555 Timer Calculator (Monostable Mode)
Calculate the one-shot pulse width for a 555 timer in monostable mode. Enter R and C to find the pulse duration T = 1.1 × R × C, or solve for any component given the desired pulse width.
How to Use
- Choose which value to solve for — T (pulse width), R, or C.
- Enter the two known values. Suffixes work: 10k, 4.7M, 100nF, 22uF.
- The waveform shows trigger (low-going pulse) and the resulting output pulse.
- Keep R between 1kΩ and 10MΩ for reliable operation.
- A trigger input narrower than T will start the pulse; longer triggers behave unpredictably.
Show Work
Formulas
History of the 555 Timer
The 555 timer was designed by Hans Camenzind at Signetics in 1971 and entered production in 1972. Camenzind worked on the chip independently after his original pitch was shelved; Signetics paid him a one-time fee and no royalties. The part went on to become one of the highest-volume integrated circuits ever manufactured — an estimated billion-plus units a year still ship decades after introduction.
The monostable configuration is arguably the chip's most intuitive application. Before triggering, the output sits low and the timing capacitor is held at 0V by the internal discharge transistor. A negative edge on pin 2 (trigger) pulls it below 1/3 Vcc, flipping the internal SR flip-flop; the output goes high and the discharge transistor turns off, releasing the capacitor to charge through the external resistor R. When the capacitor voltage reaches 2/3 Vcc, the threshold comparator resets the flip-flop, the output returns to low, and the capacitor dumps back through the discharge transistor — ready for the next trigger.
The elegance of the ratiometric design is that pulse width is independent of supply voltage. Vcc cancels out of the math because both the trigger threshold (1/3 Vcc) and the reset threshold (2/3 Vcc) scale with it. That's why a 555 monostable calibrated on a bench with 9V still behaves correctly when you later run it from 12V or 5V.
About This Calculator
This calculator solves the RC charging equation Vc(t) = Vcc × (1 − e−t/RC) at the specific threshold Vc = 2/3 Vcc, giving the classic 555 monostable pulse width T = RC × ln(3) ≈ 1.0986 RC (rounded to 1.1 in most datasheets). Enter any two of R, C, or T with standard engineering suffixes (k, M, nF, µF, pF) and the third is computed instantly; the waveform preview shows the trigger edge, the charging curve, and the output high window.
Everything runs entirely in your browser — no values are sent to a server. Modern alternatives like the 74HC123 (retriggerable one-shot), CD4538 (precision one-shot), and the CMOS LMC555 exist, but the bipolar NE555 remains the teaching standard and the default in most hobbyist and classroom projects.
Frequently Asked Questions
What is monostable mode?
Monostable means one stable state — the output sits low until triggered, then goes high for a fixed time T, then returns low. It's a timed one-shot pulse. Use it for debouncing switches, generating fixed-width pulses from variable triggers, and adding delay lines.
Where does the 1.1 come from?
It's ln(3) ≈ 1.0986. The 555 in monostable mode starts charging C from 0V through R; the output flips low when C reaches 2/3 Vcc. The time for an RC circuit to charge from 0 to 2/3 of Vcc is ln(3) × RC ≈ 1.1 × RC.
How do I trigger it?
Pull pin 2 (trigger) below 1/3 Vcc momentarily — a negative-going edge. The trigger can be shorter than T, and the output will still stay high for the full pulse width. If the trigger is held low longer than T, the output stays high until the trigger releases.
What's the useful range of R and C?
R: 1kΩ to 10MΩ. Below 1kΩ you stress the discharge transistor; above 10MΩ leakage currents dominate. C: 100pF to 1000µF. Electrolytic caps above ~10µF have significant leakage and tolerance spread — use film or tantalum for accuracy.
Can the pulse width be retriggered?
Not in standard monostable. If the trigger fires again during T, the pulse simply continues — it doesn't extend. For retriggerable behavior use a 74HC123 or CD4538, which restart the timing on each trigger.
How accurate is the pulse width?
Timing accuracy is limited by R (±1-5% metal film), C (±5-20% ceramic/electrolytic), and the 555's internal comparator threshold (±5% typical). For tight tolerance, use 1% metal film + film cap and calibrate. Worst case: expect ±10-20% across temperature and supply variation.
Common Use Cases
Switch Debounce
Filter mechanical bounce with a fixed 20-50ms pulse on every press. R=10k, C=4.7µF gives ~52ms — clean single trigger from a bouncy switch.
PIR Motion → Light Timer
Trigger a relay/transistor for N seconds when motion is detected. Classic closet or hallway light timer at 30-300 seconds.
Button-to-Pulse Converter
Press a button of any duration and generate a precise 1ms pulse for a microcontroller interrupt, camera trigger, or flash unit.
Missing Pulse Detector
Monostable re-triggered by each pulse — if a pulse is missing, the monostable times out and flags the error.
Delay Line
Chain two monostables to get an "N seconds after trigger" output. First sets delay, second sets output pulse width.
Fixed-Width Trigger for Stepper/Servo
Generate consistent 1-2ms pulses for R/C servos, or stepper-driver STEP lines, from noisy sensor inputs.
Last updated: