555 Timer Calculator
This 555 timer calculator computes the output frequency, period, duty cycle, and pulse width for NE555 / LM555 circuits in astable and monostable mode. Enter your resistor and capacitor values — results update instantly using the standard 555 timing formulas (f = 1.44 / ((R1 + 2·R2)·C) for astable, t = 1.1·R·C for monostable).
Astable Mode
The 555 timer oscillates continuously, generating a square wave. Enter R1, R2, and C to calculate frequency, period, and duty cycle.
Results
Frequency
6.871 Hz
Period
145.5 ms
Duty Cycle
52.38%
High Time (t_high)
76.23 ms
Low Time (t_low)
69.30 ms
f = 1.44 / ((R1 + 2·R2) · C)
t_high = 0.693 · (R1 + R2) · C | t_low = 0.693 · R2 · C
Frequently Asked Questions
What is a 555 timer IC?
The 555 timer (NE555, LM555) is one of the most popular integrated circuits ever made. It is an 8-pin IC that can operate as an astable oscillator (continuously generating a square wave), a monostable one-shot (producing a single timed pulse on each trigger), or a bistable flip-flop. It was designed by Hans Camenzind in 1971 and has been in continuous production ever since.
What is the astable mode of a 555 timer?
In astable mode the 555 has no stable state — it continuously switches between HIGH and LOW, generating a square wave on the output pin. The timing is set by two resistors (R1 and R2) and a capacitor (C). The output is HIGH while C charges through R1 + R2, and LOW while C discharges through R2 alone. The frequency is f = 1.44 / ((R1 + 2·R2) × C).
What is the monostable mode of a 555 timer?
In monostable (one-shot) mode the 555 has one stable state (output LOW). Each negative-going trigger pulse on pin 2 causes the output to go HIGH for a fixed duration set by t = 1.1 × R × C, after which it returns to LOW and waits for the next trigger. It is used for switch debouncing, pulse stretching, and timed delays.
How do I calculate the frequency of a 555 timer in astable mode?
Use the formula f = 1.44 / ((R1 + 2·R2) × C), where R1 and R2 are in ohms and C is in farads. For example, with R1 = 1 kΩ, R2 = 10 kΩ, and C = 10 µF: f = 1.44 / ((1,000 + 20,000) × 0.00001) = 1.44 / 0.21 ≈ 6.86 Hz. Enter your values in the Astable tab above for instant results.
How do I calculate the pulse width of a 555 timer in monostable mode?
The pulse width formula is t = 1.1 × R × C. For a 1-second pulse with C = 10 µF, rearrange to find R = t / (1.1 × C) = 1 / (1.1 × 0.00001) ≈ 90,909 Ω — use a 91 kΩ resistor. The constant 1.1 derives from –ln(1/3), the time for C to charge from 0 to 2/3 Vcc.
Why is the duty cycle of a 555 always above 50% in astable mode?
Because the capacitor charges through R1 + R2 but discharges only through R2, the charge time is always longer than the discharge time, so t_high > t_low and duty cycle > 50%. To achieve 50% or less, add a diode in parallel with R2 so the capacitor charges through R1 only and discharges through R2 only; the duty cycle then becomes R1 / (R1 + R2) × 100%.
What supply voltage does the 555 timer require?
The bipolar NE555 / LM555 operates from 4.5 V to 16 V (some variants to 18 V). The CMOS versions (TLC555, LMC555, ICM7555) operate from 2 V to 15 V and consume far less quiescent current (typically under 100 µA vs. ~6 mA for bipolar), making them ideal for battery-powered circuits. Output can source or sink up to 200 mA on the bipolar version.
What is the role of the 0.01 µF capacitor on pin 5?
Pin 5 (Control Voltage) is connected internally to the 2/3 Vcc voltage divider node. A 0.01 µF (10 nF) bypass capacitor from pin 5 to GND filters supply noise that could modulate the internal thresholds and cause jitter in the output frequency or pulse width. Always add this bypass capacitor unless you are intentionally using pin 5 for voltage-controlled frequency or pulse-width modulation.
555 Timer IC — Complete Reference
555 Timer Pinout
The NE555 / LM555 is an 8-pin DIP (or SOIC) IC. Every pin has a distinct function — knowing them is essential before building any 555-based circuit.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Ground — connect to the negative supply rail. |
| 2 | TRIG | Trigger — a falling edge below 1/3 Vcc sets the output HIGH and starts the timing cycle. |
| 3 | OUT | Output — swings between GND (~0.1 V) and Vcc (~1.7 V below Vcc). Can source/sink up to 200 mA. |
| 4 | RESET | Reset (active LOW) — pull LOW to immediately reset output to LOW. Tie to Vcc if not used. |
| 5 | CTRL | Control Voltage — directly sets the internal comparator threshold (normally 2/3 Vcc). Add a 0.01 µF bypass cap to GND if unused. |
| 6 | THR | Threshold — when this pin rises above 2/3 Vcc, the output is set LOW and the capacitor begins discharging. |
| 7 | DIS | Discharge — an internal transistor shorts this pin to GND when the timer output goes LOW, discharging the timing capacitor. |
| 8 | Vcc | Supply Voltage — typically 5 V to 15 V DC (CMOS version 2 V to 18 V). |
The CMOS equivalent (TLC555, LMC555) operates at lower voltages and consumes far less quiescent current, making it ideal for battery-powered applications.
