The first kit in the How Things Work series. There is a computer in the little gold square of a bank card. Build it from transistors: your own 8-bit processor, a ROM with the program, RAM from latches, a PIN and a secret key in a transistor matrix, one data wire and a card tester. At the end the card folds into a chip with eight legs, like a real contact pad, and answers the tester's challenge. It is a teaching model of the principle, not a real card. A standalone, free kit. 6,495 transistors.
This kit is free: all lessons are open, no sign-up needed.