A Schmitt trigger calculator determines the input, ground, and feedback resistor values needed to set exact upper and lower thresholds for a comparator’s hysteresis circuit design.
Calculate Schmitt Trigger Resistor Values for Comparator Hysteresis Circuits
This calculator solves for the input, ground, and feedback resistor values of a three-resistor Schmitt trigger network from your target upper and lower switching thresholds. Analog and embedded hardware engineers use it when converting a hysteresis specification for sensor interfacing, switch debouncing, or relaxation oscillator circuits into buildable resistor values.
Entering Your Threshold and Supply Values
Enter reference voltage Vref (V), output voltage high Voh (V) and low Vol (V), your target high threshold Vth (V) and low threshold Vtl (V), and the maximum input current Iim (mA) the source can safely supply. The tool returns hysteresis width, upper and lower trip points, and the input, ground, and feedback resistor values (R1, R2, R3 in kΩ).
How the Resistor Values Are Derived
The core relationship comes from the standard op-amp comparator with positive-feedback hysteresis, treated in Sedra and Smith’s Microelectronic Circuits and in Horowitz and Hill’s The Art of Electronics: the switching thresholds are set by how much of the output swing is fed back into the reference node relative to the fixed reference voltage.
Hysteresis width and center voltage follow directly from your target thresholds:
$$\Delta V = V_{th} – V_{tl}$$
$$V_{center} = \frac{V_{th}+V_{tl}}{2}$$
R1 is sized first as a plain current-limiting resistor against your Iim spec — Ohm’s Law, nothing more:
$$R_1 = \frac{\Delta V}{I_{im}}$$
R3 sets the hysteresis band against the output swing, from Kirchhoff’s Current Law at the summing node:
$$R_3 = R_1 \times \frac{V_{oh}-V_{ol}}{\Delta V}$$
R2 comes from that same node equation, solved alongside R1 and R3 so the network hits both Vth and Vtl at your chosen Vref. There’s no separate textbook formula for it — three resistors, three equations, Kirchhoff’s Current Law, solved together.
One common mistake: R1’s voltage drop at Iim isn’t just a noise-filtering afterthought. Because it sits in series with the same node that sets your thresholds, an undersized or oversized R1 shifts Vth and Vtl away from what you specified, even though Voh, Vol, and Vref stay fixed.
A second mistake is treating Voh and Vol as supply rail voltages rather than the comparator’s actual loaded output-high and output-low levels, which on real parts usually sit a few hundred millivolts inside the rails. A third is ignoring that R3 draws current from the output on every switch, which can overload a weak reference source if R2 is also small.
When R2 comes back as “Open (∞),” it means your chosen Vref already produces the correct summing-node voltage using only R1 and R3 — no path to ground is needed to shift the threshold further, so the shunt resistor mathematically drops out of the network. That’s specific to how Vref interacts with your Vth and Vtl targets, not something a generic hysteresis calculator would flag.
Schmitt Trigger Resistor Network Diagram
Questions About Sizing a Schmitt Trigger Resistor Network
Why does R2 sometimes show as Open (∞)?
When your Vref already produces the correct summing-node voltage using only R1 and R3, no shunt to ground is needed to shift the threshold further, so R2 mathematically drops out of the network.
Does this work for an inverting Schmitt trigger?
This tool solves the topology where Vin, Vref, and Vout all feed a shared summing node compared against a fixed internal reference — it isn’t set up for the classic two-resistor non-inverting divider circuit.
Why is R1 based on Iim instead of just noise filtering?
R1 sits in the same current path that sets your switching thresholds, so sizing it from your maximum input current keeps its voltage drop from silently shifting Vth and Vtl off target.
What happens if I set Vth lower than Vtl?
The hysteresis width ΔV becomes negative, which isn’t physically buildable — swap the two values so Vth is the higher threshold and Vtl is the lower one.
Does the calculator account for real comparator delay?
No. It solves ideal resistor math only. Real comparators add propagation delay and finite output impedance, so check your specific device’s datasheet and add margin before finalizing values.