555 Timer Calculator — astable and monostable
Design astable, monostable and 50%-duty 555 circuits. Get the schematic, the waveform, and a bill of materials in stock E24 values.
Example: An astable 555 with R1 = 10 kΩ, R2 = 68 kΩ and C = 100 nF oscillates at 98.81 Hz with a 53.4% duty cycle.
Check it against real silicon
Chiprun runs your firmware on an emulated microcontroller and hands back the UART output, so you can confirm these numbers rather than trusting them.
Formula
astable: f = 1.44 / ((R1 + 2×R2) × C) monostable: T = 1.1 × R1 × CWorked example
An astable 555 with R1 = 10 kΩ, R2 = 68 kΩ and C = 100 nF oscillates at 98.81 Hz with a 53.4% duty cycle.
t_high = ln(2) × (R1 + R2) × C
t_high = 0.693 × 78 kΩ × 100 nF
t_high = 5.407 ms
Charging goes through R1 and R2 in series.
t_low = ln(2) × R2 × C
t_low = 0.693 × 68 kΩ × 100 nF
t_low = 4.713 ms
Discharging goes through R2 only — which is why the output is never symmetric.
f = 1 / (t_high + t_low) ≈ 1.44 / ((R1 + 2×R2) × C)
f = 1 / 10.12 ms
f = 98.81 Hz
Frequently asked questions
How do I calculate 555 astable frequency?
f = 1.44 / ((R1 + 2 × R2) × C). The capacitor charges through R1 and R2 in series, then discharges through R2 alone, and those two times added together give the period.
Why can't I get a 50% duty cycle from a 555?
Because charging goes through R1 + R2 while discharging goes through R2 only, so the high time is always longer than the low time. Making R2 much larger than R1 gets you asymptotically close to 50%. For a true 50%, put a diode across R2 so charging bypasses it, or divide a doubled frequency with a flip-flop.
What is the difference between astable and monostable?
Astable free-runs, producing a continuous square wave with no input. Monostable produces one pulse of a fixed width each time it is triggered, then waits. Astable needs both R1 and R2; monostable needs only R1.
Does the 555's timing depend on supply voltage?
No, and that is its main appeal. The comparator thresholds are set at ⅓ and ⅔ of Vcc by an internal divider, so the RC charging curve crosses them at the same fraction regardless of supply. Timing is set by R and C alone.
What is the capacitor on pin 5 for?
Pin 5 exposes the internal ⅔ Vcc reference. A 10 nF capacitor from it to ground keeps supply noise off that reference. Without it, a 555 switching a heavy load can retrigger itself on its own supply ripple — a very common cause of erratic output.
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