Astable mode
In astable mode, the 555 oscillates continuously. Frequency is set by R1, R2, and C using f = 1.44 / ((R1 + 2×R2) × C). The duty cycle is always above 50% in standard wiring. To get duty cycle below 50%, add a bypass diode across R2.
555 Timer Calculator
Output Waveform — duty cycle 66.7%
555 timer calculator for astable and monostable circuits
Monostable mode
In monostable mode, a trigger pulse causes a single output pulse whose width is set by t = 1.1 × R × C. Adjust R and C to get pulse widths from microseconds to many seconds.
Component selection tip
For timing accuracy, use 1% tolerance resistors and low-leakage capacitors. Electrolytic capacitors introduce more leakage for long timing intervals; film capacitors are preferred for precision monostable applications.