NE555 Astable Calculator

NE555 Astable Multivibrator

The NE555 timer IC can generate a continuous square wave in astable mode. By choosing resistor and capacitor values, you can set the oscillation frequency and duty cycle for blinkers, clock sources, PWM experiments, and hobby circuits.

Formulas

Frequency:

$$f = \frac{1.44}{(R_1 + 2R_2)\,C}$$

Duty cycle (percentage of time output is high):

$$D = \frac{R_1 + R_2}{R_1 + 2R_2} \times 100\%$$

High and low times follow $t_{HIGH} = 0.693(R_1+R_2)C$ and $t_{LOW} = 0.693\,R_2 C$. Resistors are in ohms and capacitance in farads.

Practical Notes

Real 555 timers have tolerance and threshold behavior that slightly shifts frequency from the ideal formula. Keep $R_1 \ge 1\,\text{k}\Omega$ to avoid excessive discharge current through pin 7. For duty cycles near 50%, choose $R_1 \ll R_2$.

Related tools: Op-Amp Gain Calculator and LM317 Calculator.

Frequently Asked Questions

What is astable mode on a 555 timer?

Astable mode continuously oscillates between high and low output states with no stable resting position, producing a square wave.

Why is the frequency formula 1.44?

The constant comes from the 555 internal comparator thresholds and the 0.693 timing factor for RC charging and discharging.

Can duty cycle be exactly 50%?

Not with the standard astable circuit because charging and discharging paths use different resistors. Use a 50% duty-cycle variant if symmetry is required.

What units should I use?

Enter R1 and R2 in kiloohms (kΩ) and capacitance in microfarads (µF). The calculator converts internally to ohms and farads.

What frequency range is practical?

Typical hobby circuits run from a fraction of a hertz to a few hundred kilohertz, depending on component values and timer variant.

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NE555 Astable Multivibrator

Formulas

Practical Notes

Frequently Asked Questions

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