Schmitt Trigger Calculator

Otto Schmitt invented the trigger circuit that bears his name in 1934 while still a graduate student at Washington University in St. Louis. He was trying to solve a biological-measurement problem — detecting nerve impulses in squid axons — and needed a circuit that would fire cleanly on a slow, noisy input. His solution used two vacuum tubes with cross-coupled feedback, producing the now-classic hysteresis transfer function.

Schmitt went on to a distinguished career in biophysics at Washington University and later MIT, coining the term "biomimetics" in 1957 for engineering inspired by biological systems. His eponymous trigger became a workhorse of digital electronics in the 1950s, appearing as input stages in flip-flops, pulse counters, and the SN7414 hex Schmitt inverter (1970) that made hysteretic buffering a standard TTL function.

Modern microcontrollers include Schmitt-trigger buffers on every digital input pin — without them, slow-rising signals (mechanical switches, RC-filtered inputs, charging lines) would produce multiple spurious edges as they crossed the logic threshold. The CMOS Schmitt version (CD40106, 74HC14, 74LVC14) is essentially the same hysteresis concept implemented with cross-coupled MOSFET pairs.

Enter R1 (feedback), R2 (input), and the output high/low voltages. The tool computes the upper switching threshold VH = Vout_high × R1/(R1+R2), lower threshold VL = Vout_low × R1/(R1+R2), total hysteresis VH − VL, and the center voltage. The feedback ratio R1/(R1+R2) controls the hysteresis fraction of output swing.

Sizing tip: make hysteresis at least 2× your expected noise amplitude. For debouncing a mechanical switch, 0.5-1 V hysteresis is ample. For cleaning up a thermocouple-level signal (millivolts), use a proper instrumentation amp with offset-zero and then Schmitt the comparator output. Everything runs client-side; no values leave your browser.