INTRODUCTION

The basic idea involves driving a common cathode 7-segment LED display using combinational logic circuit The seven derived logic diagrams will be then implemented by using NAND logic Gates

1.    THEORY

The Seven segment display is most frequently used the digital display in calculators, digital counters, digital clocks, measuring instruments, etc. Usually, the displays like LED’s as well as LCD’s are used to display the characters as well as numerical numbers. But, a seven segment display is used to display both the numbers and characters. These displays are frequently driven by the output phases of digital integrated circuits like decade counters as well as latches. However the outputs of these are in the type of 4-bit BCD (Binary Coded Decimal), so not appropriate for directly operating the seven segment display. For that, a display decoder can be employed for converting BCD code to seven segment code. Generally, it has four input lines as well as seven output lines. This article discusses how to design a BCD to seven segment display decoder circuit using logic gates.

BCD

BCD stands for binary coded decimal. It is a digital numbering system in which we can represent each decimal number using 4 bits of binary numbers.

There are 10 digits in the decimal system. To represent all 10 digits we need 10 combinations of 4 bi

A digital system like a computer can understand and easily read a large number in binary format. However, a human cannot read large binary numbers. To solve this problem we need to display it as a decimal digit using 7-segment display.

7-Segment Display

It is a digital device that can be used for displaying decimal number, alphabets, and characters.

7-Segment display contains 7 LED segments arranged in a shape given in figure above. Generally, there are 8 input pins in a 7-Segment display. 7 input pins for each of the 7 LEDs and one pin for the common terminal.

There are two types of 7-Segment displays.

Common Cathode

In such type of 7-segment display, all the cathodes of the 7 LEDs are connected together to form a common terminal. It should be connected to GND or logic ‘0’ during its operation.

To illuminate any LED of the display, you need to supply logic ‘1’ to its corresponding input pin.

Common Anode

The type of 7-Segment display in which all the anode terminals of 7 LEDs are connected together to form common anode terminal. This terminal should be connected with Vcc or logic ‘1’ during its operation.

To illuminate any of the LED segments we need to provide logic ‘0’ to it.

7-Segment Display Segments for all Numbers

Display combination of decimal numbers is given below.

Digit 1: to display the digit 1 we need to turn on the segments b, c. and turn off the LED segments a, d, e, f, and g. This configuration will result in the display as shown in the figure below.

Related article: MUX – Digital Multiplexer | Types, Construction & Applications

Digit 2: to display the digit 2 we need to turn on the segments a, b, d, e, g. and turn off the LED segments c, f. This configuration will result in the display as shown in the figure below.

Digit 3: to display the digit 3 we need to turn on the segments a, b, c, d, g. and turn off the LED segments e, f. This configuration will result in the display as shown in the figure below.

Digit 4: to display the digit 4 we need to turn on the segments b, c, f, g. and turn off the LED segments a, d, e. This configuration will result in the display as shown in the figure below.

Digit 5: to display the digit 5 we need to turn on the segments a, c, d, f, g. and turn off the LED segments b, e. This configuration will result in the display as shown in the figure below.

Digit 6: to display the digit 6 we need to turn on the segments a, c, d, e, f, g. and turn off the LED segments b. This configuration will result in the display as shown in the figure below.

Digit 7: to display the digit 7 we need to turn on the segments a, b, c. and turn off the LED segments d, e, f, g. This configuration will result in the display as shown in the figure below.

·         Related article: DEMUX – Demultiplexer | Types, Construction & Applications

Digit 8: to display the digit 8 we need to turn on the segments a, b, c, d, e, g only. This configuration will result in the display as shown in the figure below.

Digit 9: to display the digit 2 we need to turn on the segments a, b, c, d, f, g. and turn off the LED segments e. This configuration will result in the display as shown in the figure below.

To display these digits using binary numbers we need to decode these binary numbers into the combination used for each pattern or display using Decoder.

Karnaugh Maps Simplification

For other combinations of input, the output is “don’t care X” as there are no more digits to display. We will derive the expression for each output using Karnaugh map (K-MAP).

For output a:

For output b:

For output c:

For output d:

For output e:

For output f:

For output g:

7-Segment Display Decoder Circuit

We have derived an expression for each output now we need to make its schematic using logic gates as shown in the figure given below. Fig: Schematic of BCD to 7-Segment Display Decoder.BC’D