555 astable formulas
In astable mode the capacitor charges through R1 + R2 and discharges through R2 alone, between ⅓ and ⅔ of the supply. That gives:
thigh = 0.693 × (R1 + R2) × C
tlow = 0.693 × R2 × C
f = 1.44 / ((R1 + 2·R2) × C)
duty = (R1 + R2) / (R1 + 2·R2)
Example: R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF → f ≈ 6.9 Hz, duty cycle ≈ 52 % — a visible LED blink.
Duty cycle below 50 %
With the standard circuit, the high time is always longer than the low time, so duty cycle is always above 50 %. To get below 50 %, add a diode across R2 (anode to pin 7) so the capacitor charges through R1 only; then thigh ≈ 0.693·R1·C.
Why simulate it?
The formulas ignore the 555's own thresholds, the capacitor's tolerance and the load on the output. A transient simulation shows the real square wave and the sawtooth on the capacitor. Press Simulate this circuit and CircuPilot draws the 555 astable with these values and an oscilloscope on the output.
More detail: 555 timer astable: calculate and simulate and blinking LED astable multivibrator.
FAQ
How do I calculate 555 timer frequency?
f = 1.44 / ((R1 + 2·R2) × C). With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF that is about 6.9 Hz.
How do I get a 1 Hz blink from a 555?
Pick C = 10 µF and R1 = 1 kΩ, then solve for R2: (1.44 / (1 × 10 µF) − 1 kΩ) / 2 ≈ 71.5 kΩ. The nearest standard value, 68 kΩ, gives about 1.05 Hz.
Why is my 555 duty cycle always above 50 %?
Because the capacitor charges through R1 + R2 but discharges through R2 only. Add a diode across R2 to charge through R1 alone.
What is the maximum frequency of a 555?
The bipolar NE555 is reliable up to a few hundred kHz; CMOS versions (TLC555, LMC555) go higher. Keep R1 and R2 above 1 kΩ and C above about 100 pF.