555 Timer Calculator
Calculate the frequency, period and duty cycle of a 555 astable oscillator, or the pulse width of a 555 monostable one-shot — with a timing waveform you can actually see.
Free-running oscillator. R1 and R2 set the charge time, R2 alone sets the discharge time, and C sets the overall speed.
About the 555 Timer Calculator
The 555 timer is the most popular integrated circuit ever made — a tiny 8-pin chip that turns a couple of resistors and a capacitor into a precise clock, blinker, tone generator or delay. This calculator covers its two classic configurations. In astable mode the 555 free-runs as an oscillator: from R1, R2 and the timing capacitor C it computes the output frequency f = 1.44 / ((R1 + 2·R2)·C), the period, the HIGH and LOW times, and the duty cycle (R1 + R2) / (R1 + 2·R2). In monostable (one-shot) mode a single trigger produces one output pulse of width T = 1.1·R·C. Each result comes with a to-scale timing waveform so you can see the duty cycle and pulse length rather than just read a number, which makes it ideal for makers, hobbyists, electronics students and anyone breadboarding an NE555, TLC555 or 7555 on an Arduino or microcontroller project.
How to use this calculator
- 1 Choose Astable for a free-running oscillator or Monostable for a single triggered pulse.
- 2 In astable mode enter R1, R2 and the timing capacitor C, each with its own unit (Ω/kΩ/MΩ and pF/nF/µF/mF).
- 3 In monostable mode enter the timing resistor R and capacitor C.
- 4 Read the frequency, period, duty cycle and HIGH/LOW times (or the pulse width), and study the timing waveform to see the result at a glance.
How the 555 timer formulas work
A 555 has two comparators watching the capacitor voltage at 1/3 and 2/3 of the supply, which is why every interval carries a factor of ln(2) ≈ 0.693. Astable (free-running): tH = 0.693 · (R1 + R2) · C — output HIGH while C charges through R1 + R2 tL = 0.693 · R2 · C — output LOW while C discharges through R2 period = tH + tL, frequency = 1 / period = 1.44 / ((R1 + 2·R2)·C) duty cycle = tH / period = (R1 + R2) / (R1 + 2·R2) Because R1 is always in the charge path but never in the discharge path, the duty cycle of a plain astable is always above 50%. Adding a diode across R2 lets charging and discharging use separate resistors, which is how you reach a 50% (or lower) duty cycle. Monostable (one-shot): A falling edge on the trigger input starts one pulse of width T = 1.1 · R · C (the 1.1 factor is ln(3) ≈ 1.0986), after which the output stays LOW until the next trigger.
Frequently asked questions
What is the 555 timer astable frequency formula?
The astable output frequency is f = 1.44 / ((R1 + 2·R2)·C). It comes from the charge and discharge times: the output is HIGH for tH = 0.693·(R1 + R2)·C while the capacitor charges through R1 and R2, and LOW for tL = 0.693·R2·C while it discharges through R2 only. The period is tH + tL = 0.693·(R1 + 2·R2)·C, and since 1 / 0.693 ≈ 1.44, the frequency simplifies to 1.44 / ((R1 + 2·R2)·C).
How do I get a 50% duty cycle with a 555?
A plain 555 astable can never quite reach a 50% duty cycle, because the duty cycle is (R1 + R2) / (R1 + 2·R2), which is always greater than 50% as long as R1 is above zero. Making R1 much smaller than R2 gets close to 50% but never exactly there. To reach exactly 50% — or a duty cycle below 50% — put a diode in parallel with R2 so the capacitor charges through R1 and discharges through R2 separately, letting you set the HIGH and LOW times independently.
What is 555 monostable (one-shot) mode?
In monostable mode the 555 sits with its output LOW until a falling edge arrives on the trigger pin. That single trigger makes the output go HIGH for one fixed pulse of width T = 1.1·R·C, then it returns LOW and waits for the next trigger. It is used for switch debouncing, timed delays, missing-pulse detection and turning a brief press into a fixed-length pulse.
What capacitor do I need for a 1 Hz 555 blinker?
For a 1 Hz astable the timing product must satisfy (R1 + 2·R2)·C = 1.44, since f = 1.44 / ((R1 + 2·R2)·C). With a 10 µF capacitor that means R1 + 2·R2 ≈ 144 kΩ — for example R1 = 4.7 kΩ with R2 ≈ 68 kΩ gives close to 1 Hz. A larger 100 µF capacitor drops the resistance to about 14.4 kΩ total, while a small 1 µF capacitor would need around 1.44 MΩ. Larger capacitors let you use smaller, more common resistor values for slow blink rates.
Related electronics tools
The 555 timer sits at the heart of the electrical cluster: use the capacitor code calculator to decode the timing capacitor's printed value, the resistor color code calculator to read or build R1 and R2, Ohm's law for the surrounding circuit, and the LED resistor calculator when the 555 output drives an LED.
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