The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point.

The Schmitt trigger action uses a comparator to produce stable level-crossing switches in contrast to the action of a straight reference comparison.

Schmitt trigger action is a double threshold comparator process. The current equation at A gives:

where

This dependence upon the output voltage gives the dual threshold. The two output states give the thresholds shown at right.