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Calculator · 555

555 Timer Calculator (Astable & Monostable)

Calculate the frequency, period, and duty cycle of an NE555 astable oscillator from R1, R2, and C, or the output time of a monostable (one-shot) from R and C.

Between VCC and pin 7. 1k or more recommended
Between pin 7 and pins 6 and 2
10u = 10µF, 100n = 100nF
Frequency
6.87 Hz
Period
146 ms
High time (output on)
76.2 ms
Low time (output off)
69.3 ms
Duty cycle
52.38 %

The NE555 timer IC is the classic part for making LED blinkers, alarm tones, and delay circuits from just two resistors and a capacitor. The calculator above covers two modes. Astable is an oscillator whose output keeps turning on and off, and monostable is a one-shot timer that turns on for a set time every time it is triggered.

Astable (oscillator) formulas

Connect R1 between the supply and pin 7, R2 between pin 7 and pins 6 and 2, and C between pins 6 and 2 and ground.

High time    t_H = 0.693 × (R1 + R2) × C
Low time     t_L = 0.693 × R2 × C
Period       T   = t_H + t_L = 0.693 × (R1 + 2 × R2) × C
Frequency    f   = 1 / T ≈ 1.44 / ((R1 + 2 × R2) × C)
Duty cycle   D   = (R1 + R2) / (R1 + 2 × R2)

For example, with R1 = 1 kΩ, R2 = 10 kΩ, and C = 10 µF, the period is about 0.146 s, the frequency about 6.9 Hz, and the duty cycle about 52%. The LED blinks nearly seven times a second.

Monostable (one-shot) formula

Connect R between the supply and pins 7 and 6, and C between pin 6 and ground. Briefly pull pin 2 (the trigger) toward ground and the output turns on.

t = 1.1 × R × C

With R = 100 kΩ and C = 10 µF, it stays on for about 1.1 seconds.

Choosing component values

  • Use 1 kΩ or more for R1 and R2. If they are too small, excess current flows into the discharge pin and strains the IC. On the other hand, several MΩ or more makes the timing inaccurate because of leakage current.
  • Use a large C for slow operation and a small C for fast operation, and adjust the resistors within the range above. Use an electrolytic capacitor (mind the polarity) for slow blinking at 1 Hz or below, and a ceramic or film capacitor for several hundred Hz and up.
  • Electrolytic capacitors have large tolerances. At around ±20%, the real frequency can differ a lot from the calculated one. If you need an accurate period, use a film capacitor or replace a resistor with a potentiometer and tune it.

Circuit construction and perfboard layout are covered step by step in the guides Blinking an LED with a 555 Timer and 555 Timer Monostable Circuit. For time constant calculations, see the RC time constant calculator too.

Related guides

Circuit design & debugging

Blinking an LED with a 555 Timer: Astable Circuit Design

How an NE555 astable circuit works, the frequency and duty-cycle formulas, how to choose component values, and perfboard layout tips, with a worked example.

Circuit design & debugging

555 Timer Monostable Circuit: Turn On for a Fixed Time When a Button Is Pressed

How the 555 monostable (one-shot) circuit works, the output-time formula 1.1 × R × C, trigger pulse requirements, choosing values, and common mistakes, with examples.

Understanding parts

Capacitor Types, Polarity, and Reading Values

The difference between ceramic and electrolytic capacitors, polarity markings, how to read numeric codes like 103 and 104, and how to choose by voltage rating and use.

Results assume ideal values. Real part tolerances, temperature, and supply variation will change them, so always confirm important circuits by measuring.