How this calculator works
The astable timing capacitor charges from one third to two thirds of the supply through R1 and R2, then discharges through R2. Those thresholds give ln(2) times the relevant RC product. In monostable operation charging from zero to two thirds gives ln(3)RC. Datasheets commonly round these coefficients to 0.693 and 1.1. Comparator tolerances, leakage and discharge-pin voltage introduce real timing error.
Formula
thigh = ln(2)(R1+R2)C; tlow = ln(2)R2C; f = 1/[ln(2)(R1+2R2)C]; D = (R1+R2)/(R1+2R2). Monostable: tpulse = ln(3)R1C.
Worked example
With R1 = R2 = 1000 ohms and C = 0.001 F, high time is 1.386294 s, low time is 0.693147 s and duty is 66.6667%. The same R1 and C in monostable mode give 1.098612 s.
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Frequently asked questions
Can this circuit produce duty below 50%?
The standard two-resistor astable cannot. A diode or another topology changes the charge and discharge paths.
Why are results slightly different from 1.1RC?
This tool keeps ln(3) rather than its common 1.1 approximation. Real component and IC tolerances are usually more significant.
Are very small resistors suitable?
The mathematical model allows positive values, but the datasheet constrains discharge current and supply current. Check the actual timer variant before choosing values.