Design 555 Timer Circuits Instantly

A free browser-based 555 timer calculator for astable (oscillator) and monostable (one-shot) circuits. Enter R1, R2, and C to get frequency, period, and duty cycle — or use reverse mode to find component values for a target frequency. Includes real-time waveform preview and practical presets.

How to Use the 555 Timer Calculator

Select mode

Choose Astable for continuous oscillation or Monostable for a single timed pulse.

Enter component values

Set R1, R2 (astable) or R (monostable) in kΩ and capacitor C in µF. Or use reverse mode for a target frequency.

Read results and waveform

See frequency, duty cycle, and timing. The waveform preview shows the actual output shape.

Calculate frequency, duty cycle, and components for NE555 circuits

Mode

R1 (kΩ)

R2 (kΩ)

C (µF)

Output Waveform

Result

Frequency

—

Period

—

Duty Cycle

—

Time HIGH

—

Time LOW

—

Presets

Features

Astable mode: frequency, period, duty cycle, time HIGH/LOW from R1, R2, C values Monostable mode: pulse width calculation from R and C Reverse calculator: enter target frequency and duty cycle to find R1, R2, C Real-time waveform preview on canvas and frequency presets (1Hz to 38kHz)

FAQ

How does the 555 astable mode work?

The capacitor charges through R1+R2 and discharges through R2 only, creating a continuous square wave. Frequency = 1.44 / ((R1 + 2×R2) × C). Duty cycle = (R1 + R2) / (R1 + 2×R2).

Can I get exactly 50% duty cycle?

Not with the standard astable circuit (duty is always >50%). To achieve 50%, add a diode across R2 so charge/discharge paths are equal, or use R1 much smaller than R2.

What is monostable mode?

A one-shot timer that outputs a single HIGH pulse when triggered. Pulse width = 1.1 × R × C. After the pulse, output returns to LOW until triggered again.

What capacitor values should I use?

For audio frequencies (100Hz-20kHz): 1nF-100nF. For slow blinks (0.5-5Hz): 10µF-100µF. For infrared (38kHz): 1nF-10nF. Use ceramic for high frequencies, electrolytic for low.

Does supply voltage affect frequency?

No. The 555 uses internal voltage dividers (1/3 and 2/3 of Vcc) for timing, so frequency is independent of supply voltage. However, supply must be 4.5V-16V for NE555.

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Who is this tool for?

🔌 Electronics Enthusiasts

Published 06 Apr 2026 Updated 21 May 2026