555 Timer Calculator

Astable mode (R1 from V+ to pin 7, R2 from pin 7 to pins 2+6, C from pins 2+6 to ground) oscillates continuously:

f = 1.44 ÷ ((R1 + 2·R2) × C)
t_high = 0.693 × (R1 + R2) × C    t_low = 0.693 × R2 × C

The duty cycle is (R1 + R2) / (R1 + 2·R2), which is always above 50%. For near-50% duty, make R2 much larger than R1, or add a diode across R2.

Monostable mode produces a single pulse when triggered (pin 2 pulled low):

t = 1.1 × R1 × C

Keep resistors between about 1 kΩ and 10 MΩ and capacitors above ~100 pF for reliable operation. The standard NE555 works from 4.5–16 V; CMOS versions (TLC555, LMC555) go lower and reach higher frequencies.

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