This work presents a novel algorithm to improve energy conservation in sensor networks. The algorithm is based upon Selective Activation of sensor nodes using Thresholding (SAT). The sensor continuously monitors the received signal and makes a binary decision to join the network if the average received signal power falls within the specified minimum and maximum threshold range. Thus, SAT divides all the receivers within the coverage range of the transmitter into sets of active and inactive nodes realizing a saving in power consumption proportional to the number of inactive nodes. The sensor life-time is enhanced by allowing the node to transmit at a power that is a constant fraction of the total residual energy. For two cases of linear and hexagonal networks considered in this work, it is shown that for each transmission, the fraction of inactive nodes may exceed by over 30% of the total number of nodes present within maximum one-hop distance of the transmitter. The cost associated with energy conservation through SAT is (a) Increased sensor node density or (b) Increased transmission power requirement.