This was part of a project to automatically open a door in response to a particular pattern of knocks. A microphone (not shown) picks up the sound from the knock and feeds it to the sensor amplifier (breadboard circuit just below door handle), feeding the input of an arduino (not shown). The arduino processes the sound pattern and makes a decision of whether or not to pen the door. If the knock is correct, the arduino supplies power to printer motor (left, brush end showing), which moves the printer's ink cattridge carriage (behnid metal strip.) The white wire tied to the door handle is pulled by the carriage to open the door, just as in opening by hand. The mechanism is mounted to the door by hanging on a wooden bracket, which is in turn secured to the surface of the door with double sided tape.

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