One threshold

hi, i have a circuit where the input voltage goes from 5v to 2.5, then back up, like a square wave. What i would want is that if i could have some sort of threshold at let's say 3V or so, so that when the input drops to 2.5, it brings it down to 0V instead. In other words, some sort of way to make a square wave from 5v to 2.5v, to 5v to 0v
thanks

edit: Neevermind, i figured it out myself, a zener diode at 3v would do what i wanted