555 Timer Calculator
If you need a timer in your circuit, it is highly likely that you are going to use a 555 IC. With our 555 timer calculator, you will be able to calculate the time intervals for every given 555 astable mode configuration, the pulse duration of the 555 monostable mode, and more. On this page you get a:
- 555 timer calculator;
- 555 frequency calculator;
- 555 duty cycle calculator; and
- 555 pulse calculator.
Are you ready to discover how a 555 timer works and begin designing your 555 timer circuit?
What is the 555 IC?
The 555 chip is a widely used integrated circuit that has kept the tempo in innumerable projects since the 70s. It can operate in various modes:
- Monostable mode, where a single state is destabilized for a given time;
- Bistable mode, where the chip remains in one of two stable states until prompted to change; and
- Astable mode, in which the output of the 555 oscillates between a "high" state and a "low" state in the shape of a rectangular wave.
The chip's applications are varied: you can use it to create timers, pulses, or delays in your circuit in the monostable mode, flip-flop circuits in the bistable mode, and — most commonly — as an oscillator in the astable mode. Here we focus on the 555 astable mode and the 555 monostable mode.
Circuit diagram of the 555 astable mode
In the usual depiction of a 555 timer circuit in the astable configuration, the pins of the chip do the following:
- Pin 8 — Power supply pin;
- Pin 1 — Connection to the ground;
- Pin 3 — Output pin: it can be in either a high or low state;
- Pin 2 — Active low trigger; it controls the timer, starting it when its voltage is lower than one-third of the supply voltage, and setting pin 3 to high;
- Pin 6 — Threshold pin. When the voltage on pin 6 reaches two-thirds of the supply, pin 3 is set to low and the cycle ends;
- Pin 7 — Also known as the discharge; this allows the discharge of the capacitor that controls the timings of the cycle;
- Pin 5 — The control. When connected to a capacitor (C2, usually around 10 nF) and the ground, it works to even out the noise of the supply; and
- Pin 4 — The reset pin acts as an active low trigger. When connected to the supply the 555 can operate, but if the voltage on pin 4 is low, a trigger from pin 2 is required to restart the cycle.
The 555 astable mode
In the astable mode, the output of a 555 chip remains in the state high for Thigh seconds and in the state low for Tlow seconds. These values are controlled by two resistors and a capacitor connected to the 555 (R1, R2, and C1).
How does a 555 timer work in astable mode
Assume that the chip starts with pin 3 in the high state.
- At pin 7, discharge is open, so current flows through R1 and R2, charging C1: the voltage on pins 2 and 6 increases.
- As soon as the voltage on pin 6 reaches the threshold of 2/3 of the supply, the output on pin 3 changes to low: this causes pin 7 to connect to the ground.
- C1 discharges through R2 and pin 7: the voltage on pins 2 and 6 decreases.
- Pin 2 is triggered when the voltage becomes smaller than 1/3 of the supply, changing the state of pin 3 to high and opening pin 7.
The cycle then repeats until pin 4, the reset, is triggered.
How does the 555 timer calculator and N555 duty cycle calculator work
For the astable mode, our calculator uses the standard equations:
- High time:
Thigh = 0.693 × (R1 + R2) × C - Low time:
Tlow = 0.693 × R2 × C - Period:
T = Thigh + Tlow - Frequency:
f = 1 / T = 1.44 / ((R1 + 2·R2) × C) - Duty cycle:
D = (R1 + R2) / (R1 + 2·R2)
Simply enter R1, R2, and C in any unit (Ω, kΩ, MΩ and pF, nF, µF, mF, F) and the calculator returns the frequency, period, high/low times, and duty cycle.
The 555 monostable mode
In the monostable mode, the 555 produces a single output pulse of a fixed duration whenever it is triggered. The duration of this pulse is set by one resistor (R) and one capacitor (C).
Operations of the 555 in the monostable mode
When pin 2 receives a trigger (its voltage drops below 1/3 of the supply), the output on pin 3 goes high and the capacitor C begins charging through R. Once the capacitor voltage reaches 2/3 of the supply, the output returns to low and the pulse ends. The chip then waits for the next trigger.
How does the 555 pulse calculator work?
For the monostable mode, the pulse duration is given by:
T = 1.1 × R × C
Enter R and C and the calculator returns the pulse width, displayed automatically in the most convenient unit (s, ms, or µs).
Testing our 555 timer calculator
Try the classic astable example: R1 = 1 kΩ, R2 = 10 kΩ, and C = 10 µF. The calculator returns a frequency of about 6.86 Hz and a duty cycle of about 52.4%. Switch to monostable mode with R = 10 kΩ and C = 10 µF to get a pulse of about 110 ms.
FAQs
What is the duty cycle of a standard 555 astable circuit?
Because the capacitor charges through R1 + R2 but discharges only through R2, the basic astable circuit always has a duty cycle above 50%. To get closer to 50%, make R1 much smaller than R2 or add a diode across R2.
How do I increase the frequency of a 555 oscillator?
Reduce the resistance (R1, R2) or the capacitance (C). Frequency is inversely proportional to both, so halving C doubles the frequency.
What is the maximum frequency of a 555 timer?
The standard bipolar 555 works reliably up to roughly 500 kHz. CMOS versions (such as the 7555) can reach a few MHz.
Measurement units
This calculator supports both metric/SI engineering units (Ω, kΩ, MΩ for resistance and pF, nF, µF, mF, F for capacitance) and reports time in seconds, milliseconds, and microseconds (American and metric engineering practice use the same SI electrical units, so no separate imperial conversion is required). This calculator performs no monetary calculations.