The aim of the MeteoFox project was to design an autonomous weather station, using solar energy and Sigfox connectivity. The SPSWS is the main processing board of the device.
This first prototype is not optimized at all, the goal was to design a development board to evaluate multiples architectures, select components and check the main features required to design the whole system:
- Solar energy harvesting with a full hardware trigger (DC-DC converters selection).
- Discrete RF design (MCU + transceiver) and Sigfox library integration.
- GPS module integration.
- Weather sensors selection.
See SPSWS HW2.0 project for more details:
Below is the functional block diagram of the board.
This block was an hardware experimentation. The goal was to design a timer based on capacitor discharging.
Most of devices with sensors wake-up periodically to transmit data. Instead of using an internal timer of the MCU, the idea was to charge a large capacitor at a stable voltage, and then open it to start the discharge at a very low current. A mechanical relay was used instead of a transistor to avoid any leakage current. When the capacitor voltage fell below a given threshold, a signal was triggered to wake-up the MCU (wake-up pin of STM32 while in stantby mode). By measing the effective duration with GPS time acquisition (which is generally quick, contrary to position), the threshold voltage was dynamically adjusted with a digital potentiometer with NVM, to reach the required period.
However, capacitor discharging was very sensitive to temperature variation, so that the target duration was never really reached. Even on internal clock the internal RTC was much more precise and above all, much more energy efficient! But the design of the closed loop system was fun!