Use this calculator to determine the various characteristics of square waveform output from a 555-timer astable circuit.
– % Duty Cycle
– ms
Frequency: –
Period (T): –
Duty Cycle: –
Time High: –
Time Low: –
How to Use It?
To use the calculator, simply enter the resistance and capacitance values, and the calculator will calculate the frequency, time period, duty cycle, time high, and time low of the output from an 555-timer astable circuit.
This calculator will help you design an oscillator using a 555 timer IC. Just remember these three things before you begin.
- Increase C to increase the period (reduce the frequency).
- Increase R1 to increase High Time (T1), without affecting the Low Time (T0).
- Increase R2 to increase High Time (T1), increase Low Time (T0) and decrease the duty cycle.
What is a 555 Timer?
The 555-Timer is a popular integrated circuit (IC) that is used in a variety of electronic applications such as clock timing, signal delay, pulse generation, and signal oscillation. It can produce different output waveforms by adding external RC networks.
The IC was first marketed in 1972 by Signetics and used BJTs. Since then, many companies have produced original BJT-based ICs and later low power CMOS-based ICs.
The 555 Timer is named “555” because it typically contains three 5 kilo-ohm resistors internally that form a voltage divider. It also includes two comparators, a flip-flop, a discharge transistor, and an output stage. Here is the block diagram.
The 555-Timer IC works by using three 5 kΩ resistors to divide the supply voltage in three, and two comparators to compare these voltages to the input voltage, then sets or resets a flip-flop accordingly. The flip-flop controls the output stage and a transistor that connects the discharge pin to ground.
By wiring the 555 timer with resistors and capacitors in various ways, you can get it to operate in three different modes: Monostable, Astable, and Bistable.
Astable Mode
In Astable mode, the 555-timer IC forms a Resistance-Capacitance circuit with two external resistors (R1 and R2), and an external capacitor (C). The circuit generates a continuous rectangular wave output signal, the frequency and duty cycle of which are determined by the resistance and capacitance values of the circuit. That’s why this mode is also known as a Free Running Mode.
In this mode, when the 555-timer is powered on for the first time, the capacitor (C) starts to charge through resistors R1 and R2, driving the output signal high. As the voltage across the capacitor increases, it eventually reaches the upper threshold voltage (2/3 VCC) of the internal comparator. At this point, the flip-flop is triggered, and the output pin (pin 3) goes high.
When the output pin goes high, the discharge pin (pin 7) is connected to ground, causing the external capacitor (C) to discharge rapidly through resistor R2. The capacitor discharges until it reaches the lower threshold voltage (1/3 VCC) of the internal comparator. Once the capacitor voltage drops to the lower threshold, the flip-flop is reset, and the output pin (pin 3) goes low again. The capacitor then starts charging once again, repeating the cycle.
The frequency and duty cycle of the square wave generated by the astable circuit depend on the values of resistors R1 and R2, as well as the capacitor value (C).
The frequency (f) of the output waveform can be calculated using the following equation:
Where,
f is the number of pulses per second (measured in Hertz)
R1 and R2 are resistance values (measured in ohms)
C is the capacitance value (measured in Farads)
The period is the time covered for one pulse and is just the reciprocal of the frequency:
The high time (TH) and low time (TL) are the durations of the output being at VCC and 0 volts respectively. They are given by the formulas:
Note that the period (T) is the sum of the high time and the low time.
The duty cycle of the output waveform is the ratio of time the output is high (TH) to the total period (T). It can be calculated using the following equation:
Example
Let’s say we have a 555 Astable Circuit with resistors of 10kΩ and 20kΩ of a resistance and a capacitance value of 1 µF.
The output signal would have the following characteristics:
Frequency (f):
Time Period (T):
Time High (TH):
Time Low (TL):
Duty Cycle:
So, in summary, for the given example, the 555 Timer Astable Circuit produces an output waveform with a frequency of 28.8 Hz, a time period of 34.72 ms, a duty cycle of 60%, a time high of approximately 20.79 ms, and a time low of approximately 13.86 ms.
