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Schmitt Trigger Calculator

Electronics & PhysicsPhysics Reference
Circuit
Configuration
Voltages
Resistors

Inverting: Vref to the + input. Non-inverting: signal to the + input.

From the output back to the + input.

Hysteresis width —
Upper threshold V_TH (V)
—
Lower threshold V_TL (V)
—
Window centre (V)
—

A Schmitt trigger switches at two different input voltages. For the inverting circuit, each threshold is Vref·(1−β) plus the output level times β, where β = R1/(R1+R2). With ±12 V output swing, Vref = 0, R1 = 10 kΩ and R2 = 100 kΩ, the thresholds are ±1.091 V and the hysteresis is 2.182 V.

About this tool

Comparators chatter when a slow or noisy signal crawls past a single threshold. Positive feedback fixes that by splitting the threshold in two, and the gap between them is the hysteresis. This calculator handles the two classic op-amp layouts: the inverting trigger, where the signal goes to the minus pin and a divider sets the plus pin, and the non-inverting trigger, where the signal enters through R1 and the minus pin sits at a fixed reference. Enter the real output high and low levels of your part (rail-to-rail or not), the reference and both resistors, and it returns both thresholds, the window centre and the hysteresis width, with the working. It assumes an ideal op-amp: no input bias current, no offset voltage, and an output that settles fully at its two levels. Open-collector comparators with a pull-up need the pull-up included in R2.

How to use it

  1. Pick the configuration

    Inverting if the signal drives the − input, non-inverting if it reaches the + input through R1.

  2. Enter the output levels

    Use the measured or datasheet saturation voltages, for example 0 V and 4.9 V for a rail-to-rail part on 5 V.

  3. Set Vref

    Zero for a split-supply circuit centred on ground, or the divider voltage on a single supply.

  4. Enter R1 and R2 in kΩ

    Thresholds, window centre and hysteresis width update as you type.

Examples

±12 V op-amp, 10 kΩ and 100 kΩ

Configuration
Inverting
Output high level (V_OH)
12
Output low level (V_OL)
-12
Reference voltage (Vref)
0
R1
10
R2 (feedback)
100

Result Hysteresis width: 2.1818
Upper threshold V_TH (V): 1.0909
Lower threshold V_TL (V): -1.0909
Window centre (V): 0

  1. Feedback fraction β = R1 ÷ (R1+R2) = 10 ÷ 110 = 0.09091
  2. V_TH = Vref·(1−β) + V_OH·β = 1.0909 V
  3. V_TL = Vref·(1−β) + V_OL·β = -1.0909 V
  4. Hysteresis = V_TH − V_TL = 2.1818 V

With the reference at ground the window is symmetric: β = 10/110, so the output flips at +1.09 V going up and −1.09 V going down.

Non-inverting, ±10 V, R1 = 10 kΩ, R2 = 50 kΩ

Configuration
Non-inverting
Output high level (V_OH)
10
Output low level (V_OL)
-10
Reference voltage (Vref)
0
R1
10
R2 (feedback)
50

Result Hysteresis width: 4
Upper threshold V_TH (V): 2
Lower threshold V_TL (V): -2
Window centre (V): 0

  1. V_TH = (Vref·(R1+R2) − V_OL·R1) ÷ R2 = (0×60 − -10×10) ÷ 50 = 2 V
  2. V_TL = (Vref·(R1+R2) − V_OH·R1) ÷ R2 = (0×60 − 10×10) ÷ 50 = -2 V
  3. Hysteresis = V_TH − V_TL = 4 V

Thresholds are ±V_sat·R1/R2 = ±10 × 10/50 = ±2 V, giving a 4 V dead band.

5 V single supply, 2.5 V reference

Configuration
Inverting
Output high level (V_OH)
5
Output low level (V_OL)
0
Reference voltage (Vref)
2.5
R1
10
R2 (feedback)
10

Result Hysteresis width: 2.5
Upper threshold V_TH (V): 3.75
Lower threshold V_TL (V): 1.25
Window centre (V): 2.5

  1. Feedback fraction β = R1 ÷ (R1+R2) = 10 ÷ 20 = 0.5
  2. V_TH = Vref·(1−β) + V_OH·β = 3.75 V
  3. V_TL = Vref·(1−β) + V_OL·β = 1.25 V
  4. Hysteresis = V_TH − V_TL = 2.5 V

Equal resistors give β = 0.5, so the thresholds sit at 2.5 ± 1.25 V: 3.75 V and 1.25 V.

