555 Timer Astable Calculator
Enter R1, R2 and C for the frequency and duty cycle, or enter a target and get standard E12 resistor values.
Step-by-step
- Enter valid values to begin.
Model: ideal 555 with comparator thresholds at 1/3 and 2/3 VCC, so t = ln2 × R × C = 0.693 RC; the common 1.44/((R1+2R2)C) formula is simply 1/0.693 ≈ 1.443. Real frequency also moves with comparator delay, discharge-transistor saturation and capacitor tolerance, so tolerances of 5–10% are normal and the exact frequency should be trimmed on the bench. Supply limits, the ~200 mA discharge limit, the 1 kΩ–10 MΩ resistor window and the maximum frequencies used here are typical datasheet-class values; check the datasheet of the exact part (NE555, LM555, TLC555, LMC555) before you rely on them. In design mode the E12 values come from the IEC 60063 preferred series and the results shown use those rounded resistors.
555 Astable Timing: Where the 0.693 and the Duty Cycle Come From
Core Engineering Principles
In the astable circuit the timing capacitor charges through R1 and R2, and when it reaches two thirds of the supply the internal flip-flop flips and the discharge transistor on pin 7 pulls the capacitor down through R2 alone. At one third of the supply it releases and the cycle repeats. Both thresholds are fixed fractions of VCC, so the supply cancels out, and an RC swing between those two levels always takes ln 2 = 0.693 time constants. The 1.44 in the textbook frequency formula is just its reciprocal.
The catch is that R1 sits only in the charging path, so Thigh is always longer than Tlow and the duty cycle can never fall to 50% or below. For a square wave, put a signal diode across R2 or make R1 small. On the bench we pick C first, usually 10 nF to 1 µF, then solve for resistors from 1 kΩ to a few MΩ. Use C0G or film capacitors: X7R ceramics shift with bias and temperature and move the frequency by tens of percent. The bipolar part also draws a current spike at each output edge, so decouple pin 8 with 100 nF.
Duty = (R1 + R2) / (R1 + 2R2) × 100 • Idis = VCC / R1
NEC & Standard References
The NE555 / LM555 datasheets from TI and ST give the astable equations, a supply range of 4.5 to 16 V for the bipolar part and the discharge current limit of about 200 mA. The CMOS TLC555 and LMC555 run from lower supplies with less output drive. IEC 60063 defines the E-series of preferred values, and E12 (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) is used here to round resistors. IEC 60384-1 is the generic fixed-capacitor specification.
1. Thigh = 0.693 × 20 kΩ × 100 nF = 1.386 ms.
2. Tlow = 0.693 × 10 kΩ × 100 nF = 693 µs.
3. Period = 2.079 ms, so f = 481 Hz. The 1.44 formula gives 480 Hz, the small gap being rounding.
4. Duty = 20 / 30 = 66.7%.
5. Idis = 12 V / 10 kΩ = 1.2 mA, far below the 200 mA limit.
6. A diode across R2 gives 693 µs high and 693 µs low: 721.5 Hz at 50% duty.
- Never drop R1 below about 1 kΩ. The discharge pin sinks VCC / R1 plus the capacitor current, and the part fails near its 200 mA limit.
- Respect the supply window. A bipolar 555 below 4.5 V misbehaves and above 16 V can be destroyed.
- Leakage ruins megohm designs. Resistors above about 10 MΩ are swamped by threshold bias current and board contamination, so keep high-impedance nodes clean and dry.
- Expect 5–10% error. Capacitor tolerance dominates; trim with a potentiometer if the frequency matters.