Door Lock

The aim of this project was to construct a door lock that responded
to a set of predefined knocksequences. The idea was that it is all too easy
to forget your keys and for example lock yourself out of your apartment.
Having to rembember a certain sequence instead is much easier and eliminates
the need for any physical keys. For additional security Bluetooth
reception of the owners cellphone is also required.

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Source: http://arduino.cc/en/uploads/Main/ArduinoBT400.jpg