555-Timer Rechner

Monostabiles und astabiles Timing

Operating Mode

Required Parameters

Resistor R1

Ohm

Resistor R2

Ohm

Timing capacitor C

Waiting for input data...

Ad Placement

Sidebar Adaptive Ad Slot

Quick Answer

The 555 timer IC generates precise time delays (monostable: T = 1.1 x R x C) or continuous square waves (astable: f = 1.44 / ((R1 + 2xR2) x C)).

Documentation

555 Timer Astable Calculator

Design an NE555 / TLC555 astable oscillator. Enter R1, R2, and C to get frequency, period, high/low times, and duty cycle.

Quick start: R1 = 1 kΩ, R2 = 6.8 kΩ, C = 100 nF → about 986 Hz at 53% duty. This page is astable only.

555 astable frequency formula

f = 1.44 / ((R1 + 2×R2) × C)

T_high = 0.693 × (R1 + R2) × C

T_low = 0.693 × R2 × C

Duty = (R1 + R2) / (R1 + 2×R2)

Symbol	Pin / meaning
R1	VCC → pin 7 (discharge)
R2	pin 7 → pins 6/2 (threshold / trigger)
C	pins 6/2 → GND

Period T = 0.693 × (R1 + 2 R2) × C. Frequency is 1 / T, which is the 1.44 form above.

Why duty cycle is always over 50%

The capacitor charges through R1+R2 and discharges only through R2. Duty is therefore (R1+R2)/(R1+2 R2) — always above 50% on a standard bipolar 555.

Make R1 ≪ R2 to approach 50% (R1 = 1 kΩ, R2 = 100 kΩ → 50.5%)
Or add a diode across R2 (cathode toward pin 7) so charge bypasses R2
CMOS TLC555 / LMC555 can do true 50% with output feedback

Worked example — 1 kHz square wave

Choose C = 100 nF
R1 = 1 kΩ, R2 = 6.8 kΩ
f = 1.44 / ((1k + 2×6.8k) × 100n) = 986 Hz
Duty = (1k + 6.8k) / (1k + 13.6k) = 53.4%

Need a one-shot pulse instead? Open the full 555 Timer Calculator and switch to monostable (T = 1.1 RC).

Component ranges

Parameter	Recommended range	Notes
R1, R2	1 kΩ – 10 MΩ	Below 1 kΩ draws excess current
C (timing)	100 pF – 1000 µF	Film or C0G; avoid electrolytics
C (pin 5)	10–100 nF	Always bypass Control to GND
VCC	4.5 V – 16 V	Standard NE555 range

NE555 vs TLC555

Bipolar NE555 / LM555 draw a few mA and top out near 500 kHz. CMOS TLC555 / LMC555 draw microamps and run to about 2 MHz. The astable formulas are the same if the thresholds stay at 1/3 and 2/3 VCC.

Related tools

555 Timer Calculator — astable and monostable
RC Time Constant Calculator — τ = RC charging
Frequency to Period Converter — f ↔ T
RC Filter Calculator — timing-network cutoff
LED Resistor Calculator — blinker current limit

Design Notes

The NE555 works from 4.5V to 16V and can source/sink up to 200 mA. For low-power use, the CMOS TLC555/ICM7555 draws only 60-250 uA vs 3-10 mA for the bipolar NE555. The standard astable duty cycle is always greater than 50 percent because the cap charges through (R1+R2) but discharges only through R2. Add a diode across R2 for 50 percent duty cycle.

Common Mistakes

1
Forgetting the 0.01uF bypass capacitor on pin 5 (Control Voltage), causing erratic timing from noise.
2
Using electrolytic capacitors for timing: their 20 percent tolerance and leakage make timing inaccurate. Use ceramic or film caps.
3
Exceeding the bipolar NE555 max frequency (~500 kHz). Use the CMOS TLC555 for up to 2 MHz.
4
In monostable mode, the trigger pulse (pin 2) must be shorter than the output pulse and drop below 1/3 Vcc.

Engineering Handbox

1. T_high = 0.693 x (R1+R2) x C = 0.693 x 57k x 100nF = 3.95 ms 2. T_low = 0.693 x R2 x C = 0.693 x 47k x 100nF = 3.26 ms 3. Period T = 7.21 ms, f = 138.7 Hz 4. Duty Cycle = 57k/104k x 100 = 54.8 percent

VerificationOutput: 138.7 Hz square wave with 54.8 percent duty cycle.

Knowledge Base

Was ist der Unterschied zwischen astabil und monostabil?

Astabil schwingt dauerhaft (Oszillator). Monostabil erzeugt einen einzelnen Impuls bei Triggerung (Monoflop).

Was ist die monostabile Formel?

Impulsbreite T = 1,1 × R × C. Die Konstante 1,1 entspricht ln(3).

Was ist die astabile Frequenzformel?

f = 1,44 / ((R1 + 2 × R2) × C). HIGH-Zeit = 0,693 × (R1 + R2) × C, LOW-Zeit = 0,693 × R2 × C.

Warum ist der 0,01µF-Kondensator an Pin 5 wichtig?

Er filtert Rauschen, das sonst Timing-Jitter verursacht. Weglassen ist der häufigste 555-Designfehler.

Was ist der Unterschied NE555 vs. TLC555?

NE555: bipolar, 4,5–16V, 3–10mA Ruhestrom, bis ~500kHz. TLC555: CMOS, 2–15V, 60–250µA, bis 2MHz. TLC555 für Batteriebetrieb.

Related Engineering Tools

digital embedded