The 555 timer, introduced in 1972, is still one of the most popular chips ever made. In astable mode it makes a continuous square wave for blinking LEDs, tone generators and clock signals.
How to use the 555 Timer Astable Calculator
- Capacitance C
- Resistance R₁
- Resistance R₂
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the capacitor and the two resistors.
Inputs explained
| Field | What to enter |
|---|---|
| Capacitance C | Units: µF, nF, pF, mF, F |
| Resistance R₁ | Units: kΩ, Ω, MΩ |
| Resistance R₂ | Units: kΩ, Ω, MΩ |
What each result means
- High time t_H = 0.693×([Resistance R₁] + [Resistance R₂])×[Capacitance C]
- Low time t_L = 0.693×[Resistance R₂]×[Capacitance C]
- Period = 0.693×([Resistance R₁] + 2×[Resistance R₂])×[Capacitance C]
- Frequency = 1÷(0.693×([Resistance R₁] + 2×[Resistance R₂])×[Capacitance C])
- Duty cycle = ([Resistance R₁] + [Resistance R₂])÷([Resistance R₁] + 2×[Resistance R₂])×100
Example values
How the main result changes when Capacitance C changes and everything else stays at the example values above:
| Capacitance C (µF) | Frequency (Hz) |
|---|---|
| 5 | 13.7429 |
| 7.5 | 9.1619 |
| 10 | 6.8714 |
| 12.5 | 5.4971 |
| 15 | 4.581 |
| 20 | 3.4357 |
Formula
t_H = 0.693·(R₁ + R₂)·C, t_L = 0.693·R₂·C, f = 1.44/((R₁ + 2R₂)·C), duty = t_H/(t_H + t_L)
Worked example
Inputs — Capacitance C: 10 µF; Resistance R₁: 1 kΩ; Resistance R₂: 10 kΩ.
Results — High time t_H = 76.23 ms; Low time t_L = 69.3 ms; Period = 145.53 ms; Frequency = 6.87144 Hz; Duty cycle = 52.381 %.
Key terms
- Astable
- Free-running oscillator with no stable state.
Frequently asked questions
Why can't the duty cycle go below 50%?
The capacitor charges through R₁ + R₂ but discharges only through R₂ – add a diode across R₂ to get below 50%.
What is the frequency limit?
Around 100–500 kHz for the bipolar NE555.
Why 0.693?
It is ln 2 – charging between 1/3 and 2/3 of V_CC.
What is the minimum R₁?
About 1 kΩ to protect the discharge transistor.