The sensor PCB uses a split core current transformer (CT) to sense the AC current flowing through a mains cable, a typical CT would be the SCT-013-000 (Current Type) capable of measuring up to 100A available on eBay. The current transformers output is applied to a burden resistor (22 ohm) which is biased at around half of the ESP8266's supply voltage (3.3V). The ESP8266's own A/D is then used to measure voltage developed across this burned resistor. The A/D used in the ESP8266 has an input voltage range of 0-1V but the Wemos adds a resistor divider to allow for a 0-3.3V input range to the Wemos board.
Included are the PCB files with schematic, a simple 3D printable case and some sample firmware for the ESP8266.
Using the ESP8266's own A/D (10-bit) to measure the output of the current transformer, this version measure the peak values of the mains sine wave then calculating the RMS voltage value.
This is based of the EmonLib https://github.com/openenergymonitor/EmonLib library. This works different from version 1 and as some digital filtering which improves resolution at lower power levels.