Development of a sunlight digitizer applied to an ultra low power infrared sensor for crop applications

The population driven need for food in the world will increase by 70% by 2050, while less land and natural resources such as water are available for farming. Therefore, increasing the efficiency of food production is incredibly important. Genetic studies suggest that the current production yield of crops has significant room to improve. State of the art technologies cannot be used to implement such a continuous monitoring of large-scale crop fields. Commercially available sensors continuously consume power to monitor the environment even when there is no relevant data to be detected which limits their lifetime and results in unsustainable costs of deployment and maintenance (i.e. batteries need to be replaced every few weeks). The ARPA-E team, the very same project of which this master thesis is part of, realized a new class of zero-power and low-cost sensors. They are capable of monitoring the water stress related infrared characteristics of plants (i.e. leaf temperature and reflectivity) in the crop field and communicating wirelessly with the irrigation system control center upon the detection of irrigation indicators. The starting sensor of this master thesis is the one by Qian et al., with some major contributed changes. Nevertheless, this new MEMS is still not able itself to be exploited in in-field applications. It needs a new system, the so called sunlight digitizer, to verify that the sensor is communicating a need-of-water because the plants actually requires water and not only because the irradiance of a day is too high. This last phenomenon would cause the reflectance of the leaf to be over the threshold, activating the sensor to send a need-of-water signal which is in reality a false alarm. Within this project such a digitizer is designed and simulated with LTSpice. Firstly, the simulations of the circuit show positive feedbacks. Secondly, a different schematic is simulated for the optimization of the solar cell. Then, the circuit is built and tested in the form of a breadboard. The use of a millimetric solar cell to supply the whole circuit is to keep on the philosophy of this project, the ultra-low power feature. The final results lead to the realization of a 1% irradiance window digitizer, i.e. the circuit provides a within signal set to high only when the irradiance of the sun is from 220 to 222 W/m^2. This range is modifiable according to the sunlight conditions, i.e. to the average irradiance in which the digitizer is used. This can be easily done turning the knob of a potentiometer.