Schmitt Trigger Threshold Calculator

Find the upper and lower switching thresholds and hysteresis width of an op-amp or comparator Schmitt trigger circuit.

🔌 Schmitt Trigger Threshold Calculator
Ω
Ω
V
V
Upper threshold (VTH+)
—
Lower threshold (VTH-)
—
Hysteresis width
—
Step-by-step working

🔌 What is the Schmitt Trigger Threshold Calculator?

The Schmitt Trigger Threshold Calculator finds the upper switching threshold VTH+, lower switching threshold VTH-, and hysteresis width of an inverting comparator Schmitt trigger circuit, from its positive feedback resistors R1 and R2 and the comparator's output saturation voltages.

Digital and analog designers use Schmitt triggers to clean up noisy or slow-edged signals before feeding them into logic gates, debounce mechanical switches, and build free-running oscillators and function generators. Knowing the exact threshold voltages is essential to confirming the circuit switches at the intended input levels.

A common point of confusion is assuming a single fixed trip point like an ordinary comparator. A Schmitt trigger's positive feedback makes its switching threshold depend on the current output state, giving two distinct trip points (VTH+ for rising input, VTH- for falling input) rather than one, which is exactly what eliminates output chatter from noise.

This calculator computes both thresholds and the hysteresis gap between them from your resistor values and output swing, and plots the resulting hysteresis loop so you can see the switching behavior visually.

📐 Formula

VTH+ = Vsat+ × R2/(R1+R2)      VTH- = Vsat- × R2/(R1+R2)
VTH+ = upper (rising) threshold, VTH- = lower (falling) threshold
R1 = feedback resistor (Ω), R2 = ground resistor (Ω)
Vsat+, Vsat- = comparator output saturation voltages (V)
Example: R1 = 10,000 Ω, R2 = 1,000 Ω, Vsat+ = 12 V, Vsat- = -12 V → VTH+ ≈ 1.0909 V, VTH- ≈ -1.0909 V.

📖 How to Use This Calculator

Steps

1
Enter the feedback resistors. Type R1, the resistor from output to the non-inverting input, and R2, the resistor from the non-inverting input to ground.
2
Enter the output saturation voltages. Type Vsat+ (output high) and Vsat- (output low, use 0 for a single-supply design).
3
Read the thresholds. The calculator reports the upper threshold VTH+, lower threshold VTH-, and the hysteresis width between them.

💡 Example Calculations

Example 1 — Dual-Supply Comparator (±12 V)

R1 = 10,000 Ω, R2 = 1,000 Ω, Vsat+ = 12 V, Vsat- = -12 V

1
ratio = 1,000 / (10,000 + 1,000) = 0.09091
2
VTH+ = 12 × 0.09091 = 1.0909 V, VTH- = -12 × 0.09091 = -1.0909 V
VTH+ = 1.0909 V, VTH- = -1.0909 V, Hysteresis = 2.1818 V
Try this example →

Example 2 — Single-Supply Comparator (0 V to 5 V)

R1 = 4,700 Ω, R2 = 1,000 Ω, Vsat+ = 5 V, Vsat- = 0 V

1
ratio = 1,000 / (4,700 + 1,000) = 0.17544
2
VTH+ = 5 × 0.17544 = 0.8772 V, VTH- = 0 × 0.17544 = 0 V
VTH+ = 0.8772 V, VTH- = 0 V, Hysteresis = 0.8772 V
Try this example →

Example 3 — Wider Hysteresis Design

R1 = 22,000 Ω, R2 = 2,200 Ω, Vsat+ = 15 V, Vsat- = -15 V

1
ratio = 2,200 / (22,000 + 2,200) = 0.09091
2
VTH+ = 15 × 0.09091 = 1.3636 V, VTH- = -15 × 0.09091 = -1.3636 V
VTH+ = 1.3636 V, VTH- = -1.3636 V, Hysteresis = 2.7273 V
Try this example →

