Intro to Schmitt Triggers 

Comparator circuits can be sensitive around the threshold point. If the input signal fluctuates around the threshold point, the output will flicker on and off. To prevent this flickering, hysteresis can be built into the input circuit by using a Schmitt Trigger.
This video describes how Schmitt triggers work when you have a single power supply or a double power supply and shows the effects of using one.
For more videos like this as well as tutorials, online text books and example problems, see the website: www.electronx.ca