NE555 Timer Calculator - Astable & Monostable Modes

How to use this tool

1. Select astable (oscillator) or monostable (one-shot) mode.
2. Enter the resistor and capacitor values.
3. View the computed frequency, period, duty cycle, or pulse width.

NE555 Astable vs Monostable

In astable mode, the 555 timer continuously oscillates between high and low, generating a square wave. Frequency and duty cycle are set by R1, R2, and C.

In monostable mode, a trigger pulse starts a single output pulse whose width is determined by R and C alone (T = 1.1 × R × C).

Astable mode formulas

In astable mode, the oscillation frequency and duty cycle are determined by the resistors R1 and R2, and the capacitor C:

• Frequency: f = 1.44 / ((R1 + 2×R2) × C)
• Duty cycle: D = (R1 + R2) / (R1 + 2×R2)

Worked example: With R1 = 10 kΩ, R2 = 10 kΩ, C = 100 nF:
f = 1.44 / ((10,000 + 20,000) × 0.0000001) = 1.44 / 0.003 ≈ 480 Hz
D = (10,000 + 10,000) / (10,000 + 20,000) ≈ 0.667 (66.7% duty cycle)

Monostable mode formula

In monostable mode, a single trigger pulse produces an output pulse of duration T:

T = 1.1 × R × C

Worked example - 1-second pulse: To get T = 1 s, choose R = 910 kΩ and C = 1 µF:
T = 1.1 × 910,000 × 0.000001 = 1.001 s ≈ 1 second.

555 timer pin reference

Common 555 applications

• LED flasher (astable): R1 = 1 kΩ, R2 = 47 kΩ, C = 10 µF -> ~1.4 Hz blink rate. Connect LED with current-limiting resistor (470 Ω) on pin 3.
• Debounced switch (monostable): R = 100 kΩ, C = 10 µF -> 1.1 s output pulse, long enough to ignore contact bounce on a mechanical switch.
• PWM generation (astable): Add a potentiometer between R1 and R2 to vary duty cycle. Use a diode to separate charge and discharge paths for asymmetric PWM below 50%.
• Tone / beeper (astable): R1 = 1 kΩ, R2 = 10 kΩ, C = 10 nF -> ~4.8 kHz audible tone. Connect a small speaker or buzzer on pin 3 with a 100 Ω series resistor.