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Light Emitting Diode: LED
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What is an LED?
Light-emitting diode
Semiconductor
Has polarity
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LED: How It Works
When currentflows across adiode
Negative electrons move oneway and positive holes move
the other way
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LED: How It Works
The wholes existat a lower energylevel than the
free electrons
Therefore when a freeelectrons falls it losses energy
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LED: How It Works
This energy isemitted in a formof a photon,
which causes light
The color of the light isdetermined by the fall of the
electron and hence energylevel of the photon
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Inside a Light Emitting Diode
1. TransparentPlastic Case
2. Terminal Pins
3. Diode
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Kinds of LEDs
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How to Connect a LED:
Requires 1.5~2.5V and 10 mA
To prevent overloading, use resistor
470
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How to Connect a LED:
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Connect LED to BS2
LED is on whenP0 is high
LED is on whenP1 is low
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Connect Multiple LEDs to BS2
8 LEDs are connected to BS2 eachI/O pin (P0-P7) is allowed to sink6.25mA
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Case Study: Blinking LED
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Case Study: Blinking LED
Pbasic program to make an LED blink
output 0
Main:low 0pause 1000
high 0pause 1000goto Main
Make pin0 an output
Loop begins here
Turn LED on
Pause for 1 sec
Turn LED off
Pause for 1 sec
Go back to beginning ofloop
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Experiments
Flash an LED1. Single LED
2. Multiple LEDs
Traffic Light by use of LEDs