555 Timer Astable Calculator — Frequency & Duty (Free)

NE555 / TLC555 astable: f = 1.44 / ((R1 + 2 R2) C)

Operating Mode

Required Parameters

Resistor R1

Ohm

Resistor R2

Ohm

Timing capacitor C

Waiting for input data...

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Quick Answer

Astable frequency: f = 1.44 / ((R1 + 2×R2) × C). Example: R1 = 1 kΩ, R2 = 6.8 kΩ, C = 100 nF → about 986 Hz at 53% duty. T_high = 0.693 (R1+R2) C; T_low = 0.693 R2 C.

Documentation

555 Timer Astable Calculator

Design an NE555 / TLC555 astable oscillator. Enter R1, R2, and C to get frequency, period, high/low times, and duty cycle.

Quick start: R1 = 1 kΩ, R2 = 6.8 kΩ, C = 100 nF → about 986 Hz at 53% duty. This page is astable only.

555 astable frequency formula

f = 1.44 / ((R1 + 2×R2) × C)

T_high = 0.693 × (R1 + R2) × C

T_low = 0.693 × R2 × C

Duty = (R1 + R2) / (R1 + 2×R2)

Symbol	Pin / meaning
R1	VCC → pin 7 (discharge)
R2	pin 7 → pins 6/2 (threshold / trigger)
C	pins 6/2 → GND

Period T = 0.693 × (R1 + 2 R2) × C. Frequency is 1 / T, which is the 1.44 form above.

Why duty cycle is always over 50%

The capacitor charges through R1+R2 and discharges only through R2. Duty is therefore (R1+R2)/(R1+2 R2) — always above 50% on a standard bipolar 555.

Make R1 ≪ R2 to approach 50% (R1 = 1 kΩ, R2 = 100 kΩ → 50.5%)
Or add a diode across R2 (cathode toward pin 7) so charge bypasses R2
CMOS TLC555 / LMC555 can do true 50% with output feedback

Worked example — 1 kHz square wave

Choose C = 100 nF
R1 = 1 kΩ, R2 = 6.8 kΩ
f = 1.44 / ((1k + 2×6.8k) × 100n) = 986 Hz
Duty = (1k + 6.8k) / (1k + 13.6k) = 53.4%

Need a one-shot pulse instead? Open the full 555 Timer Calculator and switch to monostable (T = 1.1 RC).

Component ranges

Parameter	Recommended range	Notes
R1, R2	1 kΩ – 10 MΩ	Below 1 kΩ draws excess current
C (timing)	100 pF – 1000 µF	Film or C0G; avoid electrolytics
C (pin 5)	10–100 nF	Always bypass Control to GND
VCC	4.5 V – 16 V	Standard NE555 range

NE555 vs TLC555

Bipolar NE555 / LM555 draw a few mA and top out near 500 kHz. CMOS TLC555 / LMC555 draw microamps and run to about 2 MHz. The astable formulas are the same if the thresholds stay at 1/3 and 2/3 VCC.

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Design Notes

The NE555 works from 4.5V to 16V and can source/sink up to 200 mA. For low-power use, the CMOS TLC555/ICM7555 draws only 60-250 uA vs 3-10 mA for the bipolar NE555. The standard astable duty cycle is always greater than 50 percent because the cap charges through (R1+R2) but discharges only through R2. Add a diode across R2 for 50 percent duty cycle.

Common Mistakes

1
Forgetting the 0.01uF bypass capacitor on pin 5 (Control Voltage), causing erratic timing from noise.
2
Using electrolytic capacitors for timing: their 20 percent tolerance and leakage make timing inaccurate. Use ceramic or film caps.
3
Exceeding the bipolar NE555 max frequency (~500 kHz). Use the CMOS TLC555 for up to 2 MHz.
4
In monostable mode, the trigger pulse (pin 2) must be shorter than the output pulse and drop below 1/3 Vcc.

Engineering Handbox

1. T_high = 0.693 x (R1+R2) x C = 0.693 x 57k x 100nF = 3.95 ms 2. T_low = 0.693 x R2 x C = 0.693 x 47k x 100nF = 3.26 ms 3. Period T = 7.21 ms, f = 138.7 Hz 4. Duty Cycle = 57k/104k x 100 = 54.8 percent

VerificationOutput: 138.7 Hz square wave with 54.8 percent duty cycle.

Knowledge Base

What is the 555 astable frequency formula?

f = 1.44 / ((R1 + 2×R2) × C). R1 is from VCC to pin 7, R2 from pin 7 to pins 6/2, C from pins 6/2 to ground. Period T = 0.693 × (R1 + 2 R2) × C.

Why is the 555 astable duty cycle always over 50%?

The capacitor charges through R1+R2 and discharges only through R2. Duty = (R1+R2)/(R1+2 R2). Make R1 much smaller than R2 to approach 50%, or add a diode across R2.

How do I get a 1 kHz 555 oscillator?

Try C = 100 nF, R1 = 1 kΩ, R2 = 6.8 kΩ → ≈ 986 Hz. Or set C = 10 nF, R1 = 1 kΩ, R2 = 68 kΩ. Use film or C0G capacitors; electrolytics drift.

What is the difference between astable and monostable?

Astable free-runs and makes a square wave. Monostable waits for a trigger and outputs one pulse T = 1.1 RC. This page is astable only — use the main 555 calculator for monostable.

NE555 vs TLC555 / CMOS?

Bipolar NE555 / LM555 draw a few mA and top out near 500 kHz. CMOS TLC555 / LMC555 draw microamps and run to about 2 MHz. The astable formulas are the same if the thresholds stay at 1/3 and 2/3 VCC.

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