[1] In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit converting an analog input signal to a digital output signal. It's called a trigger as its output retains its value til the input changes enough to cause a change.
It was invented by American scientist Otto Schmitt in 1934 as a graduate student.
[1] In non-inverting configuration (circuit's outputs change in same directions as input signals), if the input is higher than a given threshold, its output is high. If the input is under a different (lower) given threshold, its output is low, and if the input is between the 2 levels, its output retains its value.
This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bistable multivibrator (latch or flip-flop). There is a close relation between the 2 circuits types: a Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.
What fundamentally makes a Schmitt trigger a Schmitt trigger is positive feedback and hysteresis (to have 2 voltage thresholds). Normal comparators/logic gates switch outputs at a single fixed voltage line. If the input signal lingers near that line or has noise, the output will rapidly and erratically flicker.
linger: to take an unexpectedly long time to leave
A Schmitt trigger solves this via 2 rules:
Dual thresholds: It uses a high and low thresholds.
Going UP: Input voltage must exceed the high threshold to flip the output.
Going DOWN: Once flipped, the circuit won't flip back til input voltage drops below the low threshold.
The dead zone between the 2 points is the hysteresis. Any noise, ripples, or static within this gap are ignored, making it ideal for conditioning messy signals and debouncing mechanical buttons.
The 7400-series is a classic family of transistor-transistor logic (TTL) integrated circuits by Texas Instrument, with logic gates, flip-flops, MUXs, etc.
There are many 7400-series Schmitt-trigger inputs ICs.
Historically, the 7400-series started out as a family but as semiconductor technology evolved, manufacturers cloned the entire 7400-series layout via CMOS tech to be more power-efficient.
The TTL series are the classic family that are built via BJTs. Members of the family have only nubmers in their code names, unlike the CMOS series which have 'HC' in names.
7413: Dual Schmitt-trigger 4-input NAND gate
7414: Hex Schmitt-trigger inverter
7419: Hex Schmitt-trigger inverter
7424: Quad 2-input NAND gate with Schmitt-trigger inputs
74121: Monostable multivibrator with Schmitt-trigger inputs
74132: Quad 2-input NAND Schmitt trigger
74221: Dual monostable multivibrator with Schmitt-trigger input
74232: Quad NOR Schmitt trigger
74310: Octal buffer with Schmitt-trigger inputs
74340/74341: Octal buffer with Schmitt-trigger inputs
Modern ICs are built via metal-oxide semiconductor (CMOS) tech, identified with a 'HC' in the names, meaning "high-speed".
74HC14/74HCT14: Hex Schmitt-trigger inverter (This is the direct modern clone of the 7414 you listed above!)
74HC132/74HCT132: Quad 2-input NAND Schmitt trigger
74HC244: Octal buffer/line driver with Schmitt-trigger inputs
74HC7014: Hex non-inverting Schmitt trigger
The 74HC14 is a hex Schmitt-trigger inverter IC. What differs it from normal Schmitt-triggers is that it has input feature hysteresis, thus using 2 voltage thresholds, providing highly stable, clean, and noise-immune digital outputs.
The 74HC04 is a hex Schmitt-trigger inverter IC. What differs it from normal Schmitt-triggers is that it has 1 fixed (constant/unadjustable) internal switching threshold built in its silicon, unlike stadnard Schmitt-triggers that have adjustable thresholds.
For having only 1 threshold, the 74HC04 has no hysteresis, giving a slow/noisy signal causing its output to quickly flicker. This also makes it digitally-operated IC, compared to standard models that are analog.
74HC14 vs 74HC04 The 74HC14 is much better than the 74HC04 for many real-world uses .
Noise immunity: When real-world inputs, the 74HC14 can flawlessly clean it. The 74HC04 can choke on the noise, causing the microcontroller to glitch/double-trigger.
"Linear" power waste absence: If a signal slowly changes from 0 to 5 V, a standard inverter like the 74HC04 gets stuck in a middle zone where its internal transistors both turn partially on, wasting power and heating up the chip. The 74HC14's "snap action" prevents this.
[1] Wikipedia
[2] What Is Schmitt Trigger and How It Works - How To Mechatronics (YouTube)
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