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555 timer calculator

Why the duty cycle is always above 50 %

In the classic circuit the capacitor charges through both resistors and discharges through the second one alone. So the time high is always longer than the time low, and exactly half is never achievable — unless you add a diode across the second resistor. The calculator says so plainly rather than returning a negative value.

555thightlow
The duty cycle of the classic circuit is always above 50 %: the discharge goes through R2 alone
Mode
Capacitance C (nF)
R1 (kΩ)
R2 (kΩ)
Result
Frequency
Period
Time high
Time low
Duty cycle

📐 The formulas​

Astable mode:

t(high) = ln2 · (R1 + R2) · C
t(low)  = ln2 · R2 · C
f = 1 / ( ln2 · (R1 + 2·R2) · C )  ≈ 1.4427 / ((R1 + 2·R2)·C)
duty cycle = (R1 + R2) / (R1 + 2·R2)     always > 50 %

Monostable (one-shot):

T = ln3 · R · C  ≈ 1.0986 · R · C

A check point:​

R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF → f = 687.0 Hz, duty cycle 52.4 %.

The rounded constant of 1.44 from the datasheet gives 685.7 Hz; the calculator works through ln 2, which is more accurate.

Where this calculator stops:
  • The duty cycle cannot be 50 % or below without a diode across R2, and the calculator warns you
  • An R1 below 1 kΩ overloads the chip's discharge pin
  • The spread of the 555's internal thresholds (hence a few percent of error) and capacitor leakage are not accounted for
  • With electrolytics, a ±20 % capacitance tolerance makes the calculation indicative only

🎯 In practice​

  • A tone generator for testing an audio chain.
  • A flasher, a delay timer, a pulse shaper.
  • A clock for simple logic where a microcontroller would be overkill.
Practical advice:
  1. The CMOS 7555 works from 2 V and up to a megahertz; the bipolar original throws current spikes onto the supply.
  2. A 100 nF capacitor at the supply pin is not optional — without it a 555 injects interference into everything nearby.
  3. For exactly 50 % duty, put a diode across R2: charging goes through R1, discharging through R2.
  4. A film capacitor gives a far steadier frequency than an electrolytic.

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UR3PKI. «555 timer calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/timer_555 (licence CC BY-NC-SA 4.0).

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