How it is calculated

Inverting: β = R1/(R1+R2); V_TH = Vref(1−β) + V_OH·β; V_TL = Vref(1−β) + V_OL·β. Non-inverting: V_TH = (Vref(R1+R2) − V_OL·R1)/R2; V_TL = (Vref(R1+R2) − V_OH·R1)/R2. Hysteresis = V_TH − V_TL.

V_OH, V_OL
Output high and low saturation levels
Vref
Reference voltage
R1, R2
Input/reference resistor and feedback resistor
β
Positive feedback fraction

The voltage at the + input is found by superposition of the two sources feeding it through R1 and R2. The output flips when the input crosses the other pin's voltage, and because the output level appears in that sum, the crossing point depends on the current output state. That gives two thresholds. In the inverting layout the hysteresis is β·(VOH − VOL); in the non-inverting layout it is (R1/R2)·(VOH − VOL).

When not to use it

  • Logic-gate Schmitt inputs such as the 74HC14 have fixed thresholds from the datasheet, not from resistors.
  • Comparators with built-in hysteresis add their own offset that this model ignores.
  • Transistor-pair Schmitt triggers follow different equations.

Common mistakes

  • Using the supply rails as VOH and VOL when the op-amp cannot swing rail to rail.
  • Swapping R1 and R2, which changes the hysteresis by a large factor.
  • Forgetting the pull-up resistor on an open-collector comparator output.
  • Entering resistors in ohms while the fields expect kilohms.

Frequently asked questions

What does hysteresis do in a Schmitt trigger?

It makes the input threshold depend on the output state. Once the output has gone high, the input must fall well below the point where it switched up before the output returns low. Noise smaller than the hysteresis width can no longer cause repeated toggling near the threshold.

How much hysteresis should I design for?

A common starting point is a few times the peak-to-peak noise on the signal. Too little and the output still chatters; too much and the circuit ignores real small changes. Many designs land between 50 mV and a few hundred millivolts for sensor signals.

Why does the inverting output go low when the input rises?

The signal drives the inverting pin. When it climbs above the upper threshold set on the + pin, the difference turns negative and the output drops to its low level, which in turn lowers the + pin voltage to the lower threshold.

How do I centre the window away from zero?

Set Vref. In the inverting layout the centre sits at Vref·(1−β) plus the average output level times β. On a 5 V single supply with output 0–5 V, Vref at 2.5 V and equal resistors, the window is centred on 2.5 V.

Do the resistor values matter or only their ratio?

For the thresholds only the ratio matters. The absolute values set the current drawn from the output and reference, and the input impedance seen by the signal in the non-inverting layout. Values from 10 kΩ to a few hundred kΩ are typical.

Can a threshold be outside the output swing?

Yes, mainly in the non-inverting layout when R1 is larger than R2. If the input can never reach that threshold the output latches in one state. The calculator flags that case.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 23). Schmitt Trigger Calculator. Tools Directory. Retrieved October 26, 2026, from https://tools.milunir.com/electronics-physics/physics-reference/schmitt-trigger-calculator/

MLA

Tools Directory Editorial Team. “Schmitt Trigger Calculator.” Tools Directory, 23 Oct. 2026, https://tools.milunir.com/electronics-physics/physics-reference/schmitt-trigger-calculator/. Accessed 26 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Schmitt Trigger Calculator.” Tools Directory. Last modified October 23, 2026. Accessed October 26, 2026. https://tools.milunir.com/electronics-physics/physics-reference/schmitt-trigger-calculator/.

Link<a href="https://tools.milunir.com/electronics-physics/physics-reference/schmitt-trigger-calculator/">Schmitt Trigger Calculator – Tools Directory</a>