The second kit in the How Things Work series, the sequel to The Payment Card. Around the same processor that sits in your card you build an ATM: a keypad, a segment screen, a bank with the card keys and balances, a card port and a slot for two cards. The ATM asks for the PIN, checks with challenge-response that the card is genuine and gives out money — and does not let in a fake card with the right number and PIN. At the end the ATM folds into one chip. It is a teaching model of the principle, not a real ATM. 15,062 transistors.
The introduction and lessons 0–1 are free; the rest opens after purchase.