555 Timer Calculator

How it works

In astable mode the 555 continuously charges and discharges the timing capacitor C. The capacitor charges through R1 and R2 until it reaches two-thirds of the supply voltage, and the output is high during this time. The discharge pin then switches on and the capacitor discharges through R2 only until it falls to one-third of the supply voltage, and the output is low. The cycle repeats, producing a square-wave style output.

In monostable mode the 555 produces a single output pulse each time the trigger input (pin 2) is pulled below one-third of the supply voltage. The output goes high and the capacitor charges through R until it reaches two-thirds of the supply voltage; the output then returns low and the capacitor is discharged, ready for the next trigger.

Design mode works backwards from the frequency you need. It calculates the missing component, picks the nearest practical standard value and then recalculates the actual frequency, timing and error with that standard part.

555 Astable Formula

f = 1.44 / ((R1 + 2 × R2) × C)

T = 1 / f

TH = 0.693 × (R1 + R2) × C

TL = 0.693 × R2 × C

D = (R1 + R2) / (R1 + 2 × R2) × 100 %

555 Monostable Formula

T = 1.1 × R × C

Design Equations

C = 1.44 / (f × (R1 + 2 × R2))

R2 = ((1.44 / (f × C)) − R1) / 2

R1 = (1.44 / (f × C)) − 2 × R2

Error % = |factual − ftarget| / ftarget × 100

When searching for R1, R2 and C, every standard capacitor and R2 value within the ranges is tried; the exact R1 needed is calculated and the standard R1 values on either side are evaluated. Results are sorted by frequency error. Resistors come from the E12 or E24 series; capacitors from the common 1 – 2.2 – 3.3 – 4.7 – 6.8 series in every decade.

Effect of Component Tolerance

The frequency range shows the approximate worst case when all resistors and the capacitor are at the limits of their tolerance. The real frequency will also vary with capacitor characteristics, temperature, supply voltage and the particular 555 IC.

fmin = 1.44 / ((R1 + 2R2)(1 + tR) × C(1 + tC))

fmax = 1.44 / ((R1 + 2R2)(1 − tR) × C(1 − tC))

Electrolytic capacitors often have −20 %/+80 % tolerance and noticeable leakage current, so circuits using them can be well outside this range.

Design Notes

• The formulas are ideal approximations; they do not guarantee an exact real-world frequency.
• R1 should not normally be extremely low: keep it at 1 kΩ or more to protect the discharge transistor.
• Very large resistor values (several MΩ) are affected by leakage currents of the capacitor, the PCB and the 555 inputs.
• Large electrolytic capacitors can have significant tolerance and leakage.
• The actual frequency depends on the particular 555 device. CMOS 555 (e.g. TLC555, LMC555) and bipolar 555 (NE555) devices behave differently, especially at high frequencies and with large resistors.
• The standard astable circuit cannot produce a duty cycle of 50 % or less; a diode across R2 or a different configuration is required.
• Always verify the calculated result on real hardware.

Notes

• The constant 1.44 is the rounded value of 1 / 0.693, so T = 1 / f and T(H) + T(L) may differ by about 0.2 %.
• Use film or C0G ceramic capacitors for accurate timing, and connect a 10 nF capacitor from pin 5 (control voltage) to ground for better stability.
• In monostable mode the trigger pulse must be shorter than the output pulse, and a new trigger during the output pulse is ignored.