This paper proposes an ultra-low power sensor network system for energy harvesting environments. The system is composed of two types of sensor nodes: master nodes, which can acquire relatively sufficient energy from batteries or mains, form the backbone of the entire system; and slave nodes, which are powered by using energy harvesting technology, are scattered around the master nodes. To overcome the limitation of harvestable energy, a slave node is usually turned off and is activated when a master node sends a request signal which contains the ID of the slave node. To verify the energy effectiveness of the proposed architecture and techniques, a simple pilot system is implemented and tested. The experimental results show that the proposed techniques drastically reduce energy consumption of the sensor nodes.