An autonomous multisensor system powered by an energy harvester fabricated with a flat-panel solar thermoelectric generator with an ultralow-power management circuit is presented. The multisensor system was tested in an agricultural application, where every 15 min the values of the temperature, air humidity, and solar radiation have to be measured and stored in a mass memory device (a Secure Digital card), with their respective time stamp. The energy-harvesting switching dc-dc converter is based on a low-input-voltage commercial integrated circuit (LTC3108), which charges a 1.65-F supercapacitor up to 5.0 V. A novel ultralow-power management circuit was developed to replace the internal power management circuitry of the LTC3108, and using this circuit, the operation of the system when no energy can be harvested from the environment is extended from 136 h to more than 266 h. The solar thermoelectric generator used for the energy harvesting is composed of a bismuth telluride thermoelectric generator with a 110-mV/°C Seebeck coefficient sandwiched between a 40 cm × 40 cm anodized aluminum flat panel and an aluminum heatsink. On a sunny winter day in the southern hemisphere (12 August 2014, at Campinas, SP-Brazil, Latitude: 22°54'), the energy supplied by the harvesting system to the supercapacitor was 7 J.

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Reducing energy consumption is mandatory in self-powered sensor nodes of wireless sensor networks that obtain all their energy from the environment. In this direction, one first step to optimize the network is to accurately measure the total energy harvested, which will determine the power available for sensor consumption. We present here a technique based on an embedded circuit with an ultra-low-power microcontroller to accurately measure the efficiency of flat-panel solar thermoelectric generators operating with environmental temperature gradients. Experimental tests showed that when a voltage of 180 mV (best case in an environmental flat-panel solar thermoelectric generators) is applied to the input of the DC-DC converter, the proposed technique eliminates a measurement error of 33% when compared with the conventional single supercapacitor strategy.

A temperature-stable heat pulse driver for low-voltage single supply soil moisture sensors based on junction transistors is presented. The temperature drift of the heat pulse, caused by the variations dV BE /dT in the transistors placed in the feedback loop, is fully compensated for. A circuit measures the variations dV BE /dT of the transistors in the feedback loop and generates a current which is proportional to the − (1/R0) dV BE /dT that, when passed through another resistor inside the feedback loop (R 1 = R 0), provides full temperature compensation. The topology of the circuit also provides an output voltage (the voltage used to define the heat pulse) which does not need to be calibrated, since it depends only on a reference voltage external to the feedback loop.