Square wave generator using 555 IC

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A square wave is one of the most common waveforms that is used in circuits. Let’s brush up a bit of theory.

A square wave is a periodic waveform which varies between two states – HIGH and LOW. May be 1/0, 5/0 or 9/0. Considering the HIGH state as the active state, the percentage of time period the waveform remains in the HIGH state is called its duty cycle. The figure below will simplify things.

A square wave generator is often called a a multivibrator. Depending on the stability of each state, it works in three modes –

  • Monostable – Where only one state is stable, say LOW. When a trigger pulse is given, it changes the output from LOW to HIGH for a fraction of the time interval and then it drops back to LOW.
  • Bistable – Where both states are stable. When a trigger pulse is given, the output changes from LOW to HIGH and stays there till another trigger pulse is given which drops the output back to LOW.
  • Astable – Where no state is stable. The output periodically changes between HIGH and LOW.

Here we are going to make an astable multivibrator using 555 timer IC where we will be able to vary the duty cycle as well as the frequency.

What you need

You will need the following parts

  1. 555 IC – 1pc
  2. 100k variable resistor – 1pc
  3. 10k resistor – 1pc
  4. 1k resistor – 1pc
  5. .01uf capacitor – 1pc
  6. 10uf/1uf/.1uf capacitor. Each of these will give a different frequency of the square wave [ 10uf -> 1HZ, 1uf -> 10HZ,, .1uf -> 100HZ ]. You can use any one of them. Here I will use the 10uf capacitor which will give a 1 HZ frequency.
  7. 9v battery

Following is an image of the parts.

What to do

Lets get to the circuit. Wire the following circuit on the breadboard.

Now, vary the potentiometer. This will vary the duty cycle of the output waveform. You can see the changes from the behaviour of the LED glowing in different time intervals. Enjoy!!

What next

If you are done with this try changing the frequency select capacitor and see how it affects the output. Check out the following if you want –

Feel free to ask any queries in the comment section below.



Square wave generator using 555 IC

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