It is well documented and includes not only a schematic and code, but an explanation of the design considerations used during the build. The project uses an ATmega32 and the parts list priced out ...

The Proxmark3 PCB 125kHz antenna., GNU GPL version 2, GitHub link. Picture this: It’s the end of the year, and a few hardy souls gather in a hackerspace to enjoy a bit of seasonal food and hang out.