UTAA EE Department
EE 205 Lab.
Experiment 3: Construction of simple combinational circuits
Objectives

To design and build BCD-to-7 segment converter

To learn and understand the working of IC 7447

To learn and understand the working of Seven-Segment Display
Equipment

Trainer board, IC7447, 7-Segment display (Common Anode), resistors.
Introduction
Short for Binary Coded Decimal, BCD is also known as packet decimal and is numbers 0
through 9 converted to four-digit binary.
A seven-segment display (SSD), or seven-segment indicator, is a form of electronic display
device for displaying decimal numerals that is an alternative to the more complex dot matrix
displays.
A light emitting Diode (LED) is a PN junction diode. When the diode is forward biased, a
current flows through the junction and the light is emitted, see Fig.1.
Fig.1. Circuit with an LED.
BCD-to-Seven Segment Converter:
A seven segment LED display contains 7 LEDs. Each LED is called a segment and they are
identified as (a, b, c, d, e, f, g) segments.
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Fig.2. Seven Segment Display Pin Diagram
The display has 7 inputs each connected to an LED segment. Seven-Segment displays are of
two types – Common Anode and Common Cathode. When all anodes of LEDs are tied
together and joined to 5 volts this type is called common anode type, and when all cathodes of
LEDs are tied together and connected to ground this type is called common cathode type. The
Seven-Segment displays that we are going to use are common anode. A limiting resistance
network must be used at the inputs to protect the 7-segment from overloading.
Common Anode (CA) & Common Cathode (CC) 7-Segment Display:
As we mentioned, there are two types of LED 7-segment displays: common cathode (CC) and
common anode (CA). The difference between the two displays is the common cathode has all
the cathodes of the 7-segments connected directly together and the common anode has all the
anodes of the 7-segments connected together. Shown below is a common anode seven
segment.
Fig.3. Connection for common anode type
As shown above all the anode segments are connected together. When working with a CA
seven segment display, power must be applied externally to the the anode connection that is
common to all the segments. Then by applying a ground to a particular segment connection
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(a-g), the appropriate segment will light up. An additional resistor must be added to the circuit
to limit the amount of current flowing thru each LED segment.
A common cathode seven segment is different from a common anode segment in that the
cathodes of all the LEDs are connected together. For the use of this seven segment the
common cathode connection must be grounded and power must be applied to appropriate
segment in order to illuminate that segment.
Fig.4. Connection for common cathode type
The 74xx47 chip is used to drive 7 segment display. You must use the 74xx47 with a
common anode 7-segment display. The input to the 74xx47 is a binary number DCBA
where D is 8s, C is 4s, B is 2s and A is 1s. The inputs DCBA often come from a binary
counter.
7447
Fig.5. Connection for common anode type with 7447 integrated circuit
The display is only sensible if the binary number is between DCBA=0000 (0) and
DCBA=1001 (9); this is called Binary Coded Decimal or BCD for short. If the number is
larger than 9 you get a strange output on the display
The inputs ��/𝑅
𝐵𝑂𝐵 and
𝑇 𝐿 are connected to 5V.
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Fig.6. The IC7447 pin diagram
In order to get the values on 7-segment display, we need to connect the 7 output pins of the
IC7447 to the 7 pins of the 7-segment display (Fig.7). As you can see in Fig.7, the 7 output
pins of the IC7447 are driven by the 4 BCD input pins.
Fig.7. Circuit Diagram for IC 7447 and a 7-Segment Display
Experiment Work
1) To start with, we will construct a circuit as seen in Fig.7 and Fig.5.
2) Place the IC 7447 on board and check its datasheet, pin diagram (Check Fig.6). Make
the appropriate connections for each pin.
3) Place the common anode type 7 segment display on board and check its pin diagram.
Make sure you connected the common pin to Vcc and other pins to the a, b, c, d, e, f, g
output pins of 7447 by using a resistor (Check Fig.7 & Fig.5).
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4) Take the input pins A, B, C, D of 7447 to a side so that by changing them we can
observe the 7 segment display. For the inputs A, B, C, D you can directly connect
them to 5 volt (Vcc) or ground by a wire.
5) Change the inputs of 7447 and fill the table below.
Prelab
1) Take a look at the datasheet of the IC7447. Study the inputs and outputs of the
integrated circuit. (http://www.davidswinscoe.com/electronics/components/7447/ )
2) Check the pin diagram of common anode 7 segment display online.
Report
1) Show your circuit to your instructor. (50%)
2) What are the differences between common anode and common cathode? (10%)
3) How do we need to connect a common anode Seven Segment Display (SSD) to a
curcuit? How do we need to connect a common cathode SSD to a curcuit? (10%)
4) Why do we need to use IC7447? What does it do? (10%)
5) In order to get digit “1” in the 7 segment display what should the inputs and outputs of
IC 7447 be? (10%)
6) Fill the table below. For “IC Outputs” measure the output values by using a
multimeter and write either 0 (if the value is close to zero) or 1. For “Display” part
write the character you see on the 7 segment display. (10%)
D
IC Inputs
C
B
A
0
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
a
b
IC Outputs
c
d
e
5
f
g
Display