Can duty cycle be below 50%?

Not in standard configuration. Place a diode across R2 so it is bypassed during discharge to achieve sub-50% duty cycle.

How accurate is a 555 timer?

Typically within a few percent. Temperature and component tolerance cause drift.

What is the maximum frequency?

Standard 555 works reliably up to about 500 kHz. CMOS versions can reach several MHz.

Astable 555 Timer Calculator

Calculate frequency, period, and duty cycle for a 555 timer in astable (free-running) mode.

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kΩ

Capacitor

µF

Frequency

480.0 Hz

Period2,083.333 ms

Duty Cycle66.7 %

Frequency vs R1

View formula & how it works →

555 Timer Astable Mode

Formulas

f = 1.44 / ((R1 + 2R2) x C)

Duty Cycle = (R1 + R2) / (R1 + 2R2) x 100%

Duty cycle is always above 50% in standard configuration. Use a diode across R2 for below 50%.

Example Calculation

R1 = 10k, R2 = 10k, C = 0.1 uF.

01f = 1.44 / ((10000 + 20000) x 0.0000001) = 1.44 / 0.003 = 480 Hz
02Duty cycle = (10 + 10) / (10 + 20) x 100 = 66.7%

Frequently Asked Questions

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