Page 56 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Interfacing 7-Segment Display Circuit Diagram

BCD to 7 Segment Decoder
Prerequisite – Number System and base conversions, BCD(8421). In Binary Coded Decimal (BCD) encoding
scheme each of the decimal numbers(0-9) is represented by its equivalent binary pattern(which is generally of
4-bits). Whereas, Seven segment display is an electronic device which consists of seven Light Emitting Diodes
(LEDs) arranged in a some definite pattern (common cathode or common anode type), which is used to display
Hexadecimal numerals(in this case decimal numbers,as input is BCD i.e., 0-9). Two types of seven segment LED
display.

1 Common Cathode Type: In this type of display all cathodes of the seven LEDs are connected together to the
ground or -Vcc(hence, common cathode) and LED displays digits when some ‘HIGH’ signal is supplied to the
individual anodes.
2 Common Anode Type: In this type of display all the anodes of the seven LEDs are connected to battery or
+Vcc and LED displays digits when some ‘LOW’ signal is supplied to the individual cathodes.
But, seven segment display does not work by directly supplying voltage to different segments of LEDs. First, our
decimal number is changed to its BCD equivalent signal then BCD to seven segment decoder converts that sig-
nals to the form which is fed to seven segment display. This BCD to seven segment decoder has four input lines
(A, B, C and D) and 7 output lines (a, b, c, d, e, f and g), this output is given to seven segment LED display which
displays the decimal number depending upon inputs.

Truth Table for common cathode type BCD to seven segment decoder

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CITS : E & H - Electronics Mechanic - Lesson 9 - 29