1-cm3 event-driven wireless sensor nodes

This paper describes a 1-cm³ event-driven wireless sensor node (WSN) consisting of a solar battery with a capacitor, vibration sensor, wireless circuit, and antenna. The WSN transmits data using energy in the millimeter-size capacitor charged by the solar battery. The concept of the wireless transmission scheme is to not only reduce 1-bit energy while transmitting data continuously but also to reduce all energy of “one-time data transmission” from circuit start-up to the end of data transmission. We have already fabricated and evaluated a wireless chip that uses ring oscillators as clocks and transmits OOK pulse trains. In this paper, we show that these architectures are suitable for reducing energy of “one-time data transmission” and for WSN operation by limited energy because they can reduce the start-up time and activated time in 1-bit symbol of RF circuits. The fabricated sensor node can transmit 36-bit data 3 m by charged energy of 8 μJ in the capacitor when the sensor detects the event. This enables us to achieve 1-cm³ WSN and paves the way to the future millimeter-size battery less WSNs.