555 Timer Calculator - Astable and Monostable Modes

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Astable: the output continuously oscillates between HIGH and LOW. Monostable: a single HIGH pulse is produced each time the trigger pin is pulsed LOW.
Resistor connected between VCC and the discharge pin (pin 7). In astable mode R1 must be at least 1 kΩ to limit discharge current. In monostable mode R1 sets the pulse width with C.
kΩ
Resistor connected between the discharge pin (pin 7) and the threshold/trigger pins. Increasing R2 lengthens both high and low intervals and lowers the duty cycle.
kΩ
Timing capacitor connected between threshold/trigger pins and GND. Larger values give lower frequencies and longer pulse widths.
nF
Frequency
480.0000 Hz

Number of complete cycles per second

Period (T)2.0790 ms
Duty cycle66.67%
Time HIGH (t₁)1.3860 ms
Time LOW (t₂)693.0000 us
Frequency (Hz)480
Duty cycle (%)66.67
Time HIGH (s)0.001386
Time LOW (s)0.000693
Time HIGH0.001386
Time LOW0.000693

Oscillating at 480.0000 Hz, duty cycle 66.7%

  • With R1 = 10 kΩ, R2 = 10 kΩ and C = 100 nF, the output oscillates at 480.0000 Hz.
  • Duty cycle is 66.7% - the output is HIGH 66.7% of each cycle. The standard 555 astable circuit always exceeds 50% duty cycle.
  • To achieve exactly 50% duty cycle, add a small diode across R2 so the capacitor charges only through R1 and discharges only through R2.

Next stepUse the chart below to see how frequency changes as you vary component values. For LED blink at 1 Hz, try R1 = 10 kΩ, R2 = 68 kΩ, C = 10 uF.

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