We propose a power efficient system architecture that exploits the characteristics of sensor networks in order to decrease the power consumption in the network. The primary characteristic of sensor networks is that the destination of all the data packets in the network is a central data collector and this central data collector, which is usually denoted as access point(AP), has unlimited transmission power and energy whereas the sensor nodes have one battery energy to remain alive for several years. Our system uses the AP to directly synchronize and explicitly schedule the nodes? transmissions over Time Division Multiple Access(TDMA) time slots. Simulations performed in TOSSIM, a simulation environment for TinyOS, show that the battery lifetime of the network with this scheme can be increased to 1-2 years from 10 days that can be obtained from a general random access network.