555 Timer Calculator

This page helps you calculate figure out the timing of a 555-based astable timer.

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This circuit is the simplest. The duty-cycle can range from 55% to 95% (duty-cycle of 80% means that the output is on for 80% of the time).

duty-cycle can range from 55% to 95%

The output is on for t1 seconds, then off for t2 seconds.

t1 = .693 x (R1+R2) x C
t2 = .693 x R2 x C
(R1 and R2 are in Ohms; C is in Farads)

Please fill in the following values and then press "CALCULATE".


R1 resistance		Ohms (use all digits, no K or M)
R2 resistance		Ohms (use all digits, no K or M)
C1 capacitance		uFd

Requires JavaScript 1.1 or newer.

Normally, a 555 can not produce a duty cycle of under about 55%. This is because the timing capacitor C charges up through resistors R1 and R2, but discharges through R2.

If you want a duty cycle less than 55%, you can modify the basic 555 circuit with a diode to bypass R2 while charging.

no restriction on duty cycle

The output is on for t1 seconds, then off for t2 seconds.

t1 = .693 x R1 x C
t2 = .693 x R2 x C
(R1 and R2 are in Ohms; C is in Farads)

Please fill in the following values and then press "CALCULATE".


R1 resistance		Ohms (use all digits, no K or M)
R2 resistance		Ohms (use all digits, no K or M)
C1 capacitance		uFd

Requires JavaScript 1.1 or newer.

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