555 astable calculation question.

[WeekendHobbiest]

Hi, everyone!

How would I calculate the value of R2 if I have a target frequency in mind and I know what value I want to use for R1 and C?

To specify, say I want a 1kHz clock and I have a 10uF cap and a 10K trim pot. How would I calculate the value of R2 based on that?

Thanks.

[Jwillis]

Increasing C increases cycle time reducing frequency. increasing R1 increases High time (T₁) but leave Time Low (T₀) unaffected . increasing R2 will increase high time decease time low and increase duty cycle down to 50% minimum.

Time High (T₁)     0.693 × (R1+R2) × C1 = Seconds
Time Low (T₀)     0.693 × R2 × C1 =Seconds
Time Period (T)    0.693 × (R1+2×R2) × C1=Seconds
Frequency (F)      1.44 / (R1+2×R2) × C1=Hertz (Hz)
Duty Cycle           (T1/T)×100=Duty Percentage (%)   

[Zero999]

10µF is too higher capacitor value for 1kHz. Go for 100nF. For 1kHz, set R2 to 6k8 and R1 to 470R, for nearly 50% duty cycle.