Astable Multivibrator using 555 Timer (Circuit with 2 LEDs and Design – Analog Electronics Project) Leave a comment

In this blog, you will learn to use 555 Timer IC as Astable Multivibrator. We have used 2 LEDs to show the output of the Circuit, however more LEDs can be used if required.

Part List:

  1. Breadboard
  2. PCB
  3. Jumper Wire
  4. 555 Timer IC
  5. Resistor and Capacitor
  6. LEDs
  7. Battery
  8. Soldering Tools


Functions of PIN:

1) Ground

All the voltages are measured with respect to this terminal.

2) Trigger

The IC 555 uses two comparators. The voltage divider consists of three equal resistances. Due to voltage divider, the voltage of non-inverting terminal of comparator ‘B’ is fixed at Vcc/3. The inverting input of comparator ‘B’ which is compared with Vcc/3 is nothing but trigger input brought out as pin number ‘2’. When trigger input is slightly less than Vcc/3, the comparator ‘B’ output goes high. This output is given to reset input of R-S flip flop. So, high output of comparator ‘B’ resets the flip flop.

3) Output

The complementary signal output (Qbar) of the flip flop goes to pin ‘3’ which is the output. The load can be connected in two ways. One between pin ‘3’ and ground while other between pin ‘3’ and pin ‘8’.

4) Reset

This is an interrupt to the timing device. When pin ‘4’ is grounded, it stops the working of device and makes it off. Thus, pin ‘4’ provides on/off features to the IC 555.

5) Control input voltage

This pin is nothing but the inverting terminal of comparator ‘A’. The voltage divider holds the voltage of this input at (2/3)Vcc. This is reference level for comparator ‘A’ with which threshold is compared. If reference level required is other than (2/3)Vcc for comparator ‘A’, then external input is to be given to pin ‘5’.

6) Threshold

This is the non-inverting input terminal of comparator ‘A’. The external voltage is applied to this pin ‘6’. When this voltage is more than (2/3)Vcc, the comparator ‘A’ output goes high. This is given to the set input of R-S flip flop. Thus, high output of comparator ‘A’ sets the flip flop. This makes ‘Q’ of flip flop high and ‘Qbar’ low. Thus, the output of IC 555 at pin ‘3’ goes low.

It consists of potential divider, two comparator, S-R flip flop, transistor (Qd).

It has two basic operating modes –

  • Monostable
  • Astable

In this Blog, we will discuss about Astable Multivibrator only.


In astable multivibrator, the threshold input is connected to the trigger input. Two external resistances ‘RA’, ‘RB’, and a capacitor ‘C’ is used in the circuit.This circuit has no stable state. The circuit changes its state alternatively. Hence, the operation is also called free running non sinusoidal oscillator.


When the flip flop is set (‘Q’ is high) which drives the transistor ‘Qd’ into saturation and the capacitor gets discharged. Now the capacitor voltage is nothing but the trigger voltage. So while discharging, when it becomes less than (1/3)Vcc , comparator ‘B’ output goes high. This resets the flip flop. Hence, ‘Q’ goes low and ‘Qbar’ goes high.

The low ‘Q’ makes the transistor off. Thus capacitor starts charging through the resistance ‘RA’, ‘RB’ and VCC. As total resistance in the charging path is (RA + RB), the charging time constant is (RA+RB)C.

Now the capacitor voltage is also a threshold voltage. While charging, capacitor voltage increases (i.e., the threshold voltage increases). When it exceeds (2/3)Vcc , then the comparator ‘A’ output goes high which set the flip-flop. The flip flop output ‘Q’ becomes and output at pin ‘3’ is low. High ‘Q’ drives transistor ‘Qd’ in saturation and capacitor starts discharging through resistance ‘RB’ and transistor ‘Qd’. Thus, the discharging time constant is RB.C. when the capacitor voltage becomes less than (1/3)Vcc , comparator ‘B’ output goes high which in turn resets the flip flop.

Thus when capacitor is charging, output is high while and when it is discharging, the output is low. The output is a rectangular wave.

The width of the pulse is given as –

W = Ton = 0.693(RA + RB).C

The charging time of capacitor is given as –

Tc=Ton = 0.693(RA + RB).C

The discharging time of the capacitor is given as –

Td = Toff = 0.693(RB).C

Thus the total time period is given as –

T = 0.693(RA + 2RB).C


The circuit diagram for the project is as below:

Practical circuit as:

Project Working Video

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