555 Timer Calculator

Frequency, duty cycle and pulse width for a 555 in astable or monostable mode. Enter the resistors and capacitor and read off what the chip will actually do.

Mode

R1 (ohms)

R2 (ohms) - astable only

Capacitor (nF)

Output --

Shop timer ICs and passives

The formula

Astable:
  t_high = 0.693 x (R1 + R2) x C
  t_low  = 0.693 x R2 x C
  f      = 1.44 / ((R1 + 2 x R2) x C)
  duty   = (R1 + R2) / (R1 + 2 x R2)

Monostable:
  pulse = 1.1 x R x C

How it works

In astable mode the 555 free runs. The capacitor charges through R1 and R2 and discharges through R2 alone, which is why the output is always high for longer than it is low — a plain 555 cannot go below 50% duty without a diode across R2.

For a duty cycle close to 50%, make R1 small relative to R2. R1 should not go below about 1 kOhm or the discharge transistor takes the abuse.

In monostable mode one trigger gives one pulse of 1.1 x R x C, and further triggers during the pulse are ignored. Keep R between roughly 1 kOhm and 1 MOhm; outside that, leakage and the chip input current start to matter more than your maths.

More calculators

All calculators

555 Timer Calculator

+91 81231 09966

Customer Service

Our company

Calculators

Recently Viewed Products

+91 81231 09966