555 Timer Calculator

About the 555 Timer Calculator

The 555 timer makes square waves (astable mode) or single timed pulses (monostable mode) from one or two resistors and a capacitor. Enter RA, RB and C to get the frequency, period, high and low times and duty cycle of an astable — with a waveform drawing — or enter the frequency and duty cycle you want with a capacitor and get RA and RB, rounded to standard E-series values together with the frequency and duty cycle those values really give.

Duty cycles of 50% or less are not possible with the standard circuit, so the calculator switches to the diode-bypass version (a diode across RB) and includes the diode’s voltage drop. Monostable mode gives the pulse width t = 1.1 × R × C, or R for a pulse you need. The formulas and limits follow the TI datasheets of the bipolar NE555 and the CMOS TLC555.

Astable formulas (TI datasheets)

• High time, while C charges through RA and RB: tH = 0.693 × (RA + RB) × C
• Low time, while C discharges through RB: tL = 0.693 × RB × C
• Period T = tH + tL = 0.693 × (RA + 2RB) × C, frequency f ≈ 1.44 / ((RA + 2RB) × C)
• Duty cycle (output high) D = (RA + RB) / (RA + 2RB) — always above 50% in this circuit.

The capacitor swings between one third and two thirds of the supply, so the timing does not depend on the supply voltage. 0.693 is ln 2.

Duty cycle below 50%: the diode across RB

With a diode across RB (anode at the discharge pin, pin 7), C charges through RA and the diode only and discharges through RB, so the high and low times are set separately: tH = RA × C × ln((⅔Vcc − Vd) / (⅓Vcc − Vd)) and tL = 0.693 × RB × C. With an ideal diode (Vd = 0) the high time is 0.693 × RA × C; the real drop of a silicon diode makes it longer, which the calculator includes. It ignores the small current through RB during charging.

Monostable

A trigger pulse (pin 2 taken below a third of the supply) sets the output high for tw = 1.1 × R × C, after which C is discharged. The trigger must go high again before the end of the pulse — at least 10 µs before for the NE555, 1 µs for the TLC555 — or the output stays high; the same figures are the shortest pulses each chip makes reliably.

NE555 or TLC555?

Both use the same timing formulas and the same pinout. The bipolar NE555 runs from 4.5 V to 16 V, can sink or source up to 200 mA, and TI recommends keeping it at or below 100 kHz. Its threshold current limits RA + RB to about 3.4 MΩ at 5 V and 10 MΩ at 15 V. The CMOS TLC555 runs from 2 V to 15 V, draws far less supply current (1 mW typical at 5 V), allows smaller timing capacitors and works up to about 2 MHz — above 100 kHz its propagation delays lengthen the period. Decouple the supply right next to the chip: TI calls a 0.1 µF ceramic capacitor sufficient for the NE555 and recommends at least 0.1 µF ceramic in parallel with 1 µF electrolytic for the TLC555. TI also notes that a capacitor from the CONT pin to ground (0.01 µF in its circuits) can improve operation.

Sources

• Texas Instruments, xx555 Precision Timers (NA555, NE555, SA555, SE555), datasheet SLFS022K
• Texas Instruments, TLC555 CMOS Timer, datasheet SLFS043K

Limitations

• Timing is only as accurate as the parts: a ±5% resistor and a ±10% (or worse, for electrolytics) capacitor can move the frequency by 15% or more. Use film capacitors and 1% resistors for accuracy, or trim with a potentiometer.
• Electrolytic capacitors leak, which lengthens long delays and makes them less repeatable; very long times are better made with a counter.
• The formulas ignore propagation delays and the discharge transistor’s resistance, which matter above about 100 kHz or with very small RB.
• The diode-bypass result assumes a constant diode drop; measure the circuit when the duty cycle must be exact.

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