# Astable Multivibrator using 555 Timer

# Circuit and operation

Astable Multivibrator using 555 Timer has no state stable. It generated free running frequency without any output source. The IC 555 can be used as an independent multivibrator with three external components: two resistors (R1 and R2) and capacitor (C). The diagram for IC 555 timer is shown below in the figure with external components.

Pins 2 and 6 are connected with each other and no external stroke is required. Activate automatically and act as a free multi-generator. The other connections are as follows: pin 8 is connected to the supply voltage (VCC). PIN 3 is the output connection and the output is available on this pin. PIN 4 is the reset of the external pin. The bottom of this pin goes back to the timer. If leg 4 is not used, it is usually attached to VCC.

Thus, when the capacitors cut, the voltage across the capacitor increases exponentially and the output voltage on pin 3 is high. When the capacitor releases, the voltage to the capacitor drops exponentially and the output voltage on pin 3 is low. The shape of the output wave is a rectangular train. The following shows the capacitor voltage waveforms and the output in stable mode.

During a charge, the capacitor is charged via the resistors R1 and R2. Therefore, C is the charge invoice time (R1 + R2), because the total resistance of the billing path is R1 + R2. During discharge, the capacitor is only released via the R2 resistor. R2C is the loose time frame.

# Duty Cycle

The charging time and the billing time depend on the quantity of counterpart R1 and R2. Often, fasting time is greater than the time of discharge. Therefore, the HIGH output is longer than the LOW output and the output waveform is not symmetrical. Timeliness is a mathematical variable that creates a link between high energy and low energy. The work cycle is defined as the percentage of time in HIGH production, d. At the right time for the total length of the circuit.

If TON is a time of good production and T is cycle time, projection D of the work is provided

D = T_{ON}/ T

Therefore, percentage Duty Cycle is given by

%D = (T_{ON} / T) * 100

T is the sum of T_{ON} (charge time) and T_{OFF} (discharge time).

The value of T_{ON} or the charge time (for high output) T_{C} is given by

T_{C} = 0.693 * (R1 + R2) C

The value of T_{OFF} or the discharge time (for low output) T_{D} is given by

T_{D} = 0.693 * R2C

Therefore, the time period for one cycle T is given by

T = T_{ON} + T_{OFF} = T_{C} + T_{D}

T = 0.693 * (R1 + R2) C + 0.693 * R2C

T = 0.693 * (R1 + 2R2) C

Therefore, %D = (T_{ON}/ T) * 100

%D = (0.693 * (R1 + R2) C)/(0.693 * (R1 + 2R2) C) * 100

%D = ((R1 + 2R2))/((R1 + 2R2)) * 100

If T = 0.693 * (R1 + R2) C, then the frequency f is given by

f = 1 / T = 1 / 0.693 * (R1 + 2R2) C

f = 1.44/( (R1 + 2R2) C) Hz

The options R1, R2 and C1 for different risk intervals are as follows: \ t

R1 and R2 must be between 1k and 1m. It is best to choose the first C1 (because the capacitors are available at some cost).

Select R2 to specify the desired frequency (f).

R2 = 0.7 /(f × C1)

Choose R1 to be about a tenth of R2 (1k min.)