As a contractor I had to build a prototype of a RFID controlled gate for a garbage can shed. One of the requirements was that it's supposed to be battery powered (Li-Ion 3.7V) and charged from Solar. Because of this I had to use two DC-DC converters - step up to 12V for gate electromagnet and flyback 3v3 for electronics. Also later I added the output for some LED's. To communicate with RFID tags I simple used RC522 board that works with basically all cards in 13.56 MHz standard. It enables you to write and read from tags which is essential if you want to use them as a replacement for a standard key 😄. Hardest part of this project was to program the state machine for unlocking the door or adding new tags to the system. I also discovered that STM32 micro-controllers have EEPROM memory (not all of them). And using EEPROM in ARM couldn't be more different than using it in AVR, because of unified memory map in ARM architecture.