Summary

You have built the computer of a payment card from transistors and folded it into a chip with eight legs.

The processor — 8 bits, 16 commands, one command per clock: the ALU, register A, flags, the program counter, jumps, the stack.
Memory — a ROM with the program, RAM from latches (writing in the second half of the clock), the personal data — a transistor matrix.
One data wire — an open drain and a resistor: they talk in turn, a byte in 160 clocks, a guard interval.
The card program — receiving and sending a byte by commands of 16 clocks, the PIN and the try counter, challenge-response.
Two cards — one program, different identities: 29 transistors of difference — and a completely different answer to the same challenge.

The card is 6,495 transistors and 8 resistors, the tester 1,729 and a resistor.

The ATM. The tester will become its card port, with a screen and a keyboard around it: enter the PIN with buttons, see ****, withdraw money from the account. That is the next kit.
Your own card. Come up with your own number, PIN and key — and place the matrix transistors. The cipher (lesson 7) will compute the answer.