How Astable Multivibrator using Logic Gates work | Example Circuits

Do you know a multivibrator circuit? Yes, you may use it in a transistor pattern or 555 timers. Also, we will learn how does the astable multivibrator using logic gates digital. What is it for?

It is a type of oscillator that generates pulse or square signals. It is a simple circuit. Which consists of IC, resistors, and capacitors.

How Astable Multivibrator using Logic Gates work | Example Circuits

How many types of multivibrators? There are 2 major types are:

  • Astable Multivibrator—generated the continuous pulse signals
  • Monostable Multivibrator—generated the single pulse signals when receiving external triggers

Biatable Multivibrator
Some people say that the RS-Flip Flop is one kind of multivibrator too. He calls it a bistable multivibrator. It has two stable conditions. When triggering, the condition will change. But now we often call these circuits as Flip Flop.

The first type is very popular. Such as the pulse generator, the clock signal, control the system to work in rhythm, and control the sequence of work and timing of circuits, etc.

Because the name is long. We often call in short it that Clock.

See right the clock generator to the 4017 counter.

What is more? Learn it works.

How does astable multivibrator work

Look at the simple pulse generator circuit using CMOS Gates.

simple pulse generator circuit using CMOS Gates

This circuit is a basic circuit of a multivibrator that uses the characteristics of the inverter and the capacitance and discharge of the capacitor C in the circuit.

Although the circuit looks easy. But it is very challenging. To explain it to be easy to understand.

I name the points A, B, C, and D in the circuit, where you will see points A and D are the same. But for convenience so explained separately.

The feature of the inverter or Not gate is that the input and output will have the opposite signal all the time.

Here is step by step a process.

First, Suppose at point C the signal is “1”. Therefore, at B must be “0”.

When C is High B is LOW

The capacitor charges a high voltage from point C to B.

It certainly has the current flowing from C through the capacitor through the resistor to point B.

The current charge via C and R to B

Now, the high voltage drop across the resistor.

It causes points A or D will have high voltage for just a brief moment.

High voltage for just brief

The charging of this capacitor will take less time. It depends on its resistance and capacitance. If the value is large then it will last longer. But if there is a small value, it will speed up.

But when the capacitor is fully charged, the current will stop flowing causing no voltage across the resistor. So, The voltage at point D is “0”.

This was where the change occurred. This time at point A is “0”, therefore B is “1”, and C is changed to “0”. Every point has the opposite status as the first state.

a full charge at all points, change status

When B is “1” and C is “0”, it will charge the capacitor again.

But this time the current will flow from point B through the resistor through the capacitor to point C.

So, It has the direction of flow of the current as opposed to the first charge.

The current flows backwards

When the current flows in the reverse direction. The voltage across the resistor will have the opposite direction. It causes the voltage at point D to be lower than B, which is “0”.

This status will be temporary. Until the capacitor is fully charged. And the current stops flowing.

The voltage at point D gradually creases.

The voltage gradually increases

That is the voltage at point D is as high as B. As a result, A is changed to “1”, therefore B has to be “0” and C is “1”. Back to the original condition, we started to explain.

Charging the capacitor and changing the voltage at various parts will continue this way.

Back to the original condition

Also:

Not only that. if you want to use this circuit. We can design it in simple.

  • The resulting waveform will vary with the product of R and C in the circuit.
  • Try changing the resistor (R) between 10K to 1M and Capacitor (C) between 0.001uF to 0.1uF.
  • The frequency of the generated pulse (F) is approximately 1 / 1.4 RC.

Do you understand? even my English is quite bad. But you probably see my efforts.

See the example circuits below. They may are useful to you.

Basic clock Circuits

From the above principles Are you bored with it? Let’s see 7 examples of simple circuits

1# Basic 1KHz clock generator circuit using CD4049 NOT Gates

We try to design the circuit to produce 1KHz frequency. By using the formula above.

We can use other CMOS NOT gates such as CD4049, CD4069, etc. Be careful with difference pinouts.

Basic 1KHz clock generator circuit using CD4049 NOT Gates

Read: 4049 datasheet and circuits

We try to design the circuit to produce 1KHz frequency. By using the formula above.

Read other: astable multivibrator using logic gates

2# 1KHz Oscillator circuit using CD4011 NAND Gates

We know that we can use NOR and NAND Gates are wired as the inverters gates.

So we can use a not chip for the oscillator, too.

