Self-Powered Soil Moisture Monitoring Sensor Using a Picoampere Quiescent Current Wake-Up Circuit

Low-power wireless stand-alone sensors that can operate without any wiring have received significant attention in sensor network applications. These devices harvest environmental energy resources to supply their power and transfer their collected data using wireless RF links. In many instances, the power supplied from the environment is far less than the power required by the sensor device. In this case, the main solution is to accumulate energy on a storage element, and when enough energy is stored, the sensor node is instantaneously activated using an undervoltage lockout (UVLO) circuit. During this short time burst, the sensor performs the required acquisition, transmits the results, and enters a sleep mode until again enough energy is collected. The quiescent current of the UVLO is the main factor that determines the minimum power level at which the sensor node is still operational. Most wake-up circuits used in conventional devices suffer from a quiescent current of a few hundred nanoamperes. In this article, using a zero-bias-current MOSFET-based approach, a new wake-up circuitry is presented, which lowers the quiescent current down to the picoampere range. As a practical application, the effectiveness of the proposed circuit is shown in the soil moisture monitoring sensor setup.