UNIVERSAL COLLEGE OF ENGINEERING &TECHNOLOGY

SUBJECT-DIGITAL

ELECTRONICS

PRESENTED BYANUPRIYA KASHYAP

130460111001

EC 3RD SEM

SEVEN SEGMENT

LED DISPLAY

CONSTRUCTION Display any number from 0 to 9 by turning on various

combinations of the segments . Actually each segment (a to f) is nothing but an LED

in a segment form. Their connection leads are brought out ,and by

applying a forward voltage to the segments to be turned on we can display any number between 0 and 9.

For example if we want to display the number 7 then the segments a , b and c should be turned on and all other segments should be off. Similarly the other numbers can be displayed .

TYPES OF SEVEN SEGMENT DISPLAYS

COMMON ANODE COMMON CATHODE

COMMON ANODE DISPLAY

A common anode seven segment display.

The name indicates the anode terminals of all LED segments are connected together.

Common Cathode Display

The cathode terminal is made common and all the anode terminals are brought out separately.

A current limiting resistor is externally connected in series with each segment.

The common cathode point is connected to ground and the anodes of the segments to be illuminated are connected to positive supply voltage Vcc.

USE OF A DECODER FOR DRIVING THE SEVEN SEGMENT

DISPLAYMany times the seven segment LED

displays are connected at the output of digital ICs such as counters.

Now as the counter output is in the BCD (binary coded decimal) form ,which has only four lines , it cannot drive the seven segment display.

The common anode type display is being used.

The decoder accept a four bit BCD count from a counter, converts it to a seven bit code suitable for seven segment display(a to g)and drives the display.

To turn on a segment the corresponding decoder output goes low , and sinks current (for common anode display).

A current limiting resistance is connected in series with each segment.

DRIVING A COMMON SEVEN SEGMENT DISPLAY

The outputs of this decoder are active high.

The common anode point is connected to Vcc i.e. positive supply voltage.

A current limiting resister is externally connected in series with each segment.

The cathodes segments to be turned on are connected to ground.

BCD to seven segments decoder

A number of MSI chips are available in market to perform the decoder / driver function.

IC 7449A,7447A or 74ls47 are 16 pin TTL Ics. They accept four BCD inputs A3 ,A2,A1,A0 with A3 represents MSB and A0 LSB.

RBI (Ripple blanking input)BI (Blanking input)RBO (Ripple blanking

output)Lamp test (LT)

Use of 7447 to Drive a Common Anode Display

The common anode point of the display is connected to positive end of the supply and the cathode are connected to the outputs of IC 7447.

The BI/RBO,RBI and LT pins are connected to +5 V so as to make them inactive.

The BCD inputs are applied to the inputs A3,A2,A1,A0.

CASCADED DISPLAYS

Non multiplexed cascaded display

Multiplexed cascaded display

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