What is astable mode on a 555 timer?

In astable mode the 555 continuously charges and discharges the timing capacitor between 1/3 and 2/3 of Vcc, producing a square wave with no external trigger needed. Frequency = 1.44 / ((Ra + 2Rb) × C).

What is monostable mode on a 555 timer?

In monostable (one-shot) mode the output goes HIGH for one fixed period when a falling edge appears on the trigger pin. Pulse width = 1.1 × R × C. After the pulse the output returns LOW and waits for the next trigger.

How do I get a 50% duty cycle with a 555?

A standard astable 555 always produces a duty cycle above 50% because the capacitor charges through Ra+Rb but only discharges through Rb. To reach exactly 50% use a Schmitt-trigger inverter modification, or use a diode across Rb so the charge path is Ra only and set Ra = Rb.

What capacitor value range works best for a 555?

Capacitor values from 100 pF to 1000 µF all work. Very small capacitances (< 100 pF) make the timing sensitive to stray capacitance on the PCB. Electrolytic capacitors above 10 µF introduce leakage that reduces accuracy.

What is the minimum timing resistor for a 555?

The datasheet recommends Ra and Rb each be at least 1 kΩ to limit peak discharge current through the internal transistor. Values above 1 MΩ may cause timing errors due to CMOS input bias currents or leakage.

Does supply voltage affect the 555 frequency?

No. The 555 uses internal 1/3 Vcc and 2/3 Vcc thresholds, so the timing period (and therefore frequency) depends only on R and C — not on Vcc. This makes the 555 useful over a 5 V to 15 V supply range.

555 Timer Calculator

Enter R and C values for astable or monostable mode and instantly get frequency, duty cycle, and pulse width for the 555 timer IC.

Operating Mode

R_A — Charge resistor

R_B — Discharge resistor

C — Timing capacitor

Enter component values and click Calculate

Results appear here

Summary

The 555 timer IC is one of the most widely used integrated circuits in electronics. In astable mode it acts as a free-running oscillator whose frequency and duty cycle are set by two resistors (Ra, Rb) and a capacitor (C). In monostable mode a single resistor and capacitor set a one-shot pulse width triggered by a falling edge on the trigger pin. This calculator covers both modes and shows all derived timing values instantly.

How it works

Select the operating mode: Astable (continuous oscillation) or Monostable (single pulse on trigger).
For Astable mode enter Ra, Rb, and C with their units.
For Monostable mode enter R and C with their units.
Click Calculate to see frequency, period, duty cycle, pulse width, and charge/discharge times.
Adjust component values and recalculate to hit a target frequency or pulse width.
Use the Reset button to clear all inputs and start a new design.

Use cases

Design a clock oscillator for a microcontroller prototype.
Set the blink rate of an LED flasher circuit.
Generate a PWM signal with a specific duty cycle.
Create a one-shot delay pulse for debouncing or triggering.
Verify component values against a 555 circuit from a datasheet example.
Teach or learn 555 timer theory with instant visual feedback.
Size components for a missing-pulse detector.
Build a tone generator at a specific audio frequency.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu

555 Timer Calculator

Use the 555 Timer Calculator

Output Waveform

Summary

How it works

Use cases

Frequently Asked Questions