There are plenty of howto guides on how to use the RDM6300 reader with Arduino (which is a 5V board), however, I had seen not many addressing the Raspberry PI ecosystem.
Now if you are looking to use RFID for your project choosing the RDM6300 board with 128 kHz cards can be an inexpensive solution to do so. The board itself sales for at around $2 on AliExpress, while you can get 10 RFID cards as low as $1.
The reader circuit is called RDM6300, it seems to be a Serial (UART) based reader circuit (seems) compatible with the EM4100 series chips.
Warning: these boards are built assuming 5V input (Arduino), please make sure you are checking this HowTo post before wiring it to your RPI.
I am also providing a python library that can be used to conveniently interface the board.