Schmitt trigger circuit and working principle detailed - Database & Sql Blog Articles

Photocoupler

What is a trigger?

Schmitt trigger circuit (abbreviation) is a waveform shaping circuit. When any waveform signal enters the circuit, the output jumps between positive and negative saturation, producing a square wave or pulse wave output. Unlike the comparator, the Schmitt trigger circuit has two threshold voltages and forms a hysteresis region to prevent noise in the hysteresis range from interfering with the normal operation of the circuit. For remote control receiving lines, the sensor input circuit will use it for shaping.
The Schmitt trigger generally has only one threshold voltage for comparison. If the input terminal has noise going back and forth across the threshold voltage multiple times, the output terminal is disturbed, and its positive and negative states produce abnormal transitions, as shown in Figure 1.

Upper threshold voltage V _TH

Lower threshold voltage V _TL

Hysteresis width (voltage) V _H

V _TL <noise < V _TH

When the input signal ν _I rises above V _TH , the trigger circuit turns ν _O

The input signal ν _I drops to less than V _TL , and the trigger circuit turns ν _O

Upper and lower threshold voltage difference V _H =V _TH -V _TL

Noise is within the allowable hysteresis width, ν _O maintains a steady state

2R1

Figure 4 non-inverting Schmitt trigger

Figure 5 (a) computer simulation diagram (b) conversion characteristic curve

The input and output waveforms and conversion characteristics are shown in Figure 5.
When the input signal falls below the lower threshold voltage V _TL , the output signal transitions from a positive state to a negative state: ν _o < V _{TL →} ν _o = - Vsat

When the input signal rises above the upper threshold voltage VTH, the output signal changes from a negative state to a positive state: ν _o > V _{TL →} ν _o =+ Vsat

The output signal transitions between positive and negative states, and the output waveform is a square wave.