1KHz Oscillator circuit using CD4011 NAND Gates

3# 1KHz Square wave generator circuit using NOR Gate CD4001

Likewise, we brought the CD4001-NOR gates CMOS chip to generated the 1KHz frequency as the NOT gates.

1KHz Square wave generator circuit using NOR Gate CD4001

4# 1KHz Oscillator using Schmitt trigger

We also use a Schmitt trigger to produce a square wave at 1kHz with the components shown in right.

When the Schmitt output is “1”, the capacitor charges via the 330K resistor.

1KHz oscillator using Schmitt trigger

Then, the voltage on the input rises to the upper trip point of the Schmitt the output drops “0”.

IC1: CD4584 or 74C14 Hex Schmitt Trigger

Next, the capacitor discharges through the resistor to the lower trip point to change the Schmitt output to “1”.

The cycle repeats at approx 1kHz. This circuit is very economical on parts.

Imagine you need too high-frequency output. approx 3MHz. And you do not have the resistor. You can use the circuit below.

5# Simplest High-frequency Oscillator circuit using NOT Gate

Let’s experiment uses NOT Gate as an oscillator circuit. We will try to take an inverter gate or NOT gate to test the fun circuit.

We use the CMOS chip CD4069B inverter gate form.

Or also use NAND gate: CD4011, MC14011B.

Then, take the input joint together into the same inverter form.

using NAND as Inverter gates

Connect the circuit as the figure below. We Just to joint pin by many wires onto the breadboard as the correct position only.

Only not Gate oscillator simplest circuit
Experiment square wave oscillator circuit using inverter pattern

See In the circuit, we connect the inverter ICa to ICd continuously series. The output of IC1c will feedback to the input of IC1a. Thus, the signal will run from ICa to IC1c and backward to IC1a in cycle form.

Then, use an oscilloscope to measure the waveform signal at the output of IC1d. We will see the square waveform signal is high frequency about 3.3 MHz.

Test without oscilloscope
But if not have the oscilloscope. You can test this circuit by applying it near the antenna of television. We will see that on-screen will have more many dots like rain. It indicates the circuit is working with high frequency.

How it works

Why is easy? We use two main principles below.

  • The input and output of the inverter will always opposite.
  • We enter the signal the input of each gate. It has a little delay time before appearing at that output.

Here is a step by step process.

Look at the circuit diagram again.

Process of inverter gate as the oscillator
  • Suppose that, at the input of IC1a, is “0” output at pin 3 will be “1”.
  • This signal “1” will come to the input of IC1b and provide the output is “0”.
  • Then it changes to “1” at pin 4 by IC1c. While this signal is sent to feedback to input IC1a again.

So, at the input of IC1a will change from “0” into “1” automatically. And this signal “1” will flow through to IC1d to output. While And it will return back again via IC1b and IC1C to input again.

Thus, the signal will be “0” and “1” alternately at all the time.

Key Tips

Connecting the inverter circuit as the oscillators. We must use the gate as an odd number such as 1, 3, 5…. etc. If even number will not cause oscillated.

Also, the frequency of the oscillator depends on the delay time of each gate and the amount of the gate, too.

  • CMOS—They will have a delay time of approximately 0.1 uS.
    For example, if we use the 3 gates connected as the delay time of 0.3 microseconds. So, the output frequency of 3.3 MHz.
  • TTL— this type will have delay time about 20 nanoseconds. Which faster than CMOS of 5 times.
    For example, if we use the 74LS00 NAND gate. It can get a frequency waveform of approximately 15 MHz.

Reducing Frequency using the capacitor

If you do not want too high-frequency output. How to reduce it.

Suppose that, in gate circuit, there is something makes long delay time up. It will make the frequencies is certainly lower.

Which we connected a little capacitor across the inverter as Figure above.

When we insert the capacitor connects between the input and output of IC1b. It will cause the delay time at the IC1b is longer up. After that, try to measure the waveform. we will see that they have lower frequencies.

We may change the larger capacitances. To find out the result reducing frequency down as table in right.

CapacitorFrequencies
NON3.3MHz
220pF720kHz
0.01uF31kHz
0.1uF3.2kHz

Now, probably understand the basics of Astable Multivibrator using the Logic Gates circuit.

In fact, there is still much more to this section, but I’m afraid that this article will be too long. You may feel bored.

Check out these related articles, too:

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