Schmitt Trigger NAND Symbol
Definition: The Schmitt Trigger NAND symbol represents a two-input NAND logic gate with integrated Schmitt trigger hysteresis on each input, depicted as a standard NAND gate shape with input pins A and B and output pin Out, distinguished by a hysteresis symbol (backlash symbol ⊡ or the letter H inscribed in the gate body) per IEC 60617-12 and ANSI/IEEE 91-1984, with the 74HC132 being the archetypal quad Schmitt-NAND device.
Also known as: Schmitt NAND gate, 74HC132, 74LS132, hysteresis NAND, noise-immune NAND gate.
What the Schmitt Trigger NAND symbol means
The Schmitt Trigger NAND symbol in a circuit diagram represents a NAND gate whose inputs have intentional hysteresis — two different threshold voltages for rising and falling transitions. The upper threshold (VT+) must be exceeded on an input to switch it from logic LOW to HIGH; the input voltage must fall below the lower threshold (VT−) to switch back from HIGH to LOW. This hysteresis prevents rapid toggling (chattering) when a slow or noisy signal crosses the logic threshold.
In digital circuit schematics, the Schmitt-NAND symbol is used wherever a NAND function must be performed on a signal that may contain noise, interference, or slow rise times — such as signals from RC networks, crystal oscillators, mechanical switches, or long cable lines. The NAND output logic function remains identical to a standard NAND: output is LOW only when both inputs are HIGH; it is HIGH in all other input combinations.
How to identify the Schmitt Trigger NAND symbol
The Schmitt Trigger NAND symbol is drawn exactly as a standard NAND gate — a D-shaped body with a small bubble on the output indicating inversion — but with an additional hysteresis symbol (a backwards-S or backlash loop, sometimes shown as a small ⊡ or a stylised letter) inscribed inside the gate body. In IEC 60617-12 notation, the hysteresis marker is a small symbol resembling two overlapping half-arrows inside the gate rectangle. Two input lines (A and B) enter on the left and one output line (Out) exits on the right with the inversion bubble. The hysteresis marker inside the gate body is the sole visual distinction from a standard NAND gate.
Function and usage boundary
A Schmitt-NAND gate performs the Boolean NAND function (Out = NOT(A AND B)) with the added benefit that each input has a hysteresis band between VT+ and VT−. When input A or B rises and crosses VT+, the input is registered as logic HIGH; when it falls and crosses VT−, it is registered LOW. The difference VT+ − VT− (hysteresis voltage, typically 0.6–1.2 V for 74HC132) defines the noise margin — any noise voltage smaller than the hysteresis band cannot cause a spurious output transition. This makes the gate ideal for interfacing with slow analog sources or creating simple oscillators with RC timing networks.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-12 (binary logic elements) defines the hysteresis symbol inscribed in a gate body as a small backlash marker. A Schmitt-input NAND gate is drawn as an IEC-style rectangular NAND function block (with '&' and the inversion indicator at the output) with the hysteresis marker on each input line or inside the block. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 91-1984 (graphic symbols for logic functions) uses the distinctive D-shaped NAND gate body with the hysteresis symbol (a backwards-S shape) inscribed inside the gate body to indicate Schmitt inputs. The 74HC132 datasheet uses this ANSI representation. |
| Key difference | IEC 60617-12 uses a rectangular logic function block with '&' label and inversion indicators; ANSI/IEEE 91 uses the curved D-shape NAND gate body. Both use an identical inscribed hysteresis symbol to indicate Schmitt-trigger inputs. The logic function and pin designations are identical. |
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Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| a | A |
| b | B |
| out | Out |
Where the Schmitt Trigger NAND symbol is used
- Crystal oscillator start-up circuits — a Schmitt-NAND provides reliable switching for slow-starting crystal oscillators and LC oscillators
- Mechanical switch debouncing — slow, bouncy switch contact signals are cleaned up by the Schmitt hysteresis, outputting a single clean edge
- RC oscillator generation — two Schmitt-NAND gates with an RC feedback network generate a stable square-wave oscillator without a crystal
- Long cable signal reception — restoring degraded logic signals with slow rise times from unterminated PCB traces or long cable runs
- Inverting logic level shifting — configuring the two inputs tied together creates a Schmitt inverter function (single NAND used as NOT gate)
- Comparator substitute — the Schmitt-NAND inputs act as a low-hysteresis comparator for simple threshold detection on analog signals
Key facts
- The Schmitt Trigger NAND performs the Boolean function Out = NOT(A AND B) — output is LOW only when both A and B are simultaneously HIGH — identical to a standard NAND gate in logic function, but with hysteresis on the inputs.
