When making an astable circuit with a 555 timber, the frequency of pin 3s pulse is calculated using this: f = 1.44 / (R1 + 2R2) x C My question is why is 1.44 included.

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1 Answer 1


In the astable configuration, the low time is given by 0.693×R2×C and the high time is given by 0.693×(R1+R2)×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×(R1+2R2)×C for the total period.

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


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