I just published the simple version of the data logger hardware and software. The simple version extends storage of the minimal version with external persistent RAM.
The additional storage provides space for more records and a higher precision for the records. Temperature and humidity is stored as float and the time as a UNIX timestamp.
I use the FRAM breakout board from Adafruit, because writing to FRAM needs way less power than writing to flash RAM. It is also faster, which increases the “sleep” time for the microcontroller and it does not require to erase a full page writing a single byte. The only catch of FRAM, it is more expensive than flash RAM.
My software has a simple abstraction of the used storage, so you can simple replace the FRAM with a flash chip or even with a SD card reader.
Building the hardware for the simple version is almost identical like building the minimal version. Just follow the instructions how to build the minimal version but add the FRAM as shown on the following page: