AN-1044 Single input security passcode entry

A passcode entry circuit is constructed without the use of any switches, touch displays, or keypads. In this example the input uses an earbud, which also has properties of a microphone. Almost any microphone or speaker can be used in a similar fashion. When the earbud is tapped, it generates a small output pulse which is amplified by an analog comparator in the GPAK2 IC. This also initiates a string of clock pulses inside the GPAK2. Each tap gets latched as a logic “one”, while the absence of a tap gets latched as a logic “zero”. Not only does the pattern need to be correct, but also the “beat”, or timing must match as well. In other words, this circuit detects a “secret knock”. Both the pattern and timing are user configurable. The circuit to perform this must be capable of serial input pattern and timing recognition.


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