Astable Mode Formula
In astable mode the 555 oscillates continuously with no stable state. The capacitor C alternately charges through R1 + R2 and discharges through R2 alone, producing a continuous square wave on the output pin.
Key Formulas
t_high = 0.693 × (R1 + R2) × C
t_low = 0.693 × R2 × C
Period T = t_high + t_low = 0.693 × (R1 + 2·R2) × C
Frequency f = 1 / T = 1.44 / ((R1 + 2·R2) × C)
The constant 0.693 is ln(2), which appears because the capacitor charges and discharges exponentially through an RC network. The voltage at the capacitor swings between 1/3 Vcc (lower threshold) and 2/3 Vcc (upper threshold), a ratio that naturally involves ln(2).
Example Calculation
Given R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF:
- t_high = 0.693 × (1,000 + 10,000) × 0.00001 = 0.07623 s
- t_low = 0.693 × 10,000 × 0.00001 = 0.0693 s
- T = 0.07623 + 0.0693 = 0.14553 s
- f = 1 / 0.14553 ≈ 6.87 Hz
- Duty Cycle = 0.07623 / 0.14553 × 100 ≈ 52.4%
Monostable Mode Formula
In monostable (one-shot) mode, the 555 produces a single output pulse of fixed duration each time a negative-going trigger pulse is applied to pin 2. After the pulse the output returns to LOW until the next trigger.
Pulse Width Formula
t = 1.1 × R × C
where t is in seconds, R in ohms, and C in farads.
The constant 1.1 comes from –ln(1/3) ≈ 1.0986, the time for the capacitor to charge from 0 to 2/3 Vcc through R. The output goes HIGH when the trigger fires and returns LOW when the capacitor reaches the 2/3 Vcc threshold.
Example Calculation
To generate a 1-second pulse with C = 10 µF:
- t = 1.1 × R × C → R = t / (1.1 × C) = 1 / (1.1 × 0.00001)
- R ≈ 90,909 Ω (use a 91 kΩ resistor)
For pulse widths below ~10 µs, stray capacitance and lead inductance become significant — consider using a dedicated one-shot IC such as the 74HC123 for precision timing at high frequencies.
Duty Cycle Explained
Duty cycle is the percentage of one period during which the output is HIGH. For the standard astable 555 circuit:
Duty Cycle = t_high / (t_high + t_low) × 100%
= (R1 + R2) / (R1 + 2·R2) × 100%
Because C charges through R1 + R2 but discharges only through R2, the standard configuration always produces a duty cycle greater than 50%. The minimum duty cycle approaches 50% as R1 approaches 0 (but R1 must be at least a few hundred ohms to protect the discharge transistor).
To achieve a duty cycle below 50%, add a diode in parallel with R2 so that C charges through R1 only and discharges through R2 only. With a bypass diode:
- t_high = 0.693 × R1 × C
- t_low = 0.693 × R2 × C
- Duty cycle = R1 / (R1 + R2) × 100%
For a precise 50% duty cycle, set R1 = R2 in the diode-bypass configuration, or use a D-type flip-flop to divide the output of an astable circuit by 2.
Common Circuits & Applications
LED Flasher (Astable)
Connect an LED (with current-limiting resistor ~470 Ω) between pin 3 and GND. Use R1 = 10 kΩ, R2 = 100 kΩ, C = 10 µF for a flash rate of ~0.65 Hz — about one flash per 1.5 seconds. A CMOS 555 running from a 3 V coin cell will last months.
Tone Generator / Buzzer (Astable)
Human hearing spans roughly 20 Hz – 20 kHz. For a 1 kHz tone, use R1 = 1 kΩ, R2 = 1 kΩ, C = 100 nF (calculated f ≈ 4.8 kHz — adjust values to hit 1 kHz exactly). Connect a piezo buzzer or small speaker (with a 0.1 µF blocking capacitor) to pin 3. Lower C for higher frequencies.
Switch Debouncer (Monostable)
Mechanical switches bounce for 5–50 ms. Trigger a monostable 555 with the raw switch signal; set the pulse width to 50–100 ms. The clean output pulse on pin 3 is guaranteed bounce-free. Use R = 100 kΩ, C = 1 µF for ~110 ms (t = 1.1 × 100,000 × 0.000001).
PWM Motor Speed Control (Astable + Low-Pass)
Drive a MOSFET gate from pin 3. Use a diode bypass to set duty cycle independently of frequency. A 10 kHz carrier with variable duty cycle smoothly controls DC motor speed. The motor coil inductance acts as a natural low-pass filter.
Missing Pulse Detector (Monostable)
Set the pulse width slightly longer than the expected interval between pulses. Each incoming pulse re-triggers (resets) the 555 before it times out. If a pulse is missing, the output goes LOW — signalling a fault. Used in industrial safety interlocks and watchdog timers.