# Astable Multivibrator: Multi Pulse Generator Circuit Design using 555 Timer IC

### Astable Pulse Generator based on 555 Timer IC:

In this post, Multi Pulse Generator circuit Design using 555 IC  is discussed in detail.
The Astable Multivirator circuit is shown below:

Before proceeding further it is highly recommended to refresh about the 555 timer IC pinout details and Internal arrangement of the 555 timer. To understand the circuit operation, assume the following

1. The output(at pin no 3) is initially in high state.
2. TR1 is in the non-conducting state.
3. The capacitor C  will begin to charge with current supplied through series resistors R1 & R2.

The internal circuit diagram of the 555 timer IC is given below. Refer this  diagram for IC1, IC2, IC3, IC4 and TR1.

• When the threshold input(pin-6) voltage exceeds 2/3 of VCC, the upper comparator (IC1) output state will change .
• It will lead to the resetting of Bistable due to voltage transition across R.
• Consequently this will make the inverted output goes to high state and turning TR1 at the same time.
• Because of the inverting action of buffer(IC4) the final output at the pin-3 goes to low state.
• Now capacitor(C)  will discharge through R2 into the collector of TR1.
• At some point, the voltage across the trigger input (pin-2) goes to (1/3)rd of the VCC.
• At this point the lower comparator will change state & the voltage transition at S will return the bistable to its original set condition.
• Then the inverted output goes to low, TR1 change into OFF state.
• Eventually the circuit output at pin no-3 goes to high  state.
• Thus the entire charge and discharge process is repeated continuously.
• The astable multivibrator circuit output waveforms are shown below.

From the waveform the following points can be concluded:

Output High state Duration: tON = 0.693 C (R1 + R2)

Output Low state Duration: tOFF = 0.693 C R2

Period of output waveform: t = tON + tOFF = 0.693 C (R1 + 2R2)

Pulse repetition frequency: P.R.F = 1.44/[C(R1 + 2R2)]

Duty cycle:
[tON/(tON + tOFF)] = [(R1+R2) / (R1+2R2)] x 100%

Here
t in seconds,
R1, R2 in ohms.
When R1 = R2  the duty cycle equation becomes

[tON/(tON + tOFF)] = [(R + R) / (R+2R)] x 100% = 67%

By changing the values of R1, R2 & C the Pulse repetition frequency of the 555 astable multivibrator circuit output can be changed.

To give the satisfactory performance normally the R1, R2 & C values should  be within the following range:
C = 10nF to 470µF
R1 = 1kΩ to 1MΩ
R2 = 1kΩ to 1MΩ

Design Requirement:
Design a square wave generator circuit with the help of 555 timer IC.

Solution:

1. Remember that for square wave form generation, the tON and tOFF should be equal.
2. But the tON time is always greater than the tOFF time.
3. So the ratio [tON/(tON + tOFF)] will never becomes unity.
4. However by selecting R2 very much larger than R1, the 555 timer IC will generate the square wave output
ie; if R2 >> R1

Pulse repetition frequency =  0.72/(CR2)

The Duty cycle equation becomes

[tON/(tON + tOFF)] = [R2 / 2R2)] x 100%  = 50%

Design Requirement:
Design a pulse generator which will produce a Pulse repetition frequency of 10 Hz with a duty cycle of 67% .
Solution
With the help of astable multivibrator circuit diagram and equations derived above we can meet this design requirement.

Pulse repetition frequency: P.R.F = 1.44/[C(R1 + 2R2)]

As this is the square wave generator  R1 = R2= R and thus the duty cycle is D=67%

substituting the values P.R.F = 0.48/CR

• Select the C value form the range given above.
• This value should be such that R should be neither too small nor too big.
• Consider C= 1μF.
• substituting the value in equation to get R value.
R =4.8kΩ