555 Astable Circuit Calculator

Resistor Ohm's Law
 
 

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

Notes:
  • 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%)