Schmitt Trigger Calculator (Comparator Hysteresis Thresholds)
By Santaji GadepatilinUpdated November 6, 2026
A Schmitt trigger adds positive feedback to a comparator so it switches at two different input levels, which stops noisy, slow signals from causing chatter at the output. This Schmitt trigger calculator gives the upper and lower thresholds and the hysteresis band for an inverting comparator — see Schmitt trigger hysteresis and comparator circuits explained.
How to use the Schmitt trigger calculator
Enter the reference voltage, the two resistors of the positive-feedback divider and the comparator’s output high and low levels. The signal drives the inverting input. The calculator returns the two switching points, the hysteresis and the centre of the band. To design, choose the hysteresis you need and adjust R₂/R₁.
Schmitt trigger formulas
VTH = (Vref R₂ + VOH R₁) / (R₁ + R₂) · VTL = (Vref R₂ + VOL R₁) / (R₁ + R₂)
VH = VTH − VTL = (VOH − VOL) R₁ / (R₁ + R₂)
Worked example
Vref = 2.5 V, R₁ = 10 kΩ, R₂ = 100 kΩ, rail-to-rail output 0–5 V:
- VTH = (2.5 × 100k + 5 × 10k) ÷ 110k = 2.7273 V
- VTL = (2.5 × 100k) ÷ 110k = 2.2727 V
- Hysteresis = 454.5 mV, centred on 2.500 V
Hysteresis vs R₂/R₁ (Vref = 2.5 V, output 0–5 V)
| R₂ / R₁ | Hysteresis | VTH | VTL |
|---|---|---|---|
| 2 | 1,667 mV | 3.333 V | 1.667 V |
| 5 | 833 mV | 2.917 V | 2.083 V |
| 10 | 455 mV | 2.727 V | 2.273 V |
| 20 | 238 mV | 2.619 V | 2.381 V |
| 50 | 98 mV | 2.549 V | 2.451 V |
| 100 | 50 mV | 2.525 V | 2.475 V |
Notes
- The output goes low when the input rises above VTH and high when it falls below VTL (inverting action).
- Positive feedback shifts the band centre above Vref; to centre it, lower Vref or add a resistor from the + input to ground.
Frequently asked questions
How do you calculate Schmitt trigger thresholds?
For an inverting comparator with R₁ from V_ref and R₂ from the output to the + input, V_TH = (V_ref·R₂ + V_OH·R₁)/(R₁ + R₂) and V_TL = (V_ref·R₂ + V_OL·R₁)/(R₁ + R₂).
What is the hysteresis of a Schmitt trigger?
The gap between the thresholds, V_H = (V_OH − V_OL) × R₁/(R₁ + R₂). A larger R₂ gives less hysteresis.
How much hysteresis should I use?
More than the peak-to-peak noise on the input — commonly 1–5% of the signal range, or a few tens of millivolts for a clean signal.
Does an open-drain comparator change the calculation?
Yes. The output high level is set by the pull-up resistor, which also sits in series with R₂, so V_OH is lower than the pull-up supply unless R₂ is much larger than the pull-up.