The 555 timer: astable and monostable, with the formulas
The 555 has been the go-to timer chip since 1972: two comparators, a flip-flop and a discharge transistor that together watch a capacitor charge and discharge between a third and two thirds of the supply. Wired one way it oscillates (astable) and gives you a square-ish wave for blinking, beeping, clocking or PWM; wired another it fires one clean pulse of a set length per trigger (monostable). This calculator does both, draws the circuit with your parts, shows the output and capacitor waveforms, and finds the capacitor that lands on a frequency or pulse you want.
How to use the 555 timer calculator
- Pick the mode in the menu row: Astable for a continuous oscillation, Monostable for one pulse per trigger.
- Type the parts: R1 and R2 and the timing capacitor (astable), or R and C (monostable), as they are marked (10k, 4k7, 10u, 15n). The supply is optional and only scales the waveform.
- Read the frequency, period, high and low times and duty, or the pulse width; the drawing shows the chip wired up with your values and the waveforms with the times marked.
- Optional: type the frequency or pulse you want, see the capacitor that gives it with your resistors, and put it in with one click. Other capacitors lists the standard values; Calculation writes the maths out; click the chip for its pins.
Inside the 555
Three equal resistors inside the chip divide the supply into ⅓ and ⅔ VCC. One comparator watches the threshold pin (6): when it rises above ⅔ VCC the flip-flop resets, the output goes low and the discharge transistor on pin 7 turns on. The other watches the trigger pin (2): when it falls below ⅓ VCC the flip-flop sets, the output goes high and the discharge transistor turns off. Everything the chip does follows from those two trip points, which is why the timing depends only on R and C and not on the supply voltage.
Astable: the oscillator
f ≈ 1.44 / ((R1 + 2 R2) C) thigh = 0.693 (R1 + R2) C tlow = 0.693 R2 C
R1 runs from the supply to pin 7, R2 from pin 7 to the capacitor, and pins 2 and 6 are joined to the capacitor. The capacitor charges through R1 + R2 until it reaches ⅔ VCC, then pin 7 grounds the top of R2 and it discharges through R2 alone until ⅓ VCC, and the cycle repeats. Each half covers half of the remaining voltage gap, which on an RC curve takes ln 2 = 0.693 time constants; 1 / 0.693 is the 1.44 in the frequency formula. The duty cycle is (R1 + R2) / (R1 + 2 R2), always over 50%: make R2 much larger than R1 for a near-square wave, or put a diode across R2 (anode to pin 7) so the charge path skips R2 and the two times become 0.693 R1 C and 0.693 R2 C independently.
| R1 | R2 | C | Frequency | Duty | Use |
|---|---|---|---|---|---|
| 1 kΩ | 10 kΩ | 10 µF | 6.87 Hz | 52% | a slow blink |
| 1 kΩ | 47 kΩ | 15 nF | 1.01 kHz | 51% | a 1 kHz tone |
| 10 kΩ | 100 kΩ | 1 µF | 6.87 Hz | 52% | a few hertz, near-square |
| 1 kΩ | 10 kΩ | 1 nF | 68.7 kHz | 52% | a 69 kHz clock |
Monostable: one pulse
t = 1.1 R C
One resistor from the supply to pins 6 and 7 (joined), the capacitor from there to ground, and the trigger on pin 2 held high through a pull-up. At rest the discharge transistor keeps the capacitor empty and the output low. A short low pulse on the trigger sets the output high and lets the capacitor charge; when it reaches ⅔ VCC — ln 3 = 1.0986 time constants later — the output drops and the capacitor is emptied again, ready for the next trigger. Triggers that arrive during the pulse are ignored, so the output is a clean pulse of fixed width whatever the input looks like: a switch debouncer, a missing-pulse detector, a delay, a touch-on timer.
Choosing the parts
The timing resistors should be between about 1 kΩ and 1 MΩ: below that the discharge pin has to sink too much current and the chip heats; above it the chip's own input current (a microamp or so on the bipolar part) spoils the timing. The capacitor should be at least 100 pF, so stray capacitance does not matter, and for long times a film or low-leakage electrolytic rather than a cheap electrolytic, whose leakage stretches the pulse. The formula is independent of the supply, but the chip itself needs 4.5–16 V (bipolar) or 2–15 V (CMOS). Put 10 nF from pin 5 to ground and 100 nF across the supply near the chip, and expect ±5–10% from part tolerances; a trimmer in series with R2 (astable) or R (monostable) takes care of the rest.
Your circuit, step by step
- High: 0.693 × (1 kΩ + 10 kΩ) × 10 µF = 76.25 ms.
- Low: 0.693 × 10 kΩ × 10 µF = 69.31 ms.
- Period and frequency: 145.6 ms → 6.87 Hz; duty 52%.
Worked example: 1 kΩ, 10 kΩ and 10 µF
High time 0.693 × (1 kΩ + 10 kΩ) × 10 µF = 76.25 ms; low time 0.693 × 10 kΩ × 10 µF = 69.31 ms; period 145.6 ms, so the LED on the output blinks at 6.87 Hz with a 52% duty — the classic first 555 circuit. Swap the capacitor for 100 nF and the same resistors give 687 Hz, an audible tone; for a monostable, 100 kΩ and 10 µF give a pulse of 1.1 s.
Questions
Why does the supply voltage not change the frequency?
Because both trip points are fractions of the supply, and so is the voltage the capacitor is charging towards. Raise the supply and the capacitor charges faster, but it also has further to go, and the two effects cancel exactly.
How do I get a 50% or lower duty cycle?
A diode across R2 (anode to pin 7) lets the charging current bypass R2, so the high time is set by R1 and the low time by R2, and any duty is possible. Or take the output from a divide-by-two flip-flop, which is always exactly 50%.
Can the output drive a speaker or a relay directly?
The bipolar 555 sinks or sources up to 200 mA, enough for a small speaker through a capacitor, a relay with a flyback diode, or a handful of LEDs. The CMOS versions manage a few milliamps and need a transistor.
What is the fastest a 555 will go?
The bipolar part works to a few hundred kilohertz, CMOS versions to about 2–3 MHz; above that the internal delays dominate and the formula drifts. For slow timing, hours are reachable with a few megohms and a low-leakage capacitor on a CMOS 555, but a microcontroller or a counter chip is more accurate.