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555 Timer Calculator

Compute astable frequency and duty or monostable pulse width, and reverse-design resistors for a target.

Calculator

Configuration
Frequency
687 Hz
Period
1.46 ms
Duty cycle
52.4 %
Time high
762 µs
Time low
693 µs

Reverse design — pick R1/R2 for a target

R1
3 kΩ
R2
6.2 kΩ
Actual frequency
937 Hz
Actual duty
59.7 %

How 555 Timer Calculator works

The 555 is a comparator-and-flip-flop timing circuit built around an internal resistor chain that sets two thresholds at one third and two thirds of the supply. An external capacitor charges toward the supply through your resistors; when it crosses the upper threshold the output flips low and an internal transistor discharges the capacitor; when it falls past the lower threshold the output flips high and charging resumes.

In astable mode the chip free-runs as an oscillator. The capacitor charges through R1 and R2 in series and discharges through R2 alone, which is why the output is asymmetric: the high time is always longer than the low time. Frequency is f = 1.44 / ((R1 + 2R2) × C), and duty cycle is (R1 + R2) / (R1 + 2R2) — a value that can approach 50% as R2 grows relative to R1, but can never reach or go below it in the classic configuration.

In monostable mode the chip produces one pulse per trigger. A falling edge on the trigger pin starts the capacitor charging through a single resistor, and the output stays high for T = 1.1 × R × C until the upper threshold is reached. The 1.1 factor is ln(3), which is the time constant multiple needed to charge from zero to two thirds of the supply.

Notably, none of these equations contain the supply voltage. Because the thresholds are ratios of the supply set by an internal divider, the timing is supply-independent — a genuinely useful property that makes the 555 tolerant of an unregulated rail.

Reference

  • Astable frequency: f = 1.44 / ((R1 + 2 × R2) × C)
  • Astable high time: t_high = 0.693 × (R1 + R2) × C
  • Astable low time: t_low = 0.693 × R2 × C
  • Astable duty cycle: D = (R1 + R2) / (R1 + 2 × R2) — always above 50%
  • Monostable pulse width: T = 1.1 × R × C

How to use this calculator

  1. Pick a mode

    Astable for a continuous oscillator, monostable for a single timed pulse per trigger.

  2. Enter components or a target

    Supply R1, R2, and C to find the resulting timing, or enter the frequency and duty you want and let the tool reverse-design the resistors.

  3. Choose realistic values

    Keep resistors between about 1 kΩ and 1 MΩ, and prefer a larger capacitor over a very large resistor for long periods.

  4. Check the duty cycle

    Confirm the achievable duty suits your application, or plan for the diode modification if you need 50% or below.

Worked examples

A 1 Hz blinker

Given
R1 = 10 kΩ, R2 = 68 kΩ, C = 10 µF
Result
f ≈ 0.99 Hz, duty ≈ 53%

The classic LED flasher. Making R2 much larger than R1 pushes the duty cycle close to the 50% floor.

A one-second pulse

Given
Monostable with R = 91 kΩ, C = 10 µF
Result
T = 1.0 s

Standard E24 values landing on a round result. Electrolytic capacitor tolerance of ±20% dominates the real accuracy.

Why duty cannot go below 50%

Given
R1 = 1 kΩ, R2 = 1 MΩ
Result
Duty ≈ 50.05%

The charge path always includes R1, the discharge path never does. A diode across R2 bypasses R1 on charge and breaks the limit.

When to use it

  • Building a blinking indicator or a simple LED flasher.
  • Generating a clock for a counter, shift register, or other logic without a microcontroller.
  • Producing a fixed-length pulse from a momentary button press — a debounced one-shot.
  • Creating an adjustable tone for a piezo buzzer or small speaker.
  • Making a power-on delay or a simple timeout circuit.

Things to watch out for

  • The timing capacitor dominates accuracy. Electrolytics are typically ±20% and drift with temperature and age; use film or C0G ceramic where the period matters.
  • The classic astable cannot reach 50% duty or below. A diode in parallel with R2 gives the charge path its own route and removes the restriction.
  • The 555 draws a large current spike at every output transition. Fit a 100 nF decoupling capacitor directly at the supply pin or the chip will retrigger itself on its own noise.
  • The bipolar NE555 sinks and sources up to 200 mA but drops well over a volt doing so. CMOS variants such as the TLC555 are cleaner and run at lower voltages, with far less supply disturbance.

Frequently asked questions

What is the 555 astable frequency formula?

f = 1.44 / ((R1 + 2 × R2) × C). The high time is 0.693 × (R1 + R2) × C and the low time is 0.693 × R2 × C, because the capacitor charges through both resistors and discharges through R2 alone.

Why can the duty cycle not be 50% or less?

The charge path runs through R1 and R2 while the discharge path runs through R2 only, so the high time always exceeds the low time. Adding a diode across R2 lets the capacitor charge through R1 alone, which makes duty cycles at and below 50% achievable.

How do I calculate a monostable pulse width?

T = 1.1 × R × C, with R in ohms and C in farads. The 1.1 is ln(3), the number of time constants needed for the capacitor to reach two thirds of the supply from fully discharged.

Does supply voltage affect 555 timing?

No, and that is one of its best features. The comparator thresholds are derived from the supply by an internal divider, so both scale together and cancel. The 555 keeps its period across its whole supply range.

What are practical component limits?

Keep resistors roughly between 1 kΩ and 1 MΩ. Below that the discharge transistor cannot cope; above it the chip’s own input bias current becomes significant relative to the charging current. For long periods increase the capacitor rather than the resistor.

Should I use an NE555 or a CMOS version?

Use a CMOS part such as the TLC555 or LMC555 for battery work and clean supplies: microamps of quiescent current, operation down to about 2 V, and no large switching spike. The bipolar NE555 is preferable only when you need its higher output drive.

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