These are the PCB schematics for the weatherstation project. It is designed to work with an ESP-01 tuned for deep sleep. The circuit is powered by any 9V power source (6xAA works best). A linear voltage regulator (MCP1755S) breaks that 9V down to 3.3V. The MCU communicates through I2C with the on board voltage sensor (MCP3421) and an air quality sensor like the BME-280. Additionaly any other I2C device that runs on 3.3V may be connected through an extra pair of VCC, GND, SCL and SDA.
The implementation can be found here.