555 Timer Calculator (Missing Pulse Detector)

Missing-pulse detection has its roots in 1950s aerospace telemetry: ground stations needed to know within seconds if a satellite's telemetry beacon failed. Vacuum-tube retriggerable one-shots watched the pulse stream; when it stopped, an alarm fired. The same principle governs watchdog timers on modern MCUs and safety-critical industrial controllers.

The 555 variant appeared in Signetics' 1972 application notes shortly after the chip's release: add an NPN transistor from the input to the timing-capacitor side (collector on C, base through a small cap to the input), and each input pulse briefly shorts C to ground, restarting the timer. The output then stays high as long as input pulses arrive faster than T = 1.1·R·C; it drops when the interval exceeds the timer.

Modern MCU watchdog timers are conceptually identical: a counter increments on each clock tick and resets each time the software "pets the watchdog" by writing a specific register. If the software hangs, the counter reaches its terminal value and fires a reset. The underlying principle — retriggerable timeout with a specific expected period — dates straight back to the 555 application in 1972.

Enter the nominal input pulse period T_in (for a 10 Hz tachometer this is 100 ms), the detection margin k (typically 1.5 means "trigger alarm if a pulse is 50% late"), and either R or C. The tool computes the required timeout T_RC = k × T_in, then solves for the remaining component via T_RC = 1.1 × R × C. The readout shows the expected input frequency and margin ratio for sanity checking.

Remember to add the retrigger path — a small NPN transistor (2N3904, 2N2222) with its collector on the timing capacitor, emitter on ground, base driven by the input through a ~10 kΩ resistor. Without this path the 555 behaves as a plain (non-retriggerable) monostable and ignores input pulses during the timing window. Everything runs client-side; no values leave your browser.