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Schmitt Trigger Thresholds Calculator

Enter the saturation voltage and resistors to get the thresholds.

How it is calculated

For the classic non-inverting op-amp Schmitt trigger the two switching thresholds are equal in size and opposite in sign: VT = ± (R1 ÷ R2) × Vsat. Vsat is the output saturation voltage in volts (the level the op-amp output swings to, taken as symmetric, +Vsat and −Vsat), R1 is the resistor from the input signal and R2 is the feedback resistor from the output. Only the ratio matters, so both resistors can be in ohms, kilohms or any other unit as long as they match. The tool shows the upper threshold, the lower threshold and the hysteresis width 2 × VT. Both resistors must be above zero and Vsat must not be negative. It assumes an ideal op-amp and a symmetric supply: with a single supply or an output that does not reach the rails, the real thresholds shift.

Worked example

An op-amp that saturates at ±12 V with R1 = 10 kΩ and R2 = 47 kΩ. The ratio is 10 ÷ 47 ≈ 0.2128. Upper threshold: 0.2128 × 12 ≈ 2.55 V. Lower threshold: −2.55 V. The hysteresis band is twice that, 2 × 2.553 ≈ 5.11 V (computed from the unrounded value, so it is not exactly 2 × 2.55). The output flips high only once the input rises above +2.55 V and flips low only after it falls below −2.55 V.

Try this example →

Common mistakes

  • Swapping R1 and R2. The thresholds scale with R1 ÷ R2, so inverting the ratio gives completely different values (47 ÷ 10 × 12 = 56.4 V).
  • Using the supply voltage as Vsat. Many op-amps saturate one or two volts short of the rails, which shifts both thresholds.
  • Using this formula for a single-supply circuit or a reference-biased trigger. The tool assumes symmetric ±Vsat and thresholds centred on zero.
  • Entering resistances in different units (one in ohms, the other in kilohms). The ratio is then wrong by a factor of 1,000.
  • Reading the hysteresis as one threshold. The band between the two thresholds is 2 × VT, the full gap the input must cross to flip the output.

About this calculator

A Schmitt trigger is a comparator circuit with built-in hysteresis — it switches output states at a different input voltage going up than coming down, which prevents a noisy or slowly-changing input signal from triggering multiple false transitions. For the classic non-inverting op-amp version, the upper and lower thresholds are VT = ±(R1/R2)·Vsat, where Vsat is the op-amp's output saturation voltage and R1/R2 sets the feedback ratio; the gap between the two thresholds is the hysteresis band.

Analog and digital design engineers use Schmitt triggers to clean up slow or noisy signals — squaring up a sine wave into a clean digital pulse, debouncing a mechanical switch, or building reliable oscillators — and calculating the exact thresholds up front is essential for setting the right noise margin for a given application.

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