LED Basics

Last updated 10/30/18

2 © tjEE 1910

LED Basics

• Light Emitting Diode

• Light is emitted from special diodes when they are sufficiently forward biased

• No light is emitted if the forward bias (voltage) is below the diode turn on voltage

• No light is emitted if the diode is reverse biased

• Typical LED “turn on” voltage is 1.6v – 2.4v

3 © tjEE 1910

LED Basics

• Light Emitting Diode

• Schematic symbol

• Physical construction

Anode Cathode

Anode Cathode

4 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

Anode Cathode

+ -V D

ID

VDVon0v

I D

5 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

3.3V Battery

+

-

VD and the batteryvoltage are not matched

6 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

3.3V Battery

R

+

-

Von

+

-

Some of the batteryvoltage is used acrossthe resistor – allowingthe diode voltage to beVON

7 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

Loop Equation:-3.3v + I*R +Von = 0I*R = 3.3v – VonR = (3.3v – Von)/I3.3V Battery

R

+

-

Von

+

-

8 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

Von = 1.8vImax = 6mA

R = (3.3v – 1.8v)/6mAR = 250Ω3.3V Battery

R

+

-

Von

+

-

9 © tjEE 1910

LED Basics

• Light Emitting Diode

• Operation

We have 330Ωresistors

Solve for II = 4.5mA

< Imax

3.3V Battery

R

+

-

Von

+

-

10 © tjEE 1910

LED Basics

• Light Emitting Diode

• Use our output pin instead of the battery

DigitaloutputPin 5

gnd

Output = “1” (3.3v) → ONOutput = “0” (0.0v) → OFF

MSP432 Board330Ω

+

-

Von

11 © tjEE 1910

LED Basics

• Light Emitting Diode• Use our output pin instead of the battery

330 ΩResistor(direction doesnot matter)

Anode(long wire)

Cathode(short wire)

GND

12 © tjEE 1910

LED Basics

• Light Emitting Diode• Alternate hookup A

DigitaloutputPin 5

gnd

Output = “1” (3.3v) → ONOutput = “0” (0.0v) → OFF

MSP432 Board

330Ω

+

-

Von

13 © tjEE 1910

LED Basics

• Light Emitting Diode• Alternate hookup B

DigitaloutputPin 5

3.3v

Output = “1” (3.3v) → OFFOutput = “0” (0.0v) → ON

MSP432 Board330Ω

+

-

Von

14 © tjEE 1910

LED Basics

• Light Emitting Diode• Alternate hookup C

DigitaloutputPin 5

3.3v

MSP432 Board

330Ω

+

-

Von

Output = “1” (3.3v) → OFFOutput = “0” (0.0v) → ON