555 Timer Calculator
The 555 timer is low-cost and highly reliable. With just a few external resistors and capacitors it can build astable oscillators, monostable (one-shot) circuits, and Schmitt triggers. It is widely used as a timer in instrumentation, home appliances, electronic measurement, and automatic control.
This 555 timer circuit calculator determines the square-wave output characteristics of a 555 timer circuit in monostable (one-shot) mode and astable (free-running) mode.
In monostable mode (also called one-shot mode), the output is a single short positive pulse generated for each input event, with a duration determined by the circuit's resistance and capacitance.
In astable mode (also called free-running mode), the 555 timer outputs a continuous rectangular wave whose frequency and high/low times can be calculated from the circuit's resistance and capacitance.
Select configuration
Formula
Understanding the 555 Timer
The 555 timer IC is one of the most common and widely used integrated circuits, suitable for clock timing, signal delay, pulse generation, and oscillation.
In monostable mode, the 555 timer IC forms an RC circuit with an external resistor and capacitor. When an input signal is applied to the trigger pin, the IC becomes active and starts charging the capacitor. When the voltage across the capacitor reaches 2/3 of the supply voltage, charging stops and the output returns low, ready for the next input signal.
Adjusting the resistance and capacitance changes the capacitor's charging speed, which shortens or lengthens the output pulse duration.
In astable mode, the 555 timer uses two resistors and one capacitor to produce a continuous rectangular wave with a fixed high/low ratio at a specific frequency. At power-up, the capacitor begins charging, driving the output high. It keeps charging until the voltage reaches 2/3 of the supply voltage; it then discharges, driving the output low. When the voltage falls to 1/3 of the supply, the capacitor charges again, driving the output high, and the cycle repeats.
As the 555 timer IC charges and discharges the capacitor, the output toggles between the high and low states. The duration of these states and the repetition rate of the cycle are functions of the resistance and capacitance.
Example
Monostable
The output signal duration can be modeled as follows: time in seconds (T) equals the constant 1.1 multiplied by the resistance R in ohms (Ω) and the capacitance C in farads (F).
T = 1.1 × R₁ × C₁
So, for a circuit with a resistance of 10,000 ohms (Ω) and a capacitance of 1,000 microfarads (1000 µF), the total output signal duration is:
T = 1.1 × 10,000 × 0.001 = 11 seconds
Astable
From the continuous output signal in astable mode, you can determine the frequency in hertz (Hz), the signal repetition period, and the time spent in the high and low states. Using the values of R1 and R2 in ohms (Ω) and the capacitance C1 in farads (F), all three values that define the output signal can be derived.
Thus, for a circuit with resistors of 10,000 (10k) ohms (Ω) and 15,000 (15k) ohms (Ω) and a capacitance of 10 microfarads (µF), the output signal has the following characteristics: