Wireless sensor networks are collection of autonomous devices comprising of sensors to sense the effects of temperature, pressure or pollutions in our environment at different locations in real time. These devices may be termed as sensor nodes that have the job of computations, sensing and communication. The network lifetime of these devices is limited by the power issues due to the use of batteries. With this paper we bring out the design and implementation with the help of centralised group aware duty cycle protocol for wireless sensor networks. In this arrangement the nodes form a set with already fixed diameter. One node like a sentry out of the group remains active while others go to sleep mode. After some time somebody else takes the responsibility of this sentry and the current sentry goes to sleep mode. This is like the duty cycle of sentry which is followed by all the nodes in the group one by one. Thus energy efficiency and fault tolerance is achieved. The group is transparent to the network except that active node.