7 Segment LED Displays

1
7 Segment LED Displays

• Keitz
• Chapter 12-6

2
Final Exam

• Allow 2 hours
• Open Book, Open Notes
• No calculator.

3
Seven Segment LED

• Seven light emitting diodes packaged for a 0-9
  numeric display.

4
Diode

• points in direction of easy current flow

anode way up
Conventional current flow
cathode way down
Remember a comes before c
5
Diode Approximations

• Assume voltage drop across a lit LED is 1.7
  volts.
• Assume 15 mA is required to light LED

6
Seven Segment LED Schematic

• Common anode internal wiring.
• Connect to ground to light.
• Thus the control is active low.

a b c d e f g
7
Current Limiting Resistor

• for 5 volt logic need a current limiting
  resistor for each LED.

5 v
8
Display with external resistors

• Connect the resistors to ground and LED will
  light.

5v
a b c d e f g
9
LED Connection

• Why not use only one resistor?

5v
10
7 Segment Module

• 74LS240
• Tri-state octal buffer inverter.
• Tri-state
• Output can be high, low or floating.
• floating to attach two or more to a bus
• octal 8 buffer inverters
• buffer circuit to isolate input from output.
• current drawn by load is supplied by buffer, not
  input.

11
7 Segment Module

• Fabrication Laboratory
• Input at high or floating
• Inverter is low
• Potential difference from 5 volt supply to
  inverter low drives current through LED.
• gt LED on

Input
12
Display Example

• To indicate a 4
• requires b,c,f,g segments are on.

a
f
b
g
e
c
d
13
LED Decoder

• Need to change 4 bit BCD output to proper 7
  segment signal
• 410 01002 1001100 (abcdefg)
• Could build the decoder from logic gates.
• Make truth table.
• Write SOP expression.
• Simplify with a Karnaugh map.

14
LED Decoder 7447

• Integrated Circuit Decoder
• Pins not in order

g
f
e
d
c
b
a
Low on LT is a Lamp Test to check all segments.
7447
RB I
RB0
LT
A1
A2
A3
A4
BCD Inputs
15
Zero Suppression

• RBI - Ripple Blanking Input
• low on RBI will prevent zero from displaying.
• use on Most Significant Digit to suppress
  leading zeros.
• 06 6
• RBO - Ripple Blanking Output
• low output if zero is suppressed
• send to next lower digit driver
• 006 6

16
Multiple Segments
BCD
BCD
BCD
7447
7447
7447
RBI
RBO
RBI
7 segments
7 segments
7 segments
G
MSB
LSB
17
Decimal Counter
7447
7490
Clock
7 segments
BCD