How to use the 555 timer calculator
- Choose Astable for a continuous square wave — blinking LEDs, beepers, clock pulses, PWM — or Monostable for a single timed pulse after a trigger.
- Enter the resistors and capacitor with their units.
- Read the frequency, period, high time, low time and duty cycle (astable) or the pulse width (monostable). The waveform redraws as you type.
Astable mode formulas
In astable mode the capacitor charges through R1 and R2, and discharges through R2 only.
t_high = 0.693 × (R1 + R2) × C
t_low = 0.693 × R2 × C
f = 1.44 ÷ ((R1 + 2 × R2) × C)
Duty = (R1 + R2) ÷ (R1 + 2 × R2)
Monostable mode formula
In monostable mode the 555 waits for a trigger on pin 2, then holds the output high for a fixed time:
T = 1.1 × R × C
100 kΩ and 10 µF give 1.1 seconds; 1 MΩ and 100 µF give about 110 seconds.
555 pin guide
| Pin | Name | What it does |
|---|---|---|
| 1 | GND | Ground (0 V) |
| 2 | TRIG | Starts a cycle when it falls below ⅓ of VCC |
| 3 | OUT | Output — drives LEDs, buzzers or a transistor |
| 4 | RESET | Hold high (to VCC) for normal operation |
| 5 | CTRL | Usually 10 nF to ground for stability |
| 6 | THR | Ends the high time at ⅔ of VCC |
| 7 | DIS | Discharges the timing capacitor |
| 8 | VCC | Supply, 4.5 V to 15 V for the NE555 |
Choosing component values
- Keep R1 at 1 kΩ or more to protect the discharge pin.
- Keep timing resistors below about 1 MΩ — above that, leakage current makes timing unpredictable.
- Electrolytic capacitors are only accurate to about ±20%. Use film capacitors when timing matters.
- The NE555 is reliable up to a few hundred kHz. For faster or more accurate clocks, use a crystal oscillator or a microcontroller timer.
Need to size the LED resistor on the output? Use the LED resistor calculator.
