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:	1.023	Hz	Cycle Time:	977.130	ms
Duty Cycle:	66.67	%
Time High:	651.420	ms	Time Low:	325.710	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%)

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