About: Electronics and Communication engineer

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Here we are going to make an astable multivibrator with ic555. it is also possible to make astable multivibrator with transistors.

Step 1: Materials Required

1. IC 555

2. Capacitors

3. Connectors

4. LED

5. PCB

6. Power source

7. 10K Resistor pot

Step 2: Circuit Diagram and Explanation

IC 555 is an 8 pinout chip. It generally operates in 3 modes. A-stable, Mono-stable and Bi-stable modes

1. Astable mode
This means there will be no stable level at the output. So the output will be swinging between high and low. This character of unstable output is used as clock or square wave output for many applications.

2.Mono-stable mode

This configuration consists of one stable and one unstable state. The stable state can be chosen either high or low by the user

3. Bi-stable mode

In bistable mode, both the output states are stable.

Here we are using astable mode for frequency pulse output

Function of different Pins


This pin is used to provide a zero voltage rail to the Integrated circuit to divide the supply potential between the three resistors shown in the diagram.


The trigger input is used to set the output of the F/F to ‘high’ state by applying a voltage equal to or less than Vin/3 or any negative pulse, as the voltage at the non-inverting end of the comparator is Vin/3.


It is the output pin of the IC, connected to the Q’ (Q-bar) of the F/F with an inverter


This pin is used to reset the output of the F/F regardless of the initial condition of the F/F and also it is an active low Pin so it connected to ‘high’ state to avoid any noise interference

5.Control Voltage

it is connected to the inverting input having a voltage level of (2/3) Vin. It is used to override the inverting voltage to change the width of the output signal irrespective of the RC timing network.


The pin is connected to the non-inverting input of the first comparator. The output of the comparator will be high when the threshold voltage will be more than (2/3) Vin thus resetting the output (Q) of the F/F from ‘high’ to ‘low’.


This pin is used to discharge the timing capacitors to the ground when the output of Pin 3 is switched to ‘low’.


This pin is used to provide the IC with the supply voltage


Frequency = 1.44 / (( R1 + 2R2 ) C)

Time period = T1+T2

T1 = (0.69 x (R1+R2) C)

Here R1 and R2 used together since the capacitor is charging through both resistor

T2 =(0.69 x R2 x C)

Here R2 is alone used since the capacitor is discharging through only R2 resistor


R1 is the resistance between VCC and pin7

R2 is the resistance between pin 7 and pin 2/pin 6

C is the capacitance connected from pin6/pin2 to ground

working of the circuit

1. Pin 8 is the power supply input and Pin 4 is active low reset pin, thus both are connected to VCC

2. Pin 1 is ground so it is connected to the negative side of battery

3. Pin 7 is connected to VCC through a resistor thus it will help to avoid shorting of VCC and GND. when the output at Pin 3 goes low, the internal transistor connected to the pin 7 will switch ON and it will be grounded. since the resistor are connected to pin 7, the voltage at pin 7 will lesser than VCC. thus it avoids direct contact of VCC and GND

4. pin 2 and pin 6 are shorted together and the same input voltage greater than 2/3vcc is applied,This is because when a voltage greater than 2/3vcc is applied to the first internal comparator of the ic555 the output of the first comparator will be 'HIGH' and that of the second comparator will be 'LOW' ( refer the internal diagram of ic555 )

The output of the first comparator is connected to reset of flipflop

similarly, The output of the second comparator is connected to set of flip-flop

Thus the output Q of flip-flop will be 'LOW' and other output Q-BAR will be 'HIGH'

when the Q-BAR becomes HIGH, the internal transistor will be switching ON and thus the input voltage of both comparators goes below 2/3VCC

when a voltage lesser than 2/3vcc is applied to the first internal comparator of the ic555 the output of the first comparator will be 'LOW' and that of the second comparator will be 'HIGH', which will switch off the internal transistor and the input voltage at both comparators goes above 2/3vcc and the process continues

Thus the output will be pulse

5. A resistor is connected in series with a LED at the output. this resistor will help to feed working voltage to LED

6. Here we are not using control voltage, thus pin5 is connected to GND through a small capacitor. The voltage at pin 5 is 2/3VCC. Aswe know that capacitor block the DC, it will help to not grounding the 2/3VCC directly to GND

Step 3: Wiring and Soldering

Step 4: Working of IC555 With Different Frequency

Step 5: Application