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7-Segment Display

September 12th, 2015

Seven segment display is seven individual LED's within a single small compact  package.

7-segments_Indicator

So it consist of 7 LED's and each LED is known as segment.It allows to display decimal digit from 0 to 9 but actually you can display all hexadecimal digits apart from 0 to 9.Eighth LED represent decimal point(DP) can be used when two or more seven segment displays are connected together to form number greater than ten.

By forward biasing the appropriate pins you can get desired numerical digit.There are two types of 7 segment display

1.Common Cathode

2.Common Anode

At each LED there are two pins anode and cathode respectively.The individual LED pins are represented by "a" through "g" and by forward biasing these pins as desired you can get to display decimal digits 0 to 9.

1.Common Anode

segment2

In common anode configuration all pins of LED(a to g) are connected together and given logic 1 and LED's are illuminated by giving 0 to set of pins according to our need.For example to display 7 we need to give 0 to a,b and c.

2.Common Cathode:-

segment1

In common cathode configuration cathode pins are grounded together and LED's are illuminated by logic 1.

Need For Series Register:

segment4

Practically we always use Vcc as +5V  or higher.Here is how register voltage is determined

a.Forward bias voltage=+2V

b.Forward Current=20mA

Then Resister value R could be

R=(Vcc-2)/0.02

ie R=150 ohm .

So in above figure diode acts like a switch that closes when forward biased and open when reversed biased and in series with register so that LED,s don't get burn't due to high voltage.

 

Connecting more 7 segment together:

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In above figure we can see that four 7 segments have been connected together.To select any one of the four digit you make corresponding common cathode low via NPN transistor which you can drive from I\O pin via resistor.

Advantage:

1.small size

2.Cheap

3.Long life

4.Easy to interface with other electronic components.

5.Energy efficient

6.Can be controlled for brightness and color

7.Environment friendly.

 

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