Wireless sensor networks are being used widely for various security and surveillance applications. To be used in a more practical scenario, sensor networks support human decisions by preliminarily analyzing and interpreting data. The main design goal of a sensor network is to minimize power consumption. This paper proposes a two-level network to assist a human operator so that he/she is needed only to confirm the system's decision whether a detected intruder is a potential danger or not. At the first level, relatively simple nodes with basic sensing devices are used as primary detectors of intrusions. The second-level sensor node is composed of a high performance FPGA that can be dynamically reconfigured for different applications, interfaced with the camera. Thus the proposed technique reduces redundant transmission, consumes less power and bandwidth.