A Schmitt trigger converts an irregular shaped waveform into a square wave. It is a special type of comparator in which the output changes from one saturation level to another saturation level depending on differential input voltage.
Consider the circuit shown below to which an a.c. voltage is applied. The potential at B is positive and at point A is zero. The differential voltage is positive, so the output voltage is driven to + V sat. At this instant, the potential at B is: VB = R2/(R1 +R2)*(+Vsat.).This voltage is called upper trigger point (VUTP).When input becomes more positive than VUTP, the differential input is negative. Therefore the output is driven to – VSAT. Now the potential at B is: VB = R2/(R1 +R2)*(-Vsat.) This voltage is called lower trigger point (VLTP).The output remains at –Vsat until input voltage becomes more negative than VLTP.When input becomes more negative thanVLTP, the differential input is again positive. Therefore the output is driven to + VSAT. the circuit diagram and waveforms are as shown.