Using a Schmitt-trigger gate as a pulse generator

摘要

Gate IC1d of the 74HC132 Schmitt-trigger input NAND chip generates the pulses that clock the counter(IC2).So what is a Schmitt-trigger gate,and why are we using one?An'ordinary'non-Schmitt-trigger gate or inverter is effectively a high-gain but mostly linear amplifier.As a result,the output transition from high-to-low or low-to-high takes place over a narrow input voltage range,as shown in Fig.a,a plot of the output voltage versus the input voltage.As a result,the negative-feedback RC circuit shown in Fig.b will typically reach equilibrium at some point(E),with the output'stuck'at an intermediate voltage.If the input were to increase in voltage,as indicated by the arrow,the output would respond by decreasing and would restore the circuit to point E with a time constant determined by R and C.The reverse is true if the input voltage decreases.