555 timer astable time constants
In the astable configuration, the low time is given by 0.693&times;R2&times;C and the high time is given by 0.693&times;(R1+R2)&times;C.

The factor of 0.693 is the scale factor (relative to the RC time constant) for the time that the capacitor takes to charge or discharge exponentially between 1/3 Vcc and 2/3 Vcc. The precise value is -ln(1/2) = 0.693147...

If you add the two half periods together, you get 0.693&times;(R1+2R2)&times;C for the total period.

If you take 1/period to get frequency, you can write it as 1/(0.693&times;(R1+2R2)&times;C), or you can move the constant to the top and write it as 1.442695/((R1+2R2)&times;C).