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Calculation of the frequency of 555 oscillator


The oscillation frequency of this oscillator is fixed by the value of capacitor (C) and resistor (Ra) and resistor (Rb).
Roughly oscillation frequency can be calculated by the following formula.
Because there is an error of the part, the oscillation frequency of the actual circuit is sometimes different from the calculated value little.


Unit f : Hz / C : Farad / Ra and Rb : ohm

It is possible to calculate by the following formula in the time that the output becomes H level and the time which is made an L level.

The time that the output becomes H level (tH)


The time that the output becomes L level (tL)


The frequency (f)




It is as follows when calculating at the value of the circuit this time.

The time that the output becomes H level (tH)
tH = 0.693 x ( Ra + Rb ) x C
= 0.693 x ( 5.6 x 103 + 47 x 103 ) x 0.01 x 10-6
= 0.365 x 10-3


The time that the output becomes L level (tL)
tL = 0.693 x Rb x C
= 0.693 x 47 x 103 x 0.01 x 10-6
= 0.326 x 10-3


The frequency (f)
f = 1 / ( tH+tL ) = 103 / ( 0.365 + 0.326 )
= 1.45 x 103

= 1.45 (kHz)


The oscillation frequency of the made circuit was to 1,114 Hz be with the +5V power.
The difference with the calculated value seems to be the factor that the error of the part is big. I attempted to change 555 (different manufacturer) but the oscillation frequency was the same approximately.



I measured the change of the oscillation frequency by the change of the line voltage.


The oscillation frequency is stable at more than +3V voltage.
It is the most stable in the voltage for 7V to 9V and it is the change of about 1Hz(0.1%).