- Hysteresis voltage for 74HC132 at VCC = 5 V is typically 1.2 V (VT+ = 2.4 V, VT− = 1.2 V), providing substantially better noise immunity than a standard 74HC00 NAND (which has no hysteresis).
- Tying both inputs together converts the Schmitt-NAND into a Schmitt inverter, producing Out = NOT(A); this is the most common single-gate usage of a 74HC132.
- The hysteresis symbol inscribed in the gate body (a backlash or reverse-S shape) is defined by ANSI/IEEE 91-1984 and IEC 60617-12 and is the sole visual distinction from a standard NAND gate symbol.
- The 74HC132 contains four independent Schmitt-NAND gates in a 14-pin DIP or SOIC package; equivalent devices include 74LS132, 74HCT132, and SN74HC132.
- The propagation delay of a Schmitt-NAND is slightly longer than a standard NAND because the hysteresis comparator requires additional switching time (14 ns vs 10 ns typical for 74HC at 4.5 V).
- Reference designator for logic gates in schematics is U followed by a number with pin group identifier (e.g., U1A, U1B for gates A and B of the same IC package).
Frequently asked questions
What does the Schmitt trigger NAND symbol mean in a circuit diagram?
The Schmitt Trigger NAND symbol represents a two-input NAND gate with input hysteresis — two different switching thresholds (VT+ and VT−) that prevent the output from toggling on noisy or slowly changing input signals. The logic function (Out = NOT(A AND B)) is identical to a standard NAND gate but with noise immunity from the hysteresis band.
What does a Schmitt NAND gate symbol look like?
The Schmitt-NAND symbol is identical to a standard NAND gate — a D-shaped body with a bubble on the output — except for a hysteresis symbol (a backwards-S or backlash loop) inscribed inside the gate body. In IEC rectangular notation, it is a box labelled '&' with an inversion indicator and the same hysteresis marker. The inscribed hysteresis symbol is the only visual distinction from a regular NAND gate.
What is the difference between a standard NAND gate and a Schmitt NAND gate?
A standard NAND gate has a single threshold voltage; small noise near the threshold causes rapid output toggling (chattering). A Schmitt-NAND gate has two thresholds (VT+ and VT−) with a hysteresis band between them; the input must cleanly cross VT+ before switching HIGH and fall below VT− before switching LOW. This prevents chattering on noisy inputs.
What IC implements a Schmitt trigger NAND gate?
The 74HC132 (and 74LS132, 74HCT132) is the standard quad two-input Schmitt-NAND gate IC, containing four independent Schmitt-NAND gates in a 14-pin package. The 74HC132 at 5 V has VT+ ≈ 2.4 V, VT− ≈ 1.2 V, and 1.2 V hysteresis.
What standard defines the Schmitt NAND symbol?
ANSI/IEEE 91-1984 (graphic symbols for logic functions) and IEC 60617-12 (binary logic elements) both define the hysteresis marker inscribed in a NAND gate body as the standard symbol for a Schmitt-input NAND gate. The symbol is identical in both standards except for the underlying gate body shape (curved ANSI vs rectangular IEC).
How do I use a Schmitt NAND as an oscillator?
Connect the two inputs together (making it an inverter), then connect a resistor (R) from output to input and a capacitor (C) from input to ground. The output charges the capacitor through R; when the capacitor voltage reaches VT+, the gate switches, discharging through R until VT− is reached, then repeating. Oscillation frequency is approximately f = 1/(1.2 × R × C) for a 74HC132-based oscillator.
What is the output logic function of a Schmitt NAND?
The output of a Schmitt-NAND is LOW only when both inputs A and B are simultaneously HIGH (above VT+). For all other input combinations (A LOW, B LOW, or A LOW and B HIGH), the output is HIGH. This is identical to the standard NAND truth table — the Schmitt hysteresis affects input switching thresholds only, not the logic function.
Related symbols
- AND Gate symbol
- NAND Gate symbol
- NOR Gate symbol
- NOT Gate / Inverter symbol
- OR Gate symbol
- XOR Gate symbol
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