Astable Multivibrator Using 555 Timer – Electronic Circuit

A multivibrator is a one type of electronic circuit, that is used to implement a two state system like flip-flops, timers and oscillators. Multivibrators are categorized by two amplifying devices like electron tubes, transistors and other devices like capacitors and cross coupled by resistors. Multivibrators are classified into three types based on the circuit operation, namely Astable multivibrators, Bistable multivibrators and Monostable multivibrators. The astable multivibrator is not stable and it repeatedly switches from one state to the other. In monostable multivibrator, one state is stable and remaining state is unstable. A trigger pulse is the root to the circuit to enter the unstable state. When the circuit enters into the unstable state, then it will return to the normal state after a fixed time. A bistable mutivibrator circuit is stable that can be changed from one stable to other stable by an external trigger pulse. This multivibrator circuit is also called as flip-flop which can be used to store one bit of data.

Astable Multivibrator
Astable Multivibrator

Astable Multivibrator Working

This type of multivibrator includes two amplifying stages that are connected with a two capacitive-resistive coupling networks in a positive feedback. The amplifying elements are FETs, JFETs, Op-Amps, vacuum tubes, etc. The astable multivibrator circuit using BJTs is drawn in the form of cross coupled pair. The o/p terminals of the multivibrator can be defined as the active devices, that will have opposite states; one will have low voltage whereas the other has high voltage.

Astable Multivibrator  Working
Astable Multivibrator Working

The above astable multivibrator circuit has two unstable states that change alternatively with a max transition rate due to accelerating +ve feedback.

It is employed by the coupling capacitors that suddenly transfer voltage variations because the voltage across a capacitor cannot change quickly. In every state, one transistor is turned on and the remaining one is turned off. So, one fully charged capacitor discharges slowly, therefore changing the time into an exponentially altering voltage. At the same time, remaining empty capacitor charges quickly. The operation of the above circuit is based on the forward-biased BE junction of the turned on BJT, that can provide a route for the restoration of the capacitor.

Astable Multivibrator Using 555 Timer

The designing and working of astable multivibrator using a 555 timer IC is done by using  ransistors and operational amplifiers. The 555 timer IC affords exact time delay from ms to hours. The oscillation frequency can be measured manually by small modification. 555 timer IC is a relatively cheap, stable and user-friendly integrated circuit and that is apt for circuit designers with for both astable and monostable applications. The first 555 integrated circuit was designed in the year 1971 by the corporation of Signetics as the SE555 or NE555. Astable multivibrator using 555 IC is a simple oscillator circuit that generates continuous pulses. The frequency of the circuit can be controlled by shifting the values of resistors R1, R2 ad capacitor C1.

Astable Multivibrator Using 555 Timer
Astable Multivibrator Using 555 Timer

Design of Astable Multivibrator

  • The designing steps of the astable multivibrator are given below.
  • The Required Components are NE 555 or SE 555, Resistors (1MΩx2, 1KΩ), Capacitors (0.01Fµ, 1Fµ) and LED
  • The capacitor in the above circuit charges through the two resistors R1 and R2 and there charging time can be calculated as Tcharges= 0.69 (R1+R2) C1. During this charging time the o/p is high that is 1.38Sec
  • The capacitor discharges through the resistor R2 then the discharging time can be Tdischarge= 0.69 R2C1. During this discharging time the o/p is low, that is 0.69 Sec.
  • The total time period is T= Tcharges+ Tdischarge => 1.38+ 0.69= 2.07Sec
  • The oscillation frequency is 0.483Hz.
  • The duty cycle can be calculated in the following manner.
  • Duty cycle= Ton /Ton + Toff => 1.38/2.07= 66%

Working of Astable Multivibrator

When the power is turned ON consider the flip flop is cleared initially, then the o/p of the inverter will be high. The charging of the capacitor will be done using two resistors R1& R2. When the voltage of the capacitor goes above 2/3 Vcc, then the output of the higher comparator will be High, it changes the control flip flop.So the control flip-flop’s Q o/p will be a LOW & Q’ will be High. So the final o/p of the Inverter is LOW. At the same time the Q1 transistor switches ON and the C1capacitor starts discharging through resistor R2.

Operation of Astable Multivibrator
Operation of Astable Multivibrator

When the voltage of the capacitor is < 1/3Vcc, then the o/p of the lower comparator will be high and control flip flop gets is set to 1. When the discharge transistor Q1 gets off, then the capacitor gets charged and continues this process. According to the status of the o/p, the LED at the output will blink. When the low voltage is applied at the 4th pin (reset pin) of the IC then it resets the IC. When the low signal is applied to the base of the Q2 transistor then it switches ON by the capacitor.

Applications of Astable Multivibrators

The applications of Astable multivibrators involve in radio gears to transmit and receive radio signals and also in time, morse code generators and some systems which require a square wave like analog integrated circuits and TV broadcasts.

Advantages and Disadvantages of Astable Multivibrators

These multivibrators are changes between one stable state to another continuously. This permits multivibrators to power themselves & achieve work at a constant rate without impact from any external forces or actions. Also, these multivibrators are low cost to produce, and simple to design

These multivibrators do not transfer the whole o/p signal to the i/p. This is due to resistance in the circuit, nonexistence of a totally closed loop at the o/p terminals, and the leaning of one transistor/capacitor to absorb energy at a somewhat different rate than the other. Although the amplifier restores the lost energy when it amplifies the signal, the signal will finally be too small to be of any use.

This is all about astable multivibrator using 555 timer, advantages,disadvantages and applications.We hope that you have got a better understanding of this concept, furthermore any doubts regarding this topic please give your suggestions in the comment section below.

Photo Credits:

  • Astable Multivibrator by tpub
  • Astable Multivibrator Using 555 Timer by visionic
  • Working of Astable Multivibrator by circuitstoday


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