# Simple 555 Timer Projects: Fixed and Variable Pulse Waveform Generator

### Simple 555 Timer Projects: Fixed & Variable Pulse Generator Circuit Design Project

In our previous posts we had seen the Single pulse generation (Monostable Multivibrator) and Multipulse generation (Astable multivibrator) operation of the 555 timer IC. With the understanding of those circuits, we can do this simple project.

In this post we will design the 555 timer IC based pulse generator circuit which can produce pulse waveform with different dimensions and voltages.

Let us first design a pulse generator circuit which can produce fixed pulse width using 555 timer IC. Then we will implement the same formula to derive the pulse generator circuit which can produce variable pulse width.

#### 555 Timer IC based Square Wave Generator Design:

To produce a perfect square waveform, the ON time (tON) of the Astable Multivibrator circuit should be equal to the OFF time (tOFF). To achieve symmetrical square wave output, very high value of resistor is selected for R2 compared to R1.

Astable_Multivibrator_using_555_Timer_IC

Design Requirement:
Design a 5V and 50Hz square wave generator using 555 timer IC.
Please click here to know the formulas used for the waveform generator circuit.

Solution:
Given Pulse repetition frequency - 50Hz

Pulse repetition frequency: P.R.F = 1.44/[C(R1 + 2R2)]
when R2 >> R1, the value of
Pulse repetition frequency =  0.72/(CR2)

General values for R1, R2 & C are given below:
R1 = 1kΩ to 1MΩ
R2 = 1kΩ to 1MΩ
C = 10nF to 470µF

Selection of Component values:
For R1, Use the minimum recommended value from the range given above to meet the condition R2>>R1
Thus R1 = 10K
For C, select low value so that R2 will be very high.
Thus C = 100nF

Substituting R1 and C values in the above equation;
R2 = 0.72 /  (50 x 100 x 10-9)

The resultant circuit is given below:

Square_wave_generator_Design_Calculation

#### Variable Pulse Generator Circuit Operation:

So far in our previous simple projects we used single 555 timer IC. This circuit requires two timer 555 timer ICs. We can use 556 IC ( It is dual timer IC) also. Please refer the below figure.

Before proceeding further it is highly recommended to read about Monostable and Astable operation of the 555 timer IC.
Click here to know about the Monostable Multivibrator Circuit design of 555 timer IC

#### Components Required to build this Circuit:

• 555 Timer IC - 2 no.
• Resistors - 1K, 100k, 100 Ohm = 1 no each
• Resistor - 10K = 3 no
• Capacitors - 100nF = 3 no,  10nF = 2 no, 1uF, 470nF, 47nF, 4.7nF = 1 no each
• 1K, 100K POT =1 no each

Variable_Pulse_Generator_Circuit_Using_555_Timer_IC

• In the above circuit, the first 555 timer(IC1) operates in astable mode.
• While the second timer (IC2) operates in monostable mode.
• The switch selected capacitors (C1 to C3) along with variable resistor(VR1) adjusts the Pulse repetition frequency generated by the IC1.
• The output of IC1(pin-3) is given as the trigger input of IC2(pin-2) through capacitor C5.
• The Pulse repetition frequency can be changed over the range of 10 Hz to 10 kHz.
• The Pulse width can be changed from 50µs to 50ms.
• The output voltage is adjustable from 0 V to 10 V.
• The 100Ω resistor is included to limit the output current. In addition to that it will provide a protection to circuit if any short-circuit occurs at the output load.

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### 4 Responses

1. amandeep singh says:

what is the working of pulse generator cum timer project

2. Kevin says:

Thanks for this circuit !dea please post more with 555IC

3. Saaritha says:

Thanks for the circuit

4. manimaran says:

hai, i want to single clock pulse generator.i have project in physics.