555 Timer Calculator
Frequency, duty cycle and pulse width for astable and monostable 555 circuits.
How it works
The 555 is a comparator pair and a flip-flop watching a capacitor charge and discharge between one third and two thirds of the supply. Every equation follows from that, and the useful consequence is that none of them contain the supply voltage — the thresholds are ratios of Vcc, so the timing is the same at 5 V as at 15 V.
Astable: the capacitor charges through R1 + R2 and discharges through R2 alone.
t(high) = 0.693 × (R1 + R2) × C
t(low) = 0.693 × R2 × C
f = 1.44 ÷ ((R1 + 2R2) × C)
Monostable: one pulse per trigger, t = 1.1 × R × C.
Because charge goes through both resistors and discharge through only one, a standard astable can never reach 50% duty — the high time always exceeds the low time. Getting to 50% or below needs a diode across R2 so charging bypasses it, or one of the CMOS variants. Design mode here accounts for the diode when the target duty is 50% or less and says so.
Practical limits are worth knowing. Above roughly 500 kHz the internal propagation delays start to dominate and the output stops being trustworthy. Below about 1 kΩ the discharge transistor is being asked for too much current; above about 1 MΩ the capacitor's own leakage becomes comparable to the charging current and timing drifts with temperature and humidity. Electrolytic capacitors have tolerances of ±20% and worse, so a 555 timing a long interval is approximate by construction — if you need accuracy, use a crystal.
The other thing every 555 circuit needs is a 100 nF decoupling capacitor across the supply, right at the chip. The output stage briefly shorts the rail during transitions and the resulting spike will retrigger the chip through its own supply if you let it.
Common questions
Why can't I get 50% duty cycle?
Because the capacitor charges through R1 + R2 and discharges through R2 only, so the high time is always longer. Put a diode across R2 — anode to the R1 side — so charging bypasses R2 entirely, and then equal resistors give equal times.
What are sensible resistor values?
Between about 1 kΩ and 1 MΩ. Below 1 kΩ the discharge transistor is overloaded and the chip heats up; above 1 MΩ capacitor leakage becomes comparable to the charging current and the timing wanders.
Does the supply voltage affect the timing?
No, and that is the elegant part. The thresholds are one third and two thirds of Vcc, and the capacitor charges toward Vcc, so the ratios cancel exactly. Timing is set by R and C alone.
Why does my circuit trigger randomly?
Missing decoupling, nearly always. Put 100 nF across the supply pins at the chip, and 10 nF from the control pin to ground. The 555's output stage causes a large supply spike as it switches, which comes straight back in through the threshold comparators.