555 astable oscillator: f = 1.44 / ((R1 + 2×R2) × C). Duty cycle D = (R1 + R2) / (R1 + 2×R2). Duty cycle is always above 50% in standard astable configuration — see notes for 50% designs.
555 Timer — Astable Mode
Parameter
Value
Frequency
--
Duty Cycle
--
Period
--
tHIGH
--
tLOW
--
Formula & Theory
Frequency: f = 1.44 / ((R1 + 2×R2) × C)
Duty cycle: D = (R1 + R2) / (R1 + 2×R2)
Period: T = 1/f = (R1 + 2×R2) × C / 1.44
High time: tHIGH = 0.693 × (R1 + R2) × C
Low time: tLOW = 0.693 × R2 × C
The factor 1.44 = 1 / (0.693 × (1 + 2)) — derived from the RC charge/discharge fractions between the 555's internal threshold voltages (1/3 Vcc and 2/3 Vcc).
Set R1 = 1 kΩ (minimum recommended for output current limiting), then 2×R2 = 13,400 → R2 = 6.7 kΩ → use 6.8 kΩ
Actual f = 1.44 / ((1 + 13.6) × 1000 × 0.1×10⁻⁶) = 986 Hz
Duty cycle = (1 + 6.8) / (1 + 13.6) = 53.4%
Assumptions & Limitations
Standard astable circuit — timing capacitor between pins 2/6 and ground, R1 from Vcc to pin 7, R2 from pin 7 to pins 2/6
No control voltage on pin 5 (leave unconnected or bypass with 10 nF to GND)
Supply voltage 5–15 V — frequency has slight Vcc dependence in bipolar 555s (less so in CMOS variants)
Duty cycle always > 50% — standard circuit cannot achieve ≤ 50% without modification
Common Mistakes
Making R1 too small: R1 must be at least 1 kΩ to limit the discharge current through the 555's internal switch (rated 200 mA max). Very low R1 can damage the IC.
Not bypassing pin 5: Leave pin 5 (control voltage) floating and noise couples in, modulating the frequency. Always add a 10 nF bypass capacitor from pin 5 to GND.
Ignoring CMOS variants for low-power: NE555 draws 5–15 mA supply current. For battery applications, use TLC555 or LMC555 (CMOS) which draw 100–250 µA.
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Why is duty cycle always above 50% in astable mode?
In the standard astable circuit, the capacitor charges through R1 + R2 and discharges through R2 only. Since R1 always contributes to the charge time but not the discharge time, the high period (charge) is always longer than the low period (discharge). To get 50% duty cycle, add a bypass diode around R1 during charging.
How do I get a 50% duty cycle from a 555 timer?
Add a diode (e.g. 1N4148) in parallel with R1, oriented to bypass R1 during the capacitor charging phase. During charging, current flows through the diode directly to the capacitor, so tHIGH ≈ 0.693 × R2 × C. Since tLOW = 0.693 × R2 × C, the duty cycle is 50%.
What is the maximum frequency a 555 timer can oscillate at?
The NE555 is typically specified to 500 kHz, though many work to 1 MHz with some degradation in duty cycle accuracy. CMOS variants like the TLC555 or LMC555 have lower maximum frequency but far lower supply current — useful for battery-powered designs.