HOW DOES A SEVEN-SEGMENT DISPLAY OPERATE?

INTRODUCTION

A seven-segment display (SSD) is a vital form of electronic display device for displaying decimal numerals. It is an alternative to the more complex dot matrix displays. Today we often see Seven-Segment displays are widely used in digital clocks, electronic meters and other electronic displays units.

Basically, SSD could be used to display single decimal digit i.e. from zero to nine. It is generally driven by a binary coded decimal nibble. Thus in order to find out how it operates, let us investigate the logic behind its input and output signals.

CONTENTS

       I.          Building the truth table for SSD 

     II.          Boolean expression for the logic function

   III.          Simplification using K-Map

   IV.          The circuit diagram of SSD

Building the truth table

•    The seven segments are referred to by the letters A to G as shown below.

•      Thus, in order to display a digit, we need to turn on all or some of the 7 segments of the SSD display by putting “1” to the appropriate SSD pins. In displaying decimal digits, due to the necessity of 10 distinct combinations of the 7 segments, the no. of binary inputs should be greater than or equal to 4. (Note: if “n” amount of binary inputs are given, maximum amount of combinations that could be obtained is 2ⁿ). Hence we supply 4 input signals to obtain 7 output signals, which is a practical application of a Decoder.
•      Let the input signals be P, Q, R and S. Then the truth table for displaying digits could be given as follows.

Boolean Expressions

• Using the above truth table, we could express the Boolean expression for each output signal A to G as given below. Each expression is given as the sum of the combinations of  input signals when a particular segment is 1.

    Since it would be more complicated and difficult to design the circuit, directly from the               above expressions, they have to be simplified further. It’s done using Karnaugh Maps.

   In filling K-Maps, considering a particular segment, all “1”s for P,Q,R & S are put down from       all the combinations. Then zeros are filled for all the other valid BCD inputs.

After completion of a K-Map and looping  the largest possible areas, simplified Boolean expression could be given. It could be further minimized using Boolean Algebra.

Simplification using K-Map

•  Now, with the use of the simplified and concluded Boolean expression, the circuit could be drawn.

The circuit diagram of SSD

drawn by A.C.L.Jayathilake©

Hope you obtain a comprehensive idea about SSD system and I believe that  if you are more curious about how work out on the circuit ,you should watch this video.

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