UNIVERSAL COLLEGE OF ENGINEERING &TECHNOLOGY

DIGITAL ELECTRONICS SUBJECT - DIGITAL ELECTRONICS

ANUPRIYA KASHYAP PRESENTED BY ANUPRIYA KASHYAP EC 3 RD SEM

SEVEN SEGMENT LED DISPLAY

CONSTRUCTION  Display any number from 0 to 9 by turning on various combinations of the segments.  Actually each segment (a to f) is nothing but an LED in a segment form.  Their connection leads are brought out,and by applying a forward voltage to the segments to be turned on we can display any number between 0 and 9.  For example if we want to display the number 7 then the segments a, b and c should be turned on and all other segments should be off. Similarly the other numbers can be displayed.

TYPES OF SEVEN SEGMENT DISPLAYS COMMON ANODE COMMON CATHODE

COMMON ANODE DISPLAY  A common anode seven segment display.  The name indicates the anode terminals of all LED segments are connected together.

 The cathode terminal is made common and all the anode terminals are brought out separately.  A current limiting resistor is externally connected in series with each segment.  The common cathode point is connected to ground and the anodes of the segments to be illuminated are connected to positive supply voltage Vcc.

USE OF A DECODER FOR DRIVING THE SEVEN SEGMENT DISPLAY  Many times the seven segment LED displays are connected at the output of digital ICs such as counters.  Now as the counter output is in the BCD (binary coded decimal) form,which has only four lines, it cannot drive the seven segment display.  The common anode type display is being used.

 The decoder accept a four bit BCD count from a counter, converts it to a seven bit code suitable for seven segment display(a to g)and drives the display.  To turn on a segment the corresponding decoder output goes low, and sinks current (for common anode display).  A current limiting resistance is connected in series with each segment.

DRIVING A COMMON SEVEN SEGMENT DISPLAY  The outputs of this decoder are active high.  The common anode point is connected to Vcc i.e. positive supply voltage.  A current limiting resister is externally connected in series with each segment.  The cathodes segments to be turned on are connected to ground.

BCD to seven segments decoder  A number of MSI chips are available in market to perform the decoder / driver function.  IC 7449A,7447A or 74ls47 are 16 pin TTL Ics. They accept four BCD inputs A3,A2,A1,A0 with A3 represents MSB and A0 LSB.

 RBI (Ripple blanking input)  BI (Blanking input)  RBO (Ripple blanking output)  Lamp test (LT)

Use of 7447 to Drive a Common Anode Display  The common anode point of the display is connected to positive end of the supply and the cathode are connected to the outputs of IC  The BI/RBO,RBI and LT pins are connected to +5 V so as to make them inactive.  The BCD inputs are applied to the inputs A3,A2,A1,A0.

CASCADED DISPLAYS Non multiplexed cascaded display Multiplexed cascaded display