Wire­less sen­sor devices are heav­i­ly duty-cycled to min­i­mize ener­gy con­sump­tion. To fur­ther reduce the ener­gy required for sens­ing, these devices often pre­fer pas­sive sen­sors that pro­duce out­put pow­er pro­por­tion­al to phys­i­cal quan­ti­ty. There­fore, while a sen­sor out­put is not being sam­pled (i.e., idle time), the out­put pow­er from the sen­sor is unused and, hence, wast­ed. Inspired by the obser­va­tion, we pro­pose a sub-thresh­old expo­nen­tial charge pump archi­tec­ture that works with an ultra-low capac­i­ty pow­er source (e.g., pas­sive sen­sor) that has an out­put volt­age and cur­rent as low as 250 mV and 6 μA. Uti­liz­ing the charge pump as an ener­gy har­vester to a sys­tem, we even­tu­al­ly design and imple­ment Sen­er­gy, a bat­tery­less wire­less sens­ing plat­form that uses a pho­to­di­ode sen­sor as a sole pow­er source. Specif­i­cal­ly, the Sen­er­gy pro­to­type reports light inten­si­ty through a 2.4 GHz radio when­ev­er it has har­vest­ed suf­fi­cient enough ener­gy from the pho­to­di­ode sen­sor, thus achiev­ing per­pet­u­al oper­a­tion. We use the pro­to­type to eval­u­ate the pro­posed sys­tem based on two appli­ca­tion sce­nar­ios, name­ly adap­tive trans­mis­sion of sen­sor data and pro­vid­ing an unin­ter­rupt­ed pow­er sup­ply to an on-board real-time clock.

• RESEARCH PUBLICATION