https://circuitdigest.com/microcontroller-projects/arduino-rfid-door-lock-code

https://openhomeautomation.net/arduino-battery/

https://arduino.stackexchange.com/questions/10059/how-to-send-the-mfrc522-rfid-module-into-sleep-mode

https://forum.arduino.cc/index.php?topic=340222.0