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555 Timer Calculator

Frequency, duty cycle and pulse width for astable and monostable 555 circuits.

kΩ
kΩ
µF

Result

Frequency
6.857 Hz
Period
145.8 ms
High time
76.23 ms
Low time
69.3 ms
Duty cycle
52.38 %
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How it works

The 555 timer is one of the most successful chips ever made and still the quickest way to make a blinking light, a clock signal or a delay. In astable mode it free-runs as an oscillator; in monostable mode it produces a single pulse of fixed length when triggered.

Pick a mode, enter your resistor and capacitor values and read off the timing. In astable mode, R1 and R2 charge the capacitor; because R1 is always in the charging path, the output is high longer than low, so the duty cycle is always above 50% unless you add a diode.

Formula

Astable frequencyf = 1.44 / ((R1 + 2 × R2) × C)
High / low timet_high = 0.693 × (R1 + R2) × C , t_low = 0.693 × R2 × C
Duty cycleD = (R1 + R2) / (R1 + 2 × R2)
Monostable pulset = 1.1 × R × C

Worked example. R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF: f = 1.44 / (21 k × 10 µ) = 6.86 Hz with 52% duty cycle.

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Frequently asked questions

How do I get a 50% duty cycle from a 555?

Use a fast diode across R2 so the capacitor charges through R1 only and discharges through R2, then pick R1 = R2. Alternatively, use the 555 output to drive a flip-flop that divides by two.

What is the difference between a 555 and a 7555 / LMC555?

CMOS versions (7555, LMC555, TLC555) draw far less current, work down to about 2 V and can oscillate at higher frequencies. Bipolar NE555 chips draw a few mA and need 4.5 V or more.

Can a 555 run from 3.3 V?

A standard NE555 usually needs 4.5 V or more. Use a CMOS variant such as the TLC555 or LMC555 for 3.3 V logic.

Do I need a microcontroller instead?

For anything beyond a simple blinker or analog trick — PWM with changing duty cycle, precise timing, communication — a microcontroller is cheaper and more flexible. The 555 shines where you have no code and no power budget for a processor.

Boards this helps with

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