Wireless sensor networks are rapidly expanding due to advances in wireless technologies but also due to their multiple applications in different fields, such as monitoring processes in industrial environments, monitoring patient's vital signs in e-health applications or traffic monitoring. One fundamental aspect in the design of wireless sensor networks is the security of the communications established within the network. As in any wireless network, an open transmission medium is used, which opens door to attackers and a wide variety of threats. For this reason, it is essential to design mechanisms to protect the information against unauthorized access and verify the identity of the parties involved in a communication. On the other hand, nodes are limited devices in terms of processing, storage, etc. and therefore they do not support most traditional security mechanisms. In this work we have implemented and evaluated a wireless sensor network for presence detection. The network is composed of two nodes: a sensor node which detects movements and sends reports to a coordinator node, which in turn sends this information to an external server. The deployed system makes use of security mechanisms that provide confidentiality and authentication in the sensor network and the communication between router and server.