555 Timer IC Operation (Astable & Monostable)

This article will show you the basic operation of the 555 timer IC. It is a most popular IC that can help you to work with many important and interesting field. The IC internal circuitry and the pin configuration was given below,

Fig(a): Internal Circuitry

Fig(b): Pin configuration

Basic Operation:
The circuit shown in figure(a) is the internal circuitry of a 555 timer IC. The device consists of two comparators, one transistor,a flip-flop and a outputs stage.The reference voltages for this two comparators inside the 555 are produced across a voltage divider consisting of three equal resistors of 5k ohms each.These three resistors produce 1/3 and 2/3 voltage levels for controlling the action of Trigger and Threshold comparators inside the IC.The Threshold comparator is referenced at 2/3 Vcc and the Trigger comparator is referenced at 1/3Vcc. The two comparators control the flip-flop which, in turn, controls the state of the output i.e. either ON or OFF states.The reset pin 4 is connected to Vcc to start the timing cycle as it 555 timer is used in timing application.

Tow Important Operation Mode:

1. Monostable (One shot) Mode;
2. Astable Mode.

These two are discussed below,

1. Monostable Mode: The circuit for designing monostable multivibrator is constructed as below,

Fig(c): Timer Ic 555 As Monostable  Mode

In this above circuit the resistor R1 and Capacitor C1 is connected externally to the IC. The pulse width of the output is controlled by the formula,
tw = 1.1R1C1 . The pin 5 Control voltage pin is connected with C2 (Decoupling capacitor) to prevent the noise at the Triggering and Threshold level.

When trigger is not applied to pin 2 the output is Low, Transistor Q1 is ON, Capacitor C1 is discharged and current is following as figure below,

When a trigger is applied to pin 2, the output goes to high at instantly, Transistor is OFF, the capacitor start to charge through R1 as figure below,

When the capacitor is charged to 1/3 Vcc the output returns to Low and the transistor turns ON immediately and the capacitor C1 starts to discharge as figure below,

The charging rate of C1 determines how long the output is High.

2. Astable Mode: The circuit for designing Astable multivibrator is connected as below,

Fig(d): The 555 Timer IC As Astable multivibrator.

This configuration can also be named as non sinusoidal oscillator.In this configuration the Threshold input (Pin 6) is connected with Trigger input (Pin 2).Two resistor R1 & R2 , One capacitor C1 is connected externally as shown in figure(d). R1 , R2 & C1 control the total pulse width T as well as the frequency f. One more capacitor C2 is connected to Control (Pin 5) as decoupling capacitor but it has no effect on the whole operation.

When you put Vcc connected, the capacitor C1 is uncharged, i.e.  both the Threshold & Trigger input is Low. This make the comparator A Low and comparator B High. See figure below,

Fig(e): 555 Timer for greater than 50% duty cycle

Thus the output of the Latch as well as the base of the transistor Q1 is Low. The output of the multivibrator is High. Now the capacitor start to charge through R1 and R2 as indicated in figure.

When the capacitor voltage Vc is equal to 1/3 Vcc, the comparator B gives Low output.This will cause no effect on output as well as to the transistor.

When Vc is greater than or equal to 2/3 Vcc,  comparator A gives High output. This will set the flip-flop. So the output becomes low and transistor becomes ON.

When Vc is less than (discharges to) 1/3 Vcc, the comparator B gives High output. This will reset the flip-flop. So output again becomes high and transistor is OFF.

The summery of the Astable Multivibration operation can be shown in a Table below,

The capacitor will charge and discharge between 2/3 Vcc and 1/3 Vcc in above ways. This cycle continuous and you will get high-low-high-low output continuously.

There are some factors of Astable multivibrator . Frequency, and Duty cycle are main things about designing an astable multivibrator or an oscillator.

Duty cycle: Duty cycle is the ratio of high pulse width to total pulse width multiplied by 100%. It specify, how unsymmetrical the output oscillation is. Mathematically the duty cycle is,

Here, T1 is the high pulse width and T is the total pulse width.

Frequency: The output of an Astable multivibrator is a non sinusoidal or square wave. It has a frequency. The frequency of the output signal is given by the formula below,

The circuit shown in figure(e) is only for the duty cycle greater than 50%. When the duty cycle is less than 50% or equal to 50% the circuit will change it's looking.

For getting the duty cycle less than 50% making R1<R2 the circuit can be either like ,

Fig(f): 555 Timer (with single diode) for getting Duty cycle less than 50%

Or,

Fig(g): 555 Timer (with two diodes) for getting Duty cycle  less than 50%

In the first circuit the capacitor will charge through R1 & D1 and discharge through only R2. In the second circuit, the capacitor will charge through R1 & D1 and discharge through D2 & R2.

For getting about 50% duty cycle (for example 49.99%) make R1=R2 at the circuit shown in figur(f) for duty cycle less than 50%. And  for example 50.001% duty cycle which is almost about 50% make R1<<R2 at the circuit shown in figure(e) for duty cycle greater than 50%. 

Or, for exact 50% duty cycle, design your circuit as below,

Fig(h): 555 timer IC for getting 50% duty cycle.

The equations for calculating duty cycle and frequency are like below.

You can calculate the value of of R1, R2 and C1 according to your expected value of duty cycle and frequency by a 555 timer calculator. It's free and easy to use. Here is the download link of 555 timer calculator at Schematica.