What is Schmitt trigger? How it works? Where is it used?

Schmitt trigger:

Schmitt trigger is an electronic circuit with positive feedback which holds the output level till the input signal to the comparator is higher than the threshold. It converts a sinusoidal or any analog signal to digital signal. It exhibits hysteresis by which the output transition from high to low and low to high will occur at different thresholds.

Invention:

The Schmitt trigger was invented by American scientist Otto H. Schmitt in 1934. By that time, Otto Schmitt was a student. In the year 1937, he published his invention in his doctoral. The name he gave was "thermionic trigger".

Schmitt Trigger Types:

The two different Schmitt trigger types are:

1. Non-inverting type, in which the input and output are both high / both low at the same time (no phase shift).
2. Inverting type, in which there is 180° phase shift between input and output.

Symbols:

There are basically two symbols for the Schmitt Trigger. The symbol is a triangle with an input and an output, just like the one used for the non-inverting buffers. Inside there is the hysteresis symbol. Depending on the type of Schmitt Trigger, inverting or non-inverting (standard), the hysteresis curve sign differs.

Figure 1: Logic Symbols of Schimitt Trigger.

Operations of Schimitt Trigger:

The Schmitt Trigger is a type of comparator with two different threshold voltage levels. Whenever the input voltage goes over the High Threshold Level, the output of the comparator is switched HIGH (if is a standard ST) or LOW (if is an inverting ST). The output will remain in this state, as long as the input voltage is above the second threshold level, the Low Threshold Level. When the input voltage goes below this level, the output of the Schmitt Trigger will switch.

The HIGH and LOW output voltages are actually the POSITIVE and NEGATIVE power supply voltages of the comparator. The comparator needs to have positive and negative power supply (like + and -) to operate as a Schmitt Trigger normally. The following drawing shows how a Schmitt Trigger would react to an AC voltage input:

Figure 2: Basic operation of a Schimitt Trigger.

The orange line is the AC input. The horizontal RED line indicates the High Threshold Level, while the BLUE horizontal line indicates the Low Threshold Level. The green line is the output of the Schmitt Trigger. When the input voltage level goes above the High Threshold Level, then the output of the ST goes High. When the input voltage level goes below the Low Threshold Level, then the output of the ST goes Low. This is the basic operation of a Schmitt Trigger.

Figure 3: The most simple Schmitt Trigger circuit is implemented with a comparator with a positive feedback.

Applications of Schimitt Trigger:  

• Squaring circuit (where a noisy signal requires squaring up).
• Sine-to-Square comparator 
• Amplitude comparator 
• Level detector
• Oscillator i.e. bistable multivibrator, relaxation oscillator etc. 

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