Sonoff is an ESP8266 based on/off switch that will let you control electrical devices around your home. Out of the box it can be only managed by the eWeLink app which is running on Chinese servers. If you want to take full control of the device you can flash the firmware and access it inside your local network.
In addition to the WeMos battery powered temperature server, it would be nice to know what the remaining level of the battery is, so we can recharge it in time. This is easily done using a voltage divider circuit and the analog input on the WeMos D1 mini module.
Blink LED is a Hello World! application for microcontrollers. If you manage to get it running, you can assume that you are all setup and ready for further developments on your board. In this blog item I will cover how to do that on a STM32F407 Discovery board.