What is a Schmitt trigger?

A Schmitt trigger is a comparator circuit with positive feedback that introduces hysteresis — two different switching thresholds for rising and falling inputs. This eliminates false triggering on noisy signals.

How do you calculate Schmitt trigger thresholds?

For a non-inverting Schmitt trigger: Vth+ = Vcc × R2/(R1+R2). For an inverting Schmitt trigger with Vref = Vcc/2: Vth+ = Vref(1+R1/R2) − Vout_low×R1/R2 and Vth− = Vref(1+R1/R2) − Vout_high×R1/R2.

What is hysteresis in a Schmitt trigger?

Hysteresis is the voltage difference between the upper threshold (Vth+) and lower threshold (Vth−). A larger hysteresis makes the circuit more immune to noise but requires a larger signal swing to change state.

When should I use a Schmitt trigger instead of a regular comparator?

Use a Schmitt trigger when the input signal has noise or slow edges that could cause a regular comparator to switch multiple times. Schmitt triggers are common in signal conditioning, touch sensors, oscillators, and digital input circuits.

Schmitt Trigger Calculator

Calculate threshold voltages and hysteresis for inverting and non-inverting Schmitt trigger circuits.

Component Values

R1 (feedback)

R2 (to ground)

Supply Voltage (Vcc)

Results

Upper Threshold (Vth+)2.500 V

Lower Threshold (Vth−)0.000 V

Hysteresis (Vth+ − Vth−)2.500 V

Transfer curve with hysteresis

How does a Schmitt Trigger work?

A Schmitt trigger is a comparator with positive feedback that creates two distinct switching thresholds. When the input rises above the upper threshold (Vth+), the output switches HIGH. It stays HIGH until the input falls below the lower threshold (Vth−). This gap between thresholds is called hysteresis.

Hysteresis is the key feature that makes Schmitt triggers so useful. On a noisy signal, a regular comparator switches back and forth many times as the signal crosses the threshold. A Schmitt trigger ignores noise smaller than the hysteresis window, producing a clean, stable output.

Non-inverting Schmitt triggers output HIGH when Vin > Vth+. Inverting configurations output LOW when Vin > Vth+. Both use positive feedback from the output to the non-inverting input to create hysteresis. The amount of hysteresis is set by the R1/R2 ratio.

Non-Inverting Vth+

Vth+ = Vcc × R2 / (R1 + R2)

Key Points

Hysteresis = Vth+ − Vth− prevents oscillation on noisy signals
Larger R1/R2 ratio → wider hysteresis window
Non-inverting: output follows input direction
Inverting: output is opposite to input direction

Applications

Touch and capacitive sensor debouncing
Square wave oscillator with RC timing
Level detection with noise immunity
Digital input signal conditioning

Practical Examples

5 V supply, 3.18 V hysteresis

Non-inverting Schmitt trigger with R1 = 10 kΩ and R2 = 47 kΩ from a 5 V supply. Gives a wide hysteresis for noisy environments.

VH = 5 × 47/(10+47) = 4.12 V · VL = 5 × 10/(10+47) = 0.88 V · Hysteresis = 3.24 V

Switch debounce circuit

A mechanical switch with 100 mV bounce on a 3.3 V logic signal. Use a Schmitt trigger input (74HC14) with a 100 Ω/100 nF RC filter for clean output.

RC time constant = 10 µs > bounce duration; 74HC14 threshold: VH ≈ 2.0 V, VL ≈ 1.2 V

Formula Reference

Schmitt trigger thresholds (non-inverting, op-amp)

V_th+ = Vcc × R1/(R1+R2)  (upper threshold, output high)
V_th– = 0  (lower threshold when Vout swings low, simplified)

More precisely with ±Vcc supply:
V_th+ = +Vout × R1/(R1+R2)
V_th– = –Vout × R1/(R1+R2)
Hysteresis: ΔV = V_th+ – V_th– = 2×Vout×R1/(R1+R2)

74HC14 (inverting Schmitt, 5V): V_th+ ≈ 3.15V, V_th– ≈ 1.55V, ΔV ≈ 1.6V

More Examples

Noise-immune push button (op-amp, ±12V supply)

R1=10kΩ, R2=100kΩ: V_th+ = 12×10/110 = 1.09V, V_th– = –1.09V, ΔV = 2.18V. Signal must cross ±1.09V to toggle — ignores noise < 1V.

Hysteresis: 2.18V · Thresholds: ±1.09V

Square wave oscillator (op-amp Schmitt)

R1=10kΩ, R2=100kΩ, R_t=10kΩ, C=100nF: f ≈ 1/(2×R_t×C×ln(1+2×R1/R2)) = 1/(2×10k×100n×ln(1.2)) ≈ 454Hz

Frequency: ≈ 454 Hz

Design tip

Use 74HC14 (6× inverting Schmitt) for simple digital noise immunity.
Op-amp Schmitt: use rail-to-rail op-amp for single-supply designs (LMV358, TLV2371).
Hysteresis should be > 3× peak noise amplitude to prevent chattering.
Add RC filter before Schmitt input for extra noise rejection.

Did you know? The Schmitt trigger was invented by Otto Schmitt in 1934 while studying nerve impulse propagation — he was trying to model biological neurons, not design electronics. The hysteresis principle he discovered is now fundamental to noise-immune digital logic and signal conditioning.