A Schmitt trigger is a special type of bistable device that has two threshold voltages (i.e. stable states). These are often used to control the input and operation of logic gates and IC’s. The Schmitt trigger inputs causes the device to change logic states abruptly when a given voltage level known as the threshold voltage is reached. This feature is exploited in many applications as an astable multivibrator or for increasing the noise immunity in circuit inputs. Essentially, it prevents erratic switching between states when the input voltages hover around the critical input level. This allows for reliable triggering to occur when the input is changing very slowly.
 | The hysteresis symbol ( |
Let Vt+ indicate the more positive voltage similarly, let the more negative voltage be indicated by Vt-. Then the hysteresis for a Schmitt trigger device is the difference between these voltages. An example of an application for the Schmitt trigger is shown in Figure 2-19 where A and B are assumed to be the input and output of a non-inverter, respectively.
Figure 2-20 Schmitt trigger application example
Copyright © Adrian Als , 1999
This page was last modified: Wednesday, April 12, 2000
) indicates a Schmitt trigger input