Calculator
Capacitor charges through R1+R2, discharges through R2 only. Minimum duty cycle ≈ 50% (R1 ≪ R2). For <50% duty cycle, add a diode across R2.
Charge path only. Keep ≥ 1 kΩ
Charge and discharge path
555 pin reference (DIP-8)
| Pin | Name | Function |
|---|
How it works
Astable formulas
tH = 0.693 × (R1+R2) × C
tL = 0.693 × R2 × C
T = tH + tL
f = 1.44 / ((R1+2×R2) × C)
DC = (R1+R2) / (R1+2×R2) × 100%
Monostable formula
t = 1.1 × R × C
Output HIGH for time t
after trigger on pin 2
Trigger: brief LOW pulse
(must be shorter than t)
Astable tips
- Minimum duty cycle is ~50% (when R1 ≪ R2)
- For 50% exactly: add diode in parallel with R2 (cathode to pin 7)
- Max frequency: ~300 kHz (NE555), ~1 MHz (CMOS 555)
- Add 0.01 µF from pin 5 to GND to reduce noise on control voltage
- Keep R1 ≥ 1 kΩ to protect discharge transistor
Applications
- Astable: LED blinker, tone generator, clock signal, PWM, motor speed control
- Monostable: Debounce, delay relay, missing pulse detector, one-shot timer
- Variants: NE555 (bipolar), CMOS TLC555 / LMC555 (lower power, higher freq)
- Supply: 5–15 V (NE555), 2–15 V (CMOS)