Top 4 uses of Astable Multivibrator using IC 555 Timer with Circuit diagram

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Astable Multivibrator [A.M.V.] is also called Free Running Multivibrator. There is no stable state and there are frequent switches between the two states without any external trigger. The IC 555 can be made to work as an astable multivibrator with the addition of 3 external components: 2 resistors (R1 and R2) and 1 capacitor (C). The schematic of IC 555 as an astable multivibrator is shown below.

astable multivibrator schematic
Source of Image: ElectronicsHub

Pins 2 and 6 are connected and therefore there is no need for external trigger pulse. It will trigger itself and will act as a free running multivibrator. The remaining connections are as follows:

  • Pin 8 is connected to supply voltage (VCC).
  • Pin 3 is the output terminal and hence the output is available on this pin.
  • Pin 4 is the external reset pin. This pin will reset a short short timer on the pin. So when not in use, pin 4 is usually tied to VCC.
  • Control voltage imposed on pin 5 will change threshold voltage level. But for normal use, pin 5 is connected to the ground through a capacitor (usually 0.01) F), so the external noise is filtered from the terminal.
  • The pin is 1 Ground Terminal. The timing circuit which determines the width of the output pulse, is formed by R1, R2 and C.

How does astable multivibrator operates?

The following schematic indicates the internal circuit of IC 555 operating in awkward mode. RC timing circuits include R1, R2 and C.

internal circuit of astable multivibrator
Internal Circuit Connection for Astable Multivibrator

Initially, on the power-up, there is a flip-flop RESET (and hence the output of the timer is low). As a result, discharge transistor saturation is induced (as it is linked to the Q). Capacitor C is connected to pin 7 and it will be discharged through the transistor. At this point the timer output is low. There is trigger voltage across the capacitor. Therefore, while discharging, if the capacitor voltage decreases by 1/3 VCC, which is the reference voltage to trigger the comparator (comparator 2), the output of the comparator 2 will be high. This will set the flip-flop and hence the timer output at pin 3 goes high.

This high output will turn the transistor OFF. As a result, the capacitor starts to charge through R1 and R2 (Resistors). Now, the capacitor voltage is similar/same to the threshold voltage.(as the pin 6 is connected to the capacitor-resistor junction). While charging, the capacitor voltage grows rapidly towards VCC and the moment it crosses 2/3 VCC, which is the reference voltage of the threshold comparator (comparator 1), its output becomes high.

As a result, flip-flops are RESET. Timer output falls to LOW. This low output will turn transistors ON once again, which provides a discharging path to the capacitor. Hence, the capacitor will discharge through R2 resistor. And so the cycle continues.

Thus, when a capacitor is charged, the voltage in the capacitor increases rapidly and the output voltage at pin 3 is high. Similarly, when the capacitor is discharging, the voltage drops rapidly in the capacitor and the output voltage at pin 3 is low. The size of the output wavelength is a train of rectangular pulses. The waveform of capacitor voltage and output are shown below (In Astable Mode).

Capacitor Voltage v/s Output waveforms in astable multivibrator mode
Capacitor Voltage v/s Output waveforms in astable mode

When charging, the capacitor charges via R1 and R2. Therefore the charging time is constant (R1 + R2)*C as the total resistance is (R1 + R2) in the charging path. When discharging, the capacitor discharges only through R2. Therefore discharge time is stable R2*C.

How to calculate Duty Cycle in Astable Multivibrator

Charging and discharging time constants depends on the values ​​of R1 and R2. Usually, the charging time is more than the discharging time constant. Therefore, high output remains longer than LOW output. And hence output wave is asymmetric. The duty cycle is the mathematical parameter that creates a connection between high output and low output. Duty cycle is defined as the ratio of the time of high output {i.e. the ON time} to the total time of the cycle.

