That is a very simple application for an Arduino. It appears that you will configure the existing RFID reader system to deliver a momentary pulse. The Arduino will receive this pulse and, depending on the current state (locked/unlocked) control a relay board to activate a chosen relay for a specific amount of time.
- The Arduino cannot tolerate a 12 volt input signal on a (GPIO) pin. You need a simple voltage divider (2 resistors) to reduce this to 5 volts.
- You have to think about what happens when the system is first powered up. What state should it be in (locked/unlocked)?
- I guess at the moment it is a sort of proof of concept. For a practical application, you’ll want to reduce the quiescent power consumption of the devices to avoid draining the battery unnecessarily.