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Timing circuit tool

555 timer calculator

Calculate standard 555 monostable pulse width or astable frequency, high time, low time and duty cycle. Design mode suggests practical E-series timing components for a target.

Isometric illustration of a timer chip, resistor, capacitor and square wave

555 timing inputs

Evaluate an ideal timing network or find practical E12 R/C candidates.

Calculated timing

Frequency

3.6075 Hz

Period

277.2 ms

Duty cycle

62.5%

High time

173.25 ms

Low time

103.95 ms

Practical range

The timing result sits inside the 0.1 Hz to 5 MHz quick-reference range.

555VCCOutputRA / RB / C

Standard 555 wiring at a glance

Connect each formula term to the real pin and component path. These drawings are functional references, not complete production schematics.

Astable circuit

Repeating oscillator using RA, RB and the timing capacitor.

Standard NE555 astable oscillator circuitRA connects VCC to discharge pin 7. RB connects pin 7 to threshold and trigger pins 6 and 2. The timing capacitor connects that node to ground, and output is taken from pin 3.VCCNE5558,4RARBC72,6OUT3
RA and RB set charging time; RB alone sets discharge time, so the standard output duty cycle stays above 50%.

Monostable circuit

Triggered one-shot using one timing resistor and capacitor.

Standard NE555 monostable one-shot circuitThe timing resistor connects VCC to the threshold and discharge node on pins 6 and 7. The capacitor connects that node to ground, trigger enters pin 2, and output leaves pin 3.VCCNE5558,4RC6,7TRIG2OUT3
A low trigger starts one pulse while C charges through R until the threshold node reaches about two-thirds of VCC.

Pinout (NE555)

DIP-8 pin roles used by both reference circuits.

NE555 DIP-8 pinoutPins 1 through 4 are ground, trigger, output, and reset. Pins 8 through 5 are VCC, discharge, threshold, and control.NE5551GND2Trigger3Output4Reset8VCC7Discharge6Threshold5Control
Confirm the physical package orientation and exact manufacturer datasheet before wiring or replacing the device.

How to use the 555 timer calculator

Select the operating mode first because monostable and astable circuits use different equations and component arrangements.

  1. 01

    Select monostable or astable

    Monostable mode produces one pulse per trigger. Astable mode continuously alternates between high and low states.

  2. 02

    Evaluate or design

    Evaluate calculates from entered values. Design searches practical E12 or E24 values for a target pulse width or frequency.

  3. 03

    Verify the device limits

    Check the proposed resistance, capacitance, duty cycle and frequency against the exact timer variant, supply and load in its datasheet.

Standard bipolar 555 timing equations

The calculator uses the conventional first-order equations for the common monostable and two-resistor astable connections.

Monostable pulse

t = 1.1 × R × C

Astable frequency

f = 1.44 / ((RA + 2RB) × C)

Astable high time

tH = 0.693 × (RA + RB) × C

Astable low time

tL = 0.693 × RB × C

Astable duty cycle

D = (RA + RB) / (RA + 2RB)

In the standard astable connection, the capacitor charges through RA and RB but discharges through RB. That topology keeps the ideal duty cycle above 50% unless the charge and discharge paths are modified.

The equations are nominal. Threshold accuracy, leakage, capacitor tolerance, timing resistance limits, output loading and device technology affect the result. Use the datasheet for the exact NE555, CMOS 555 or compatible part and validate critical timing on hardware.

Practical 555 design checks

A close calculated target is only the start of a robust timer design.

Confirm timing resistance limits

Very low resistance can overload the discharge transistor; very high resistance makes leakage and input currents more significant. Follow the selected device datasheet.

Use realistic capacitor tolerance

Large electrolytic capacitors can have wide tolerance and leakage. Choose a capacitor technology suited to the time range and required repeatability.

Decouple the supply

Place local bypass capacitance close to the timer and keep high-current output paths away from the threshold and trigger nodes.

Check the standard duty-cycle limit

The conventional two-resistor astable circuit cannot produce a nominal duty cycle below 50%. Use a modified charge/discharge path when that is required.

Frequently asked questions

What is the difference between monostable and astable mode?

Monostable mode waits for a trigger and produces one timed pulse. Astable mode free-runs and produces a repeating waveform.

Why is standard astable duty cycle always above 50%?

The timing capacitor charges through RA plus RB but discharges only through RB, so the calculated high interval is always longer than the low interval.

Why does the measured frequency differ from the calculation?

Component tolerance, capacitor leakage, timer thresholds, supply conditions, wiring parasitics and loading all contribute error.

Can I use these equations for a CMOS 555?

They are useful nominal equations for the standard topology, but operating limits and errors differ by device. Check the exact CMOS timer datasheet before choosing components.

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Technical references: Texas Instruments: NE555 product page, Texas Instruments: NA555, NE555 and SA555 datasheet