The Atmega328P Has an internal schmitt trigger on all Digital IO Pins.   The Schmitt trigger is a great way to reject the Noise of a Digital Pin Especially one that is Left Floating.  In addition adding a Comparator to the LED Circuit isolates the pin due to the high input impedance of the Comparator.

   However the ATmega 328P does not Express the hysteresis of the Low and High Logic Level.  

  So the People over at RadishLogic Ran the 328P Pins.

 RadishLogic Found that when using USB Buss as the Power the Low High Logic Levels were

Using 33k, 36k, and 150k a comparable Schmitt Trigger can Be Created

Further Testing on PCB Should be conducted to alpine the Schmitt Trigger of Atmega328 to the LED Comparator 

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Implementing into the Arduino Plus+ Pin seven is set to digital IO and  a modified Button Sketch is used. 

So the real world test of the Schmitt trigger show that

1) The LED Low input State Triggers at 2.29V      ------   At Atmega Triggers at  2.44V  --- 2.23% error

2) The LED High Input State triggers as  2.52V   ------   At Atmega Triggers at  2.52V --- 0% error