Astable Multivibrator using NE 555 timer IC -Circuit diagram and working

Astable Multivibrator can be designed by using 555 timer IC, Op Amps and also using transistors. The 555 IC provide accurate time delay from mille seconds to hours.The frequency of oscillation can be controlled manually by simple modification.

555 is suitable for circuit designers with a relatively stable, cheap, and user-friendly integrated circuit for both monostable and astable applications.

The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555.

This is a simple 555 timer circuit project. Astable Multivibrator is simply an oscillator circuit that produces continuous pulses. The frequency can be controlled by changing the values of R1, R2 and C1.

You can construct astable multivibrator using transistors also, but the 555 circuit is comparatively simple.
Read Also: 555 Timer Astable calculator

Circuit Diagram of Astable Multivibrator

Components required
NE 555 or SE 555
Resistor (1MΩx2, 1KΩ)
Capacitors (0.01Fµ, 1Fµ)

Output Waveform of Astable Multivibrator

Design of Astable Multivibrator
Capacitor charges through R1 and R2 there for charging time is given by


during this time the output is High, so

Tcharge=0.69(R1+R2)C1=1.38 sec

Capacitor discharges through R2 only there for discharging time is given by

Tdischarge=0.69 R2 C1,

during this time the output is Low, so

Tdischarge=Toff =0.69 R2 C1=0.69 sec

Time Period=Ton+Toff=2.07sec

Frequency of oscillation=0.483Hz

Duty Cycle is calculated by the following manner

Working of Astable Multivibrator
Consider the flip flop is initially cleared, when the power is switched on, then the output of inverter will be HIGH.
Now the capacitor C1 starts charging through R1 and R2. (Discharge transistor Q1 is OFF).
When the capacitor voltage exceeds 2/3 Vcc, the upper comparator output will be High, it Reset the control flip flop.
So the Q output of control flip flop will be LOW and Q’ will be High. So the final output from Inverter is LOW.
At the same time, the discharge transistor Q1 turns ON and the capacitor starts discharge through R2.
When the capacitor voltage less than 1/3 Vcc, the lower comparator output will be high, then the control flip flop get set to High. (Q=1, Q’=0, Final output=1).
Now the discharge transistor Q1 if OFF and then capacitor starts charging. This process continues.
The LED connected at the output will glows according to the output status.
4th pin is Reset pin, a Low voltage at this pin resets the IC. The Low signal is applied to the base terminal of reset transistor Q2. Then it turns ON followed by Discharge capacitor Q1 and capacitor discharges. See the images below.
Working Principle 555 timer IC with Images
555 Timer internal diagram after triggering (Capacitor charging)
Now let’s see the different process taking place while you are triggering a 555 timer IC.

Voltage at the pin 2 goes below 1/3 Vcc.
Lower comparator switches to high state (1).
Controls flip flop sets and Q’ output (inverted output of flip flop) goes to low state (0).
Final output (Not gate) goes to High state (1)
555 IC Internal diagram while discharging

Voltage at pin 6 goes above 2/3Vcc Upper comparator switches to high state (1).
Control flip flop resets and Q’ (inverted output of flip flop) output of goes high state (1).
Discharge capacitor turns ON and capacitor discharging occurs (Q2 short circuit both end of the capacitor).
Final output (Not gate) goes to Low state (0).
555 Timer internal diagram while reset

Applying + ve voltage at reset pin (4) Discharge transistor Q1 turns on.
It short circuits both end of capacitors. Voltage at pin 2 goes below 1/3 Vcc.
Control flip flops sets and final output goes Zero.