liminfo555 Timer Calculator
Free web tool: 555 Timer Calculator
Quick Presets
Frequency
480.000 Hz
Period
2.083 ms
Duty Cycle
66.7%
HIGH Time
1.386 ms
LOW Time
693.00 µs
Astable Circuit Diagram
Output Waveform
Formulas
About 555 Timer Calculator
The 555 Timer Calculator is a free, browser-based tool designed for electronics engineers, hobbyists, and students working with the NE555 or LM555 timer IC. It covers two fundamental operating modes: Astable and Monostable. In Astable mode, the 555 oscillates continuously — the calculator computes output frequency, period, duty cycle, and the HIGH (charge) and LOW (discharge) times using the standard formulas: f = 1.44 / ((R1 + 2×R2) × C), tH = 0.693 × (R1 + R2) × C, and tL = 0.693 × R2 × C.
In Monostable mode, a trigger pulse causes the output to go HIGH for a fixed time then return LOW. The calculator computes the output pulse width using the formula t = 1.1 × R × C, where R is the timing resistor and C is the timing capacitor. All component values support unit selection — resistors in Ω, kΩ, or MΩ, and capacitors in pF, nF, or µF — so you can work at any scale without manual unit conversion.
Results are displayed instantly as you change inputs, with smart unit formatting: times are shown in µs, ms, or s, and frequencies in Hz, kHz, or MHz depending on magnitude. This eliminates transcription errors common when doing calculations on paper or in spreadsheets. All computation happens client-side in your browser — no data is sent to any server.
Key Features
- Astable mode: calculates frequency, period, duty cycle, HIGH time, and LOW time using standard 555 formulas
- Monostable mode: calculates output pulse width (t = 1.1 × R × C) for one-shot timer designs
- SVG circuit diagram: interactive schematic with component values labeled live for both astable and monostable modes
- Output waveform preview: SVG square-wave display with tH, tL, frequency, period, and duty-cycle annotations
- Quick presets: LED blinker (1 Hz), piezo buzzer (1 kHz), PWM LED dimmer (~20 kHz) — click to auto-fill values
- Component unit selection — R1/R2 in Ω/kΩ/MΩ, capacitor in pF/nF/µF
- Smart result formatting — times auto-scale to µs, ms, or s; frequency to Hz, kHz, or MHz
- Real-time recalculation on every input change — no submit button needed
- 100% client-side processing — component values never leave your browser
- Dark mode support and responsive layout for use on any device
Frequently Asked Questions
What formulas does the 555 Timer Calculator use?
For Astable mode: frequency f = 1.44 / ((R1 + 2×R2) × C), HIGH time tH = 0.693 × (R1 + R2) × C, LOW time tL = 0.693 × R2 × C, duty cycle = (R1 + R2) / (R1 + 2×R2) × 100%. For Monostable mode: pulse width t = 1.1 × R × C. These are the industry-standard equations from the NE555 datasheet.
Can the 555 astable circuit produce a 50% duty cycle?
A standard two-resistor astable circuit cannot reach exactly 50% duty cycle because tH always includes R1 in addition to R2. To achieve 50%, a diode bypass technique is used (bypassing R1 during discharge), or R1 is set very small relative to R2. Some designs also use a buffered output to achieve symmetric waveforms.
What is the valid frequency range for a 555 timer?
The NE555 is typically rated for frequencies from below 1 Hz up to approximately 500 kHz in astable mode, though some CMOS variants (TLC555, LMC555) reach into the MHz range. High-frequency operation requires careful PCB layout to minimize parasitic capacitance and inductance on the timing pins.
How do I pick R1, R2, and C for a target frequency?
Start by choosing a capacitor value (e.g., 100 nF), then solve for R2 to set the approximate frequency using R2 ≈ 0.72 / (f × C). Choose R1 to be at least a few hundred ohms (to limit discharge current) but small relative to R2 if you want a duty cycle close to 50%. Use this calculator to verify the final combination.
What is the difference between Astable and Monostable modes?
In Astable mode the 555 oscillates continuously, producing a square-ish waveform. There is no stable output state — the output alternates between HIGH and LOW indefinitely. In Monostable (one-shot) mode, the output is normally LOW and only goes HIGH for a defined period (t = 1.1RC) when a falling-edge trigger is applied to pin 2.
Why does duty cycle always exceed 50% in a standard astable circuit?
Because the capacitor charges through both R1 and R2 (HIGH time), but discharges only through R2 (LOW time). Since tH = 0.693(R1+R2)C and tL = 0.693·R2·C, tH is always larger than tL when R1 > 0, making duty cycle always greater than 50%. Setting R1 to near zero is not safe as it can destroy the IC by short-circuiting pin 7 to Vcc.
Can I use this calculator for CMOS 555 variants (TLC555, LMC555)?
Yes. CMOS variants use the same RC timing formulas as the bipolar NE555. The key differences — lower supply voltage range, much lower power consumption, and higher frequency capability — do not change the underlying timing equations. This calculator is valid for all 555-compatible ICs.
What capacitor types are recommended for 555 timing circuits?
For stable timing, use film capacitors (polyester, polypropylene) or tantalum for values above 1 µF. Ceramic capacitors with X7R or C0G dielectric are acceptable for smaller values. Avoid Y5V or Z5U ceramics as their capacitance varies significantly with temperature and voltage, causing timing drift. Electrolytic capacitors can be used for long time constants but have wide tolerances.