I am building a 555 timer based astable multivibrator. How do I calculate the exact value of resistors and capacitor?

How to calculate resistor and capacitor value in 555 astable mode?

June 9, 2015 - 9:08pm

Replies: 3

Resistors and capacitor values are dependent on, how much frequency you want from the circuit. Below are the formulas:

Time High(Seconds) T1 =0.693 * (R1+R2) * C1Time Low(Seconds) T2 =0.693 * R2 * C1Time PeriodT = Time High + Time Low =0.693 * (R1+2*R2) * C1Freqeuncyf = 1/Time Period = 1/ 0.693 * (R1+2*R2) * C1 =1.44 / (R1+2*R2) * C1Duty Cycle:Duty cycle is the ratio of time for which the output is HIGH to the total time.Duty cycle %: (Time HIGH/ Total time) * 100 = (T1/T) * 100 =

(R1+R2)/ (R1+2*R2) *100Check this Article for more detail:

555 Timer Astable Multivibrator Circuit

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Replies: 1

VJ,

There's no one value of resistance and capacitance to produce a particular frequency. Your resistor values can change depending on the capacitor value you use or have on hand.

You can solve the equations for R1, R2, and C1 if you want. But if you're in a hurry I suggest you check out this 555 Timer Calculator at: http://houseofjeff.com/555-timer-oscillator-frequency-calculator/

Just enter the frequency you want the 555 to oscillate at (leave the first question on firing blank) and the calculator will produce a list of R1 and R2 based on a list of common capacitors values that you can select from.

Be aware that a Square Wave has a 50% duty cycle (it's HIGH 1/2 the period and LOW the other 1/2) and with the 555 IC the selection of R1, R2, and C1 can affect the duty cycle.

So if you want as close to a Square Wave for your frequency, when you enter your frequency value and the list appears, use the combination of C1, R1, and R2 that is as close to 50% duty cycle. If the duty cycle is not important then your can select what ever values the listing shows for your frequency.

Finally, if frequency stability is important (it usually is), because the 555 IC uses an RC circuit to produce a frequency the stability will be affected by the temperature of the resistors in particular. Ordinary resistors can vary their resistance gratly as temperature goes up and down and that will in turn affect the actual frequency. So if minor frequency variation is OK then you can use inexpensive carbon composition or carbon film or metal film resistors which are very common. If you want a more stable resistor you'll need to use a more temperature-stable type.

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