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I’ve used iButtons for a few projects (starting a car and parameter modding on a Segway) – electronics lab has a good how-to on making a lock with them – “In this project we will use iButton as a key to an electronic lock. This electronic lock can use many different kinds of iButtons and can store up to 9 different keys. One of the keys is the master key and is permanent stored in memory. With the use of master key we can add or remove slave keys.”Link.