❓ Frequently Asked Questions

What is a Schmitt trigger?+
A Schmitt trigger is a comparator circuit with positive feedback that has two distinct switching thresholds instead of one, an upper threshold VTH+ for rising input and a lower threshold VTH- for falling input, creating hysteresis that prevents rapid output chatter near a single crossing point.
What is the formula for the switching thresholds?+
For the classic inverting Schmitt trigger with feedback resistor R1 (output to non-inverting input) and R2 (non-inverting input to ground), VTH+ = Vsat+ x R2/(R1+R2) and VTH- = Vsat- x R2/(R1+R2), where Vsat+ and Vsat- are the comparator's output saturation voltages.
What is hysteresis in a Schmitt trigger?+
Hysteresis is the gap between the upper and lower switching thresholds, VTH+ minus VTH-. A larger gap makes the circuit more resistant to false triggering from noise near the switching point, at the cost of a less precise single trip point.
Why does a Schmitt trigger need two thresholds instead of one?+
A single-threshold comparator can rapidly switch output states (chatter) if a noisy input signal hovers near that threshold. By making the switching point depend on the current output state, the circuit only switches once the input has moved clearly past the previous threshold, eliminating chatter.
How do I convert this for a single-supply comparator?+
Set Vsat- to 0 V (the comparator's low output rail) and Vsat+ to its high output voltage (often close to its positive supply). The calculator then reports VTH- = 0 and VTH+ = Vsat+ x R2/(R1+R2), matching common single-supply logic-level comparator designs.
How do I get more or less hysteresis?+
Increase R2 relative to R1 (or decrease R1 relative to R2) to increase the feedback ratio R2/(R1+R2), which widens both thresholds symmetrically around the reference and increases hysteresis. Reducing that ratio narrows the hysteresis toward a single-threshold comparator.
Does the input resistor affect the threshold voltages?+
No, in the standard Schmitt trigger topology, the signal is applied directly to the inverting input with no resistor in that path affecting the threshold math, only R1 and R2 in the positive feedback network set the thresholds.
What happens if Vsat+ is not greater than Vsat-?+
The circuit requires Vsat+ greater than Vsat- for the feedback to be positive (regenerative) and produce two-state switching behavior, if this is violated the assumed topology does not function as a Schmitt trigger, and this calculator flags it as invalid input.
Is this the only Schmitt trigger topology?+
No, this calculator covers the standard inverting comparator-with-positive-feedback topology, which is the most common discrete design. Dedicated Schmitt trigger logic ICs (74HC14, etc.) have fixed threshold voltages set internally by the manufacturer, not by external resistors.
What units does this calculator use?+
Resistors R1 and R2 are entered in ohms. Output saturation voltages and the resulting threshold voltages are all in volts.
Tags: Schmitt Trigger Calculator Hysteresis Calculator Comparator Threshold Calculator Vth Calculator Positive Feedback Comparator Calculator Calculator Formula Free Online

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What is a Schmitt trigger?

A Schmitt trigger is a comparator circuit with positive feedback that has two distinct switching thresholds instead of one, an upper threshold VTH+ for rising input and a lower threshold VTH- for falling input, creating hysteresis that prevents rapid output chatter near a single crossing point.

What is the formula for the switching thresholds?

For the classic inverting Schmitt trigger with feedback resistor R1 (output to non-inverting input) and R2 (non-inverting input to ground), VTH+ = Vsat+ x R2/(R1+R2) and VTH- = Vsat- x R2/(R1+R2), where Vsat+ and Vsat- are the comparator's output saturation voltages.

What is hysteresis in a Schmitt trigger?

Hysteresis is the gap between the upper and lower switching thresholds, VTH+ minus VTH-. A larger gap makes the circuit more resistant to false triggering from noise near the switching point, at the cost of a less precise single trip point.

Why does a Schmitt trigger need two thresholds instead of one?

A single-threshold comparator can rapidly switch output states (chatter) if a noisy input signal hovers near that threshold. By making the switching point depend on the current output state, the circuit only switches once the input has moved clearly past the previous threshold, eliminating chatter.

How do I convert this for a single-supply comparator?

Set Vsat- to 0 V (the comparator's low output rail) and Vsat+ to its high output voltage (often close to its positive supply). The calculator then reports VTH- = 0 and VTH+ = Vsat+ x R2/(R1+R2), matching common single-supply logic-level comparator designs.

How do I get more or less hysteresis?

Increase R2 relative to R1 (or decrease R1 relative to R2) to increase the feedback ratio R2/(R1+R2), which widens both thresholds symmetrically around the reference and increases hysteresis. Reducing that ratio narrows the hysteresis toward a single-threshold comparator.

Does the input resistor affect the threshold voltages?

No, in the standard Schmitt trigger topology, the signal is applied directly to the inverting input with no resistor in that path affecting the threshold math, only R1 and R2 in the positive feedback network set the thresholds.

What happens if Vsat+ is not greater than Vsat-?

The circuit requires Vsat+ greater than Vsat- for the feedback to be positive (regenerative) and produce two-state switching behavior, if this is violated the assumed topology does not function as a Schmitt trigger, and this calculator flags it as invalid input.

Is this the only Schmitt trigger topology?

No, this calculator covers the standard inverting comparator-with-positive-feedback topology, which is the most common discrete design. Dedicated Schmitt trigger logic ICs (74HC14, etc.) have fixed threshold voltages set internally by the manufacturer, not by external resistors.

What units does this calculator use?

Resistors R1 and R2 are entered in ohms. Output saturation voltages and the resulting threshold voltages are all in volts.

📌 Quick Tips

💡This calculator assumes the classic inverting Schmitt trigger topology, feedback resistor R1 from output to the non-inverting input, and R2 from the non-inverting input to ground (or to a reference), with the input signal driving the inverting input.
💡For a single-supply comparator (output swings 0 V to Vcc rather than plus/minus), set Vsat- to 0, the lower threshold VTH- will also come out to 0 V in that symmetric-reference case.
💡Wider hysteresis (larger R2 relative to R1, or larger output swing) makes the circuit more immune to noise near the switching point, but also makes it slower to respond to genuine small signal changes near the thresholds.
💡Vsat+ and Vsat- should reflect the comparator's actual output swing, not the supply rails exactly, most comparators and op-amps saturate 1 to 2 volts short of their rails unless they are rail-to-rail output parts.
💡The two threshold voltages are always symmetric about zero only when Vsat+ = -Vsat-, for asymmetric or single-supply saturation voltages, use the calculator rather than assuming symmetry.