Schmitt Trigger Symbol
Definition: The Schmitt Trigger symbol represents an analog or digital circuit element with two distinct input switching thresholds — a positive-going threshold (VT+) and a negative-going threshold (VT−) — that cause the output to transition only when the input clearly crosses one threshold, depicted as a buffer or inverter gate body with a hysteresis symbol (backlash marker) inscribed inside, per IEC 60617-12 and ANSI/IEEE 91-1984, with the output snapping sharply between logic HIGH and LOW regardless of input rise time.
Also known as: Schmitt inverter, hysteresis comparator, regenerative comparator, 74HC14 inverter, backlash trigger.
What the Schmitt Trigger symbol means
The Schmitt Trigger symbol in a circuit diagram represents a circuit that introduces intentional hysteresis into a signal path — converting a slow, noisy, or ambiguous analog transition into a clean, fast digital edge. The input must rise above the positive threshold VT+ for the output to transition LOW (for an inverting Schmitt), and must fall below VT− for the output to transition HIGH again. The voltage gap between VT+ and VT− (the hysteresis band) defines the noise immunity: any noise smaller than the hysteresis voltage cannot cause a spurious output change.
In circuit schematics, the Schmitt trigger symbol appears wherever a digital signal may have compromised edges — slow rise times, reflected noise on long lines, contact bounce from mechanical switches, or gradual signals from RC networks. The Schmitt trigger cleans and reshapes the signal into a sharp-edged digital waveform suitable for reliable clocking of flip-flops, counters, and microcontroller inputs.
How to identify the Schmitt Trigger symbol
The Schmitt Trigger symbol is drawn as a buffer triangle (non-inverting Schmitt) or an inverter triangle with a bubble on the output (inverting Schmitt, the most common form). Inside the triangle body is inscribed a small hysteresis symbol — a backlash loop or backwards-S shape (similar to a hysteresis loop on a B-H curve compressed into the gate body). In IEC 60617-12 rectangular notation, it is shown as a box with input and output lines and the hysteresis marker inside. Two pins are visible: In on the left and Out on the right.
Function in a circuit
A Schmitt trigger circuit uses positive feedback to create two stable switching thresholds. When the input rises and crosses VT+, the output snaps to its low state (for an inverting Schmitt) — the positive feedback reinforces this transition, making it fast regardless of input slew rate. The output remains low until the input falls below VT−, at which point the output snaps back to high. The difference VT+ − VT− is the hysteresis width. For the 74HC14 inverting Schmitt (operating at 5 V): VT+ ≈ 2.9 V, VT− ≈ 2.2 V, hysteresis ≈ 0.7 V. In discrete circuits, a Schmitt trigger is implemented with an op-amp or comparator with a positive-feedback resistor network.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-12 (binary logic elements, 1997) defines the hysteresis marker inscribed in a gate body as the standard indicator for Schmitt-trigger input behaviour. A Schmitt inverter in IEC notation is a rectangular buffer block with '1' label and an inversion indicator at the output, plus the hysteresis symbol inside the block body. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 91-1984 (graphic symbols for logic functions) defines the Schmitt trigger symbol as a triangular buffer or inverter with a small backwards-S or backlash marker inscribed inside the triangle. The 74HC14 and 74LS14 datasheets use this ANSI representation. |
| Key difference | IEC uses a rectangular block body with '1' (buffer) or the inversion indicator, plus hysteresis marker inside. ANSI uses the triangular gate body with the same inscribed hysteresis marker. Logic function, pin designations, and hysteresis symbol are identical; only the gate outline shape differs. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
74HC14 (inverting Schmitt, 6-gate DIP-14): VT+ ≈ 2.9 V, VT− ≈ 2.2 V at VCC = 5 V; hysteresis ≈ 0.7 V. 74HC7541 (non-inverting Schmitt buffers). For op-amp Schmitt trigger: VT+ = VREF + (VREF × R1/R2); VT− = VREF − (VREF × R1/R2). Operating voltage: 2–6 V (74HC). Propagation delay: 14 ns at 4.5 V. Output drive: ±25 mA.
Where the Schmitt Trigger symbol is used
- Mechanical switch debouncing — eliminating contact bounce on keyboard and pushbutton signals before connecting to microcontroller interrupt pins
- Crystal oscillator circuits — Schmitt inverter with crystal and load capacitors forms the feedback amplifier in a Pierce-topology crystal oscillator
- RC oscillator and timer circuits — Schmitt inverter with RC feedback generates a free-running square wave without a crystal or 555 timer
- Long-line signal restoration — reshaping degraded TTL/CMOS signals with slow edges from unterminated PCB traces or cables into sharp edges
- Sensor threshold detection — converting a slowly changing photodiode current or thermistor voltage into a clean digital on/off signal
- Power-on-reset circuits — the RC + Schmitt trigger combination generates a defined reset pulse of controlled duration at power-up
Example
In an Arduino push-button debouncing circuit, the button signal — which bounces between LOW and HIGH for 10–50 ms on press — feeds the In pin of a 74HC14 Schmitt inverter; the hysteresis of 0.7 V at 5 V means neither the rapid contact bounces (which do not fully cross both VT+ and VT−) nor the slow rise from the RC pull-up cause spurious transitions, producing a single clean falling edge at the Out pin that the Arduino interrupt service routine registers as one button press.
