The Schmitt Trigger was first invented from Otto H. Schmitt in the 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"

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.

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:

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.

The most simple Schmitt Trigger circuit is implemented with a comparator with a positive feedback. Look at the following circuit:

When the non-inverting input (+) is higher than the inverting input (-), the comparator output switches to the POSITIVE voltage supply. On the contrary, the non-inverting input (+) is lower than the inverting input (-), the output switches to the NEGATIVE voltage supply.

The inverting input (-) is grounded, so someone would expect that the turn-on and off point would be the ground (0). The function of the ST comes from the feedback resistor R_FB. When for example the output of the comparator is to the POSITIVE voltage supply, then the non-inverting input has through the R_FB this voltage! The same happens when the output is to the NEGATIVE power supply.

The voltage needed to switch the output of the comparator must be above or below zero (ground), according to the POSITIVE and NEGATIVE power supply and according to the resistors R_I and R_FB. More specific, the formula to calculate the threshold voltage is:

So, if the output is to the POSITIVE voltage, the required negative voltage that must be applied to Vin is:

If the output is to the NEGATIVE voltage, the required positive voltage that must be applied to Vin is:

The above circuit is non-inverting Schmitt Trigger. It can be easily converted into an inverting Schmitt Trigger as follows:

The same formula is used to calculate the threshold level. But the output of this circuit is inverted in comparison to the previous one. When the input voltage is above the High Threshold Level, the output goes LOW.

These are the most basic Schmitt Trigger. The threshold voltage is an equal offset from the center of the POSITIVE and NEGATIVE power supply (usually this is 0). Thus, we can call it "symmetrical Schmitt Trigger". I have include a Symmetrical Schmitt Trigger calculator in the Dr.Calculus page. Do not forget to visit and try it!

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