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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
