and easy to understand. Hence, most of approaches use semi-formal methods to design systems and generally simulation to validate their properties in order to produce models without errors and conform to the system specifications. In this paper we describe our contribution, based on a case study that models a WSN system used to measure the flow of people in town. We have used System Modeling Language (SysML) to design the complete system. Besides, we have verified and validated some wireless sensors properties of the studied system and the results are encouraging.