Calculate 555 timer astable frequency, duty cycle, period, and component values. Includes reverse design from target frequency and E24 resistor matching.
The NE555 timer in astable mode is one of the most popular oscillator circuits in electronics. It produces a continuous square wave whose frequency and duty cycle are set by two resistors (R1, R2) and a capacitor (C).
This calculator computes frequency f = 1.44/((R1+2R2)·C), duty cycle, high and low times, average current draw, and power consumption. A reverse-design mode lets you enter a target frequency and get the nearest E24 standard resistor values.
The 555 astable is used for LED blinkers, tone generators, IR transmitters (38 kHz), PWM controllers, and clock sources. Preset buttons cover common applications from 1 Hz LED blinking to 38 kHz infrared carrier. The tool also recommends bypass capacitors and standard pin connections.
Understanding the duty cycle limitation is important: in the basic circuit, duty cycle is always above 50% because the timing capacitor charges through R1+R2 but discharges only through R2. This calculator clearly shows the duty cycle so you can plan accordingly.
The 555 timer is a staple of electronics prototyping and education. This calculator saves time by computing all timing parameters from component values, and the reverse-design feature helps select standard resistors for a desired frequency.
It replaces chart lookups and multiple formula evaluations with a single interactive tool. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
tH = 0.693 × (R1 + R2) × C. tL = 0.693 × R2 × C. f = 1.44 / ((R1 + 2R2) × C). Duty = tH / (tH + tL) × 100%.
Result: f ≈ 1.04 Hz, Duty ≈ 66.7%, Period ≈ 0.96 s
tH = 0.693 × (680k+680k) × 1µ = 0.942 s. tL = 0.693 × 680k × 1µ = 0.471 s. Period = 1.413 s, f = 0.71 Hz. Duty = 0.942/1.413 = 66.7%.
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In the basic astable circuit, the capacitor charges through R1+R2 (tHigh) but discharges only through R2 (tLow). Since tHigh > tLow, duty is always > 50%. Add a diode across R2 to bypass it during charging for near-50% duty.
Not with the basic circuit. Place a diode across R2 so charging goes through R1 only, then R1 = R2 gives 50% duty. Or use a CMOS 555 (ICM7555) with a different topology.
Ceramic or film capacitors for frequencies above 1 kHz. Electrolytic capacitors for low frequencies (< 10 Hz) but note their tolerance is poor (±20%).
Typical frequency accuracy is ±1-2% with good components. Temperature drift is about 50 ppm/°C. Use 1% resistors and stable capacitors for best results.
The original NE555 works up to ~500 kHz. CMOS versions like ICM7555 or TLC555 can reach 2 MHz. Above that, use a dedicated oscillator IC.
Pin 5 (Control Voltage) sets the internal threshold. A 10 nF bypass capacitor prevents noise from affecting the timing, improving frequency stability.