Page 144 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

1 Common Cathode Configuration: The Common Cathode (CC) – In the common cathode display, all the
cathode connections of the LED segments are joined together to logic “0” or ground. The individual segments
are illuminated by application of a “HIGH”, or logic “1” signal via a current limiting resistor to forward bias the
individual Anode terminals (a-g).
2 Common Anode Configuration :The Common Anode (CA) – In the common anode display, all the anode
connections of the LED segments are joined together to logic “1”. The individual segments are illuminated
by applying a ground, logic “0” or “LOW” signal via a suitable current limiting resistor to the Cathode of the
particular segment (a-g).

Driver/Decoder IC for LED Using 7-Segment
To design a circuit with four inputs A, B, C, D and seven outputs a, b, c, d, e, f and g, the input to the circuit is
the binary representation of one of the ten decimal digits. Its output is chosen so as to display the equivalent
decimal value using a seven-segment display. Figure shows a schematic of the seven-segment display. The
seven-segment display has seven Light Emitting Diodes (LEDs). By assigning a logical 1 to a given segment, the
segment will emit light.
The inputs to the display in figure are shown with labels a to g. The display is found in some calculators with the
LEDs replaced by Liquid Crystal Displays (LCDs).
The circuit we want to design has seven outputs. The outputs serve as inputs to the seven-segment display and
as a result are labeled a to g as well. The block diagram given in Fig. (b) shows the inputs and outputs to the
circuit to be designed. The circuit is called a BCD to seven-segment decoder
The inputs given to the circuit are A, B, C and D. The inputs represent a decimal digit between 0 to 9. The output
assigns values to a to g so as to display the corresponding decimal digit.

eg.. When the input ABCD = 0000 is applied, the decimal digit 0 is displayed using decoder IC. By inspection, this
occurs when the g output is assigned a value of 0 and all remaining outputs are assigned a value of 1. The truth
table associated with the BCD to seven segment decoder circuit is shown in Fig. (c).
Note that the table does not have output entries associated with binary combinations with decimal values
exceeding 9.
Truth table of the BCD to Seven segment Display Decoder:

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CITS : E & H - Electronics Mechanic - Lesson 77 - 83