Schmitt trigger is a special logical element adjusted to work with analog input signals. The primary purpose of Schmitt triggers was to restore the shape of digital signals. Because of the transmission line effect, digital shape transforms from square to trapezoid or triangle or more complex signal. Of course, during transmission, signals become noisy and distorted.
Schmitt trigger is a comparator circuit, but positive feedback results in hysteresis and memory effect.
Comparing to simple logical elements, Schmitt triggers have two threshold levels. Between these threshold values U1 and U2, the state of output doesn’t change the so-called hysteresis. Such an effect stabilizes output against rapid triggering by noise. Bellow is two characteristics compared of simple logical inverter and Schmitt trigger inverter:
We see that Schmitt trigger characteristics are more complex -the output signal threshold depends on the direction of input voltage rise/drop. When the input signal is rising towards Ucc, then the Threshold is U2. When the input voltage is dropping towards zero- the threshold becomes U1.
Lets see how this would look with real noisy signal.
Because of hysteresis, any noise that amplitude is lower than U2-U1 is removed. And another fact, that the signal front becomes square.
Standard digital chips have Schmitt triggers with the same logical functions as TTL inverters, 2NAND. For TTL IC’s thresholds are 0.9V(U1) and 1.7V(U2).
One of the wide usages of Schmitt triggers is reset signal forming. Some chips may require a delayed reset signal, so one of the simplest solutions is to use a circuit with a Schmitt trigger:
Reset signal former consists of RC circuit and Schmitt trigger. When Vcc is applied, the capacitor starts charging, and the input voltage of the trigger grows up to threshold value U1. The capacitor’s bigger capacitance and the bigger the resistance of the resistor – the longer the reset signal length.
Another interesting usage of the Schmitt trigger is the impulse generator. Using Schmitt trigger circuit becomes more simple than using regular logic elements.
And one more I think important usage is Schmitt triggers is switch debouncing.
The Schmitt trigger ensures that a clean output pulse will be produced.