How to use this forum
First, resize your image down to about 100 Kb, edit your post and swap them over, thanks.
3 Mb for a circuit is too much.
Indeed - we don't need to see every RGB pixel on your screen!
Back to the question - no a schmitt trigger like an HC14 is not suitable for this application,
it has very little precision at all (and a very large hysteresis - you need some hysteresis but
perhaps 1%, not 50!).
A comparator as suggested, but the 5.1V zener isn't going to be useful if the circuit is running at
The normal method is to set up a wheatstone-bridge arrangement, one arm of which uses two
resistors to sample the battery voltage (typically approx 50% of it). The other arm would use
a zener and a resistor to form the voltage reference (say 2.7V). You choose the actual
resistors in the divider arm to make the bridge balance when the battery voltage is at the value
you want to sense.
A comparator then takes its inputs across the bridge. Adding a little hysteresis is advised to
stop rapid fluctuations due to noise.