Key facts
- The Schmitt trigger has two input switching thresholds: VT+ (positive-going) and VT− (negative-going), with the output snapping sharply in opposite directions when each threshold is crossed — hysteresis width = VT+ − VT−.
- The 74HC14 is the standard hex inverting Schmitt trigger IC (6 gates, DIP-14 or SOIC-14); at 5 V it provides approximately 0.7 V of hysteresis. The 74HC7541 provides non-inverting Schmitt buffers.
- In op-amp or comparator Schmitt trigger circuits, hysteresis is set by a positive-feedback resistor divider from output to non-inverting input; VT+ and VT− are calculated as VOUT_HIGH × R1/(R1+R2) and VOUT_LOW × R1/(R1+R2) respectively.
- The Schmitt trigger is named after Otto Schmitt who first described the circuit in 1934 ('Schmitt trigger') based on his nerve-signal research at the University of Minnesota.
- Hysteresis eliminates multiple output transitions (chattering) when an input signal has noise amplitude smaller than the hysteresis band; without hysteresis, even millivolt noise near the threshold causes rapid output toggling.
- Non-inverting Schmitt triggers (Out = In with hysteresis) and inverting Schmitt triggers (Out = NOT(In) with hysteresis) are both common; the inverting form (74HC14) is more widely used because it provides a clean signal inversion simultaneously.
- Reference designator for Schmitt trigger ICs in schematics is U followed by a number; discrete comparator-based Schmitt triggers use the comparator designator U or CMP.
Frequently asked questions
What does the Schmitt trigger symbol mean in a circuit diagram?
The Schmitt trigger symbol represents a buffer or inverter with two input switching thresholds (VT+ and VT−). The output switches sharply only when the input clearly crosses one threshold, and does not switch back until the input crosses the other. This hysteresis converts noisy, slow, or bouncy signals into clean digital edges.
What does a Schmitt trigger symbol look like?
The Schmitt trigger symbol is a triangular buffer (non-inverting) or an inverter triangle with a small output bubble, with a backlash or backwards-S hysteresis marker inscribed inside the triangle body. In IEC rectangular notation, it is a box with '1' label, inversion indicator, and the same hysteresis marker inside. The inscribed hysteresis symbol is the defining visual feature.
What is the difference between a Schmitt trigger and a comparator?
A comparator has a single threshold and outputs the sign of (Vin − VREF); near the threshold, noise causes rapid output chattering. A Schmitt trigger adds positive feedback to create two thresholds with a hysteresis band; once triggered past VT+, the output cannot chatter back until Vin drops below VT−. Schmitt triggers are hysteresis comparators optimised for noise immunity.
What IC implements a Schmitt trigger inverter?
The 74HC14 (hex inverting Schmitt trigger, 6 gates per DIP-14 package) is the most widely used Schmitt trigger IC. It provides approximately 0.7 V hysteresis at 5 V. Equivalent devices include the 74LS14, 74HCT14 (5 V input compatible), and CD40106 (CMOS, 3–15 V supply).
What is the hysteresis voltage of a Schmitt trigger?
The hysteresis voltage is VT+ − VT− — the voltage band between the two switching thresholds. For the 74HC14 at 5 V, VT+ ≈ 2.9 V and VT− ≈ 2.2 V, giving approximately 0.7 V hysteresis. For an op-amp Schmitt trigger, hysteresis is set by the positive-feedback resistor ratio and can range from millivolts to volts.
What standard defines the Schmitt trigger symbol?
ANSI/IEEE 91-1984 (graphic symbols for logic functions) and IEC 60617-12 (binary logic elements) both define the hysteresis marker (backlash symbol) inscribed inside a gate body as the standard indicator for Schmitt-trigger behaviour. Both standards are consistent in symbol meaning; they differ only in gate body shape (triangular ANSI vs rectangular IEC).
What are the pins of a Schmitt trigger symbol?
The Schmitt trigger symbol shows two pins: In (analog or digital input signal) on the left and Out (digital output, inverted for a Schmitt inverter) on the right. Power supply (VCC and GND) pins are present on the physical IC but are typically omitted from the symbol in schematics for clarity.
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