555タイマー 計算ツール
無安定・単安定マルチバイブレータ
Required Parameters
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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)).
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
Knowledge Base
無安定と単安定の違いは？
無安定：連続矩形波（発振器）。単安定：トリガでワンショットパルス出力。
単安定の公式は？
パルス幅 T = 1.1×R×C。1.1はln(3)に相当。
無安定の周波数公式は？
f = 1.44/((R1+2×R2)×C)。HIGH = 0.693×(R1+R2)×C、LOW = 0.693×R2×C。
ピン5のコンデンサの役割は？
ノイズをフィルタしてジッターを防止。省略は最も多い555設計ミスです。
NE555とTLC555の違いは？
NE555：バイポーラ、4.5〜16V、~500kHz。TLC555：CMOS、2〜15V、低消費電力、2MHzまで。
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