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IC 555 Timer Calculator with Formulas and Equations

555 Timer Calculator with Formulas and Equations


Formulas and Equations for 555 Timer IC Calculator

Frequency (f) = 1/ [ ln(2) x C x (R1 + 2R2) ]

Frequency (f) = 1.44 / C x (R1 + 2R2) …. Where… 1/ ln(2) = 1.44

TP = ln(2) x C x (R1 x R2)

TP = 0.693 x C x (R1 x R2) …. Where… ln(2) = 0.693

TN = ln(2) x C x R2)

TN = 0.693 x R2 x C …. Where… ln(2) = 0.693


  • = Value of Capacitor in Farads
  • R1 and 2R2= Value of the Input Resistance
  • f = Frequency in kHZ
  • TP = Positive or High Time from each pulse
  • TN = Negative or Low Time from each pulse

Enter values and click on calculate. Result will display the calculated quantities. 

Enter your values:
Resistance R1:
Resistance R2:
Capacitance (C): µfarads
Positive Time Interval TP: ms (millisecond)
Negative Time Interval TN: ms (millisecond)
Frequency ( f ): kHz (kilo Hertz)

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  1. I am confused about the value for Tn , that this time required for negative cycle , i think that should be 0.676 *( R1+R2) .
    Can you please share the derivation for it .

  2. Kartik, the Tn equation is correct. According to the datasheet for the 555 Timer, astable output, the following formulas are used (this is how the calculator works). Tp = .693 * (Ra + Rb) * C and the Tn = .693 * Rb * C.

    1. Specially to A/c distribution
      Kartik in the symentic of log table in power supply and distribution the formula needs of 236vi to 252vi or average 2-3Amp on 2kw. loading in normal consumption or reciprocal in pnp IC Board.this is an average electric charging ac supply. More is available here on request.

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