555 Astable Mode

The capacitor charges through $R_1 + R_2$ and discharges through $R_2$, between ⅓ and ⅔ of $V_{CC}$:

$t_{high} = 0.693 (R_1 + R_2) C \qquad t_{low} = 0.693\, R_2\, C$

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

Example: $R_1$ = 1 kΩ, $R_2$ = 10 kΩ, C = 10 µF → $f$ ≈ 6.9 Hz, a visible blink. The duty cycle is always above 50 % unless a diode bypasses $R_2$ during charging.