Astable 555 – Free-Running Oscillator
Pins: R1 between VCC and pin 7 · R2 between pin 7 and pin 2/6 · C between pin 2/6 and GND
📐 Astable Formulas
Frequency
f = 1.44 / ((R1+2·R2)·C)
Period
T = (R1+2·R2)·C / 1.44
= T_high + T_low
HIGH Time
t_H = 0.693·(R1+R2)·C
LOW Time
t_L = 0.693·R2·C
Duty Cycle
D = (R1+R2)/(R1+2·R2)×100%
Always ≥ 50%
50% Duty
Use R1≪R2 or add diode
across R2 for ≈50%
❓ FAQ
Why is the duty cycle always above 50% in astable mode?
In a standard astable 555 circuit, capacitor charges through R1+R2 but discharges only through R2. Since charge time > discharge time, duty cycle > 50%. To achieve 50%, add a diode in parallel with R2 (cathode toward pin 7) so charging only uses R1+R2 while discharging only uses R2 effectively.
What is the maximum frequency I can achieve?
The 555 timer can run up to approximately 500 kHz, though performance degrades above ~100 kHz. For very small R and C values, propagation delays inside the IC limit the maximum frequency. Use CMOS 555 (TLC555) for better high-frequency performance.
How do I calculate component values for a target frequency?
Rearrange: (R1+2·R2) = 1.44 / (f·C). Choose C first (e.g. 100nF), set R1 = R2 for a ~67% duty cycle, then solve. Or use this calculator in reverse: try values until you hit your target frequency.
Monostable 555 – One-Shot Timer
Formula: t = 1.1 × R × C
📐 Monostable Formula
Pulse Width
t = 1.1 × R × C
R in Ω, C in F → t in seconds
Solve for R
R = t / (1.1 × C)
Solve for C
C = t / (1.1 × R)
Typical Values
100kΩ + 10µF → 1.1s
10kΩ + 100µF → 1.1s
1MΩ + 1µF → 1.1s
❓ FAQ
How does the monostable 555 work?
When a negative trigger pulse is applied to pin 2, the output goes HIGH and the capacitor starts charging through R. When the capacitor voltage reaches 2/3 Vcc, the output returns LOW. The time HIGH = 1.1 × R × C, independent of supply voltage.
Can the pulse be retriggered before it ends?
Standard 555 monostable is non-retriggerable — a new trigger while output is HIGH is ignored. For a retriggerable monostable, use the 74HC123 or 74HC221, or reset the 555 by briefly pulling pin 4 low.