NE555 Timer Calculator

Calculate frequency, duty cycle, and pulse duration for 555 timer circuits in astable or monostable mode.

⚙️ Configuration

Mode: Astable Monostable

Resistor R1 (Ohms)

Resistor R2 (Ohms)

Capacitor C (µF)

📊 Results

Frequency:

Duty Cycle:

Time High (tH):

Time Low (tL):

📈 Output Waveform

Theory & Formulas

Astable Mode

In astable mode, the NE555 acts as a free-running oscillator, generating a continuous square wave without any external trigger. The frequency and duty cycle are determined by two resistors (R1, R2) and one capacitor (C).

Time High (tH): 0.693 * (R1 + R2) * C
Time Low (tL): 0.693 * R2 * C
Frequency (f): 1.44 / ((R1 + 2*R2) * C)
Duty Cycle (%): (tH / (tH + tL)) * 100

Share & Export

Save Your Configuration

Copy a link to your calculation or export the results to a PDF.

Frequently Asked Questions

What is the difference between Astable and Monostable mode?

Astable mode creates a free-running oscillator that generates a continuous square wave at a specific frequency and duty cycle. It doesn't require a trigger. Monostable mode (or "one-shot" mode) produces a single output pulse of a specific duration only when you provide an external trigger signal.

Why can't a standard 555 astable circuit achieve a 50% duty cycle?

In the standard configuration, the capacitor charges through R1 and R2 but discharges only through R2. Since the charging path resistance (R1+R2) is always greater than the discharge path resistance (R2), the high time (tH) is always longer than the low time (tL), making a duty cycle of 50% or less impossible. To achieve 50%, a diode can be placed in parallel with R2.

What are typical component value ranges?

For reliable operation, resistors (R1, R2) are typically between 1 kΩ and 10 MΩ. The capacitor (C) is usually between 100 pF and 1000 µF. Using values outside these ranges can lead to inaccurate timing due to leakage currents or other parasitic effects.