the Schmitt trigger
/ shmit /
A Schmitt trigger is a comparator with hysteresis built in — a switch that snaps cleanly and refuses to dither. Push a slow, messy, noisy voltage into it and out comes a crisp, clean square edge. It is named after Otto Schmitt, who invented it in 1934 while studying nerve impulses. Wherever a real-world signal that drifts and wiggles has to be turned into a clean digital on/off, a Schmitt trigger is doing the job.
It works by feeding a little of the output back to the input with positive feedback, which creates the two thresholds described under hysteresis: a higher one to switch and a lower one to switch back. The positive feedback also makes the transition regenerative — the moment it begins to flip, it reinforces itself and races to the rail, so the output edge is fast and clean even when the input crawls. A non-inverting Schmitt trigger feeds the output back to the + input and follows the input's direction; an inverting Schmitt trigger feeds back to the - input and flips the output opposite to the input. You will find ready-made Schmitt-trigger logic gates such as the 74HC14 inverter, marked with a little hysteresis-loop symbol.
Schmitt triggers turn up wherever a clean edge is needed: squaring up a slow sine or ramp into a logic clock, cleaning a sensor output, conditioning a switch, and reshaping a signal blurred by a long cable. A favourite use is a simple relaxation oscillator — a Schmitt-trigger inverter, one resistor, and one capacitor make a square-wave generator. The honest limit is the same as hysteresis: the band that gives you noise immunity also sets a minimum signal swing. A signal that never travels the full band will never trip the output.
Feed a noisy, slowly rising ramp into a 74HC14 Schmitt-trigger inverter (thresholds roughly 2.7 V and 1.8 V on a 5 V supply). The ragged input becomes a single clean falling edge at 2.7 V; the output cannot bounce back until the input drops below 1.8 V.
Messy in, clean snap out — hysteresis plus regenerative switching.
A Schmitt trigger cleans electrical noise, but it does not remove a slow signal's delay and it does not guarantee a switch is debounced — mechanical bounce can outlast its hysteresis, so switches often still need an RC time delay as well.