To Analyze BCD to seven segment display decoder / Ic 74LS47D Project

 To Analyze BCD to seven segment display decoder                        

Objectives:

• To convert BCD code into decimal and display on seven segment LED

Components

• Switches
• 74LS47D
• Seven segment display

Introduction:

Most often seven-segment displays are used to display the digits in digital watches, calculators, clocks, measuring instruments and digital counters, etc. Generally, LCD and LED segments provide the display output of numerical numbers and characters.However to display the characters and numbers (in order to produce the decimal readout), seven-segment displays are most commonly used. Mostly these displays are driven by the output stages of digital ICs (to which the visual indication of the output stages has to be performed) such as latches and decade counters, etc.But these outputs are in the form of 4-bit binary coded decimal (BCD), and not suitable for directly driving the seven-segment displays.A display decoder is used to convert a BCD or a binary code into a 7 segment code. It generally has 4 input lines and 7 output lines. Here we design a simple display decoder circuit using logic gates.Even though commercial BCD to 7 segment decoders are available, designing a display decoder using logic gates may prove to be beneficial from economical as well as knowledge point of view.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)The decoder is an essential component in BCD to seven segment decoder. A decoder is nothing but a combinational logic circuit mainly used for converting a BCD to an equivalent decimal number. It can be a BCD to seven segment decoder. A combinational logic circuit can be built with logic gates which include inputs as well as outputs

Principle of Display Decoder Circuit

The basic idea involves driving a common cathode 7-segment LED display using combinational logic circuit.  The logic circuit is designed with 4 inputs and 7 outputs, each representing an input to the display IC. Using Karnough’s map, logic circuitry for each input to the display is designed.

        convert BCD code into decimal and display on seven segment LED

Ø Truth table for BCD to seven segment decoder (74LS47D)

Digit	A	B	C	D	a	b	c	d	e	f	g
0	0	0	0	0	0	0	0	0	0	0	1
1	0	0	0	1	1	0	0	1	1	1	1
2	0	0	1	0	0	0	1	0	0	1	0
3	0	0	1	1	0	0	0	0	1	1	0
4	0	1	0	0	1	0	0	1	1	0	0
5	0	1	0	1	0	1	0	0	1	0	0
6	0	1	1	0	0	1	0	0	0	0	0
7	0	1	1	1	0	0	0	1	1	1	1
8	1	0	0	0	0	0	0	0	0	0	0
9	1	0	0	1	0	0	0	0	1	0	0

Block diagram

ØLogical expression (74LS47D)

From the above truth table, the Boolean expressions of each output functions can be written as

a = F1 (A, B, C, D) = ∑m (0, 2, 3, 5, 7, 8, 9)

b = F2 (A, B, C, D) = ∑m (0, 1, 2, 3, 4, 7, 8, 9)

c = F3 (A, B, C, D) = ∑m (0, 1, 3, 4, 5, 6, 7, 8, 9)

d = F4 (A, B, C, D) = ∑m (0, 2, 3, 5, 6, 8)

e = F5 (A, B, C, D) = ∑m (0, 2, 6, 8)

f = F6 (A, B, C, D) = ∑m (0, 4, 5, 6, 8, 9)

g = F7 (A, B, C, D) = ∑m (2, 3, 4, 5, 6, 8, 9)

K-Map Simplification

                      

From the above simplification, we get the output values as

Logical diagram

 Circuit on multisim

When input A=0,B=0,C=0,D=0 then out put become 0.

When input A=0,B=0,C=0,D=1 then out put become 1.

When input A=0,B=0,C=1,D=0 then out put become 2.

 When input A=0,B=0,C=1,D=1 then out put become 3.

 When input A=0,B=1,C=0,D=0 then out put become 4.

When input A=0,B=1,C=0,D=1 then out put become 5.

 When input A=0,B=1,C=1,D=0 then out put become 6.

When input A=0,B=1,C=1,D=1 then out put become 7.

When input A=1,B=0,C=0,D=0 then out put become 8.

When input A=1,B=0,C=0,D=1 then out put become 9.