The arduino will also have to power a 5v relay for a 12v solenoid.
The Arduino can drive a transistor that can operate the solenoid.No relay needed, unless you want electrical isolation.Leo..
A relay also needs a transistor, a base resistor, and a flyback diode. Same as a solenoid.Unless you have a relay module with those parts already mounted on the circuit board.Leo..
3. Use a Resistor Voltage divider to connect Arduino TX to RFID RXD. Check the link https://learn.sparkfun.com/tutorials/voltage-dividers
Which are the arduino output pins on the spi though?
You can not use logic level converters to power a reader.You need to use a voltage regulator and capacitors to make a supply to drive the reader.
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