555 Astable Circuit Calculator

The 555 timer is capable of being used in astable and monostable circuits. In an astable circuit, the output voltage alternates between VCC and 0 volts on a continual basis.

The astable circuit is shown below.

By selecting values for R1, R2 and C we can determine the period/frequency and the duty cycle.

The period is the length of time it takes for the on/off cyle to repeat itself, whilst the duty cycle is the percentage of time the output is on.

In this type of circuit, the duty cycle can never be 50% or lower.

Capacitor (C):
Resistor 1 (R1):
Resistor 2 (R2):

Frequency: Hz Cycle Time: ms
Duty Cycle: %
Time High: ms Time Low: ms

  • Increasing C will increase the cycle time (and hence, reduce the frequency).
  • Increasing R1 will increase Time High, but will leave Time Low unaffected.
  • Increasing R2 will increase Time High, increase time low and decrease the duty cycle (down to a minimum of 50%)