Uses & Applications of Astable Multivibrator

1. Astable Multivibrator as Square Wave Generator

The duty cycle of an A.M.V is always greater than 50%. When the duty cycle is 50%, a square wave is obtained as the output. The duty cycle of 50% or anything less is not possible with IC 555 as astable multivibrator. Some modifications should be made for the circuit.

Modification is to add 2 diodes. 1 diode in the series with resistor R2, with a anode facing the capacitor. And another diode parallel to the resistor R2 with the cathode facing the capacitor. By adjusting the values ​​of R1 and R2, a duty cycle can be obtained in the range of 5% to 95%, including square wave output. The circuit for square wave generation has been given below.

Astable Multivibrator as Square Wave Generator
Astable Multivibrator as Square Wave Generator

In this circuit, capacitor charges through R1 & D1 and discharges through R2 & D2 .

Therefore, the charging time constant is.: TON = TC and it is given by:

TON = 0.693 * R1C  

the discharging time constant – TOFF = TD is given by

TOFF = 0.693 * R2C.

Hence, the duty cycle D is given by

D = R1/(R1+R2)

To get square wave, the values ​​of R1 and R2 can be made equal to 50% by doing the duty cycle. The waves of the square wave generator are shown below.

Waveforms of Square Wave Generator
Waveforms of Square Wave Generator

2. Astable Multivibrator as Pulse Position Modulator

In Pulse Position Modulation, the position of the pulse varies according to the modulation signal, while the amplitude and width of the pulse remains constant. The position of each pulse varies according to the instantaneous voltage of the modulate signal. To achieve Pulse Position Modulation, two 555 timer ICs are used, in which one is operated in astable mode and the other in monostable mode.

The modulating signal is first applied to pin 5 of IC 555 which is running in astable mode. The output of this IC 555 is a pulse width modulator wave. This PWM signal is implemented as trigger input to next IC 555 which is running in monostable mode. According to the PWM signal, the status of the output pulses of the second IC 555 is changed, which is again dependent on modulated signal.

The schematic of the pulse position modulator is shown below using two 555 timer ICs.

PPM using astable multivibrator circuit diagram
PPM Circuit

The threshold voltage for the first IC 555, which is determined by the control voltage (modulation signal), is converted into UTL (upper threshold level) and given by-

UTL = 2/3 VCC + VMOD

Since the threshold voltage changes w.r.t. the applied modulation signal, the width of the pulse varies and therefore the time delay is varied. As this Pulse Width Modulated signal applies to the trigger of the second IC, there will be no change in the amplitude or width of the output pulse, but only the state of the pulse changes.

The waves of the pulse position are shown below the modified signals.

PPM using astable multivibrator waveforms
Waveforms for Pulse Position Modulator

3. Pulse Train Generator using Astable Multivibrator

We know that A.M.V will produce a continuous stream of pulses. By using a potentiometer instead of R1, a train of pulses can be produced in different widths. Circuit of Pulse Train Generator is shown below by using the astable mode of operation of IC 555.

Pulse train generation using astable multivibrator
Pulse Train Generator Circuit Diagram

4. Astable Multivibrator used for Frequency Modulation

Astable multivibrator can be used to produce frequency modulated signals. A modulating signal is given to the pin 5. (PIN 5: control voltage). The circuit of Frequency Modulation is shown below.

Top 4 uses of Astable Multivibrator using IC 555 Timer with Circuit diagram 2

To generate pulse output with Duty Cycle ≈ 50%, a diode is connected to R2 in parallel. The modulation signal is given to pin 5 through the high pass filter containing a capacitor and a resistor. The output frequency will be modulated according to the modulated signal given/applied on Pin 5. If the voltage of the modulation signal is high, then the time period of the output signal is high. And if the voltage of the modulation signal is low, the time period is short. The waveforms are shown below.

FM using Astable Multivibrator
All the Images used in the above post are from ElectronicsHub. This post was originally found at ElectronicsHub, practiced, performed & rewritten by ITSTECHNOERA.

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