We built another prototype with the NodeMCU board and ordered ESP8266 (ESP-12F) chips to built a prototype with a standalone device.
After receiving the ESP-12 chip I soldered it to a adapter to make it more breadbroad friendly. The ESP-12 needs a regulated 3.3 V source. We used a breadboard power supply at the beginning.
To write to the chip GPIO 15 and GPIO 0 have to be connected to GND.
The tricky part was connecting the analog output hygrometer to the analog input of the ESP-12 board. The analog out provides a 0 to 3.3v current (when getting 100 to 0 % moisture respectively). The analog input of the ESP8266 can read between 0 - 1V so we need a voltage divider to get the analog output to give out a current with this range..