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How to Design 555 Astable Multivibrator

Understanding the Astable Multivibrator with the 555 Timer IC

The 555 Timer IC is one of the most versatile and widely used integrated circuits in electronics. Among its various applications, one of the fundamental configurations is the astable multivibrator. In this blog post, we'll delve into the concept of the astable multivibrator, how to design one using the 555 Timer IC.

 

What is an Astable Multivibrator?

An astable multivibrator is an electronic oscillator circuit that continuously switches between its high and low states without requiring external triggering. This makes it a perfect choice for generating a square wave signal, which can be used for various applications such as pulse generation, clock pulses, and tone generation.

 

555 Timer IC Overview

The 555 Timer IC, developed by Signetics (now part of ON Semiconductor), is a highly reliable and easy-to-use chip capable of operating in various modes: astable, monostable, and bistable. For our purposes, we'll focus on the astable mode.

 

Astable Multivibrator Configuration with the 555 Timer IC

In the astable mode, the 555 Timer IC generates a continuous square wave output. The frequency and duty cycle of this output are determined by the external resistors and capacitor connected to the IC.

 

Pin Configuration

The 555 Timer IC has 8 pins, each serving a specific function:

1.   GND - Ground

2.   TRIG - Trigger

3.   OUT - Output

4.   RESET - Reset

5.   CTRL - Control Voltage

6.   THRS - Threshold

7.   DISCH - Discharge

8.   VCC - Supply Voltage

 

Components Needed

  • 555 Timer IC
  • Resistors: R1, R2 and R3
  • Capacitor: C1, C2
  • 5mm LED: LED
  • Breadboard and connecting wires

 

Circuit Diagram

Here's a simplified circuit diagram for the astable multivibrator:


 

  • Pin 1 (GND) is connected to ground.
  • Pin 2 (TRIG) and Pin 6 (THRS) are connected.
  • Pin 3 (OUT) is the output where the square wave signal is taken, LED is connected with 470E resistor.
  • Pin 4 (RESET) is connected to Vcc to disable the reset function.
  • Pin 5 (CTRL) is connected to one end of capacitor C2, another end of capacitor is connected to the ground.
  • Pin 6 (THRS) is connected to the one end of resistors R2.
  • Pin 6 (THRS) is also connected to one end of capacitor C1, another end of capacitor is connected to the ground.
  • Pin 7 (DISCH) is connected to the junction of resistors R1 and R2. Another end R1 is connected to the VCC.
  • Pin 8 (VCC) is connected to the VCC.

 

Calculating Frequency and Duty Cycle

The frequency f and duty cycle D of the oscillation are determined by the resistors R1, R2, and the capacitor C1. The formulas are as follows:

  • Frequency (f):

f=1.44 / (R1+2R2)⋅C1

  • Duty Cycle (D):

D=(R1+R2) / (R1+2⋅R2)

 

 

where R1 and R2 are in ohms and C1 is in farads.

 

How to calculate Time Duration in Astable Multivibrator

1. Time Duration in Astable Mode

In astable mode, the 555 Timer generates a continuous square wave with a high and low state. The time duration of each state (high and low) is calculated based on the values of two resistors R1​, R2 and a capacitor C1.

 

Key Parameters:

  • THIGH: Time duration the output stays high.
  • TLOW​: Time duration the output stays low.
  • TTOTAL​: Total time period of one cycle (sum of THIGH and TLOW
  • f: Frequency of oscillation, which is the inverse of the total time period.

 






Final Remarks

The astable multivibrator configuration of the 555 Timer IC is a straightforward way to generate a continuous square wave signal. By adjusting the resistors and capacitor values, you can customize the frequency and duty cycle for various applications. Whether you're designing a pulse generator, a clock signal, or a tone generator, the 555 Timer IC's versatility makes it an invaluable tool in electronics.

Feel free to experiment with different values and configurations to explore the full potential of the 555 Timer IC in astable mode!

  

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