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Connect LEDs to PC parallel port

GaryK4

Apr 25, 2015
4
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Apr 25, 2015
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4
I have a project where I want to connect LEDs to my computers parallel port. I just ordered a multi-color package with resistors.
The description says "Forward Voltages are Clear Red, Orange and Yellow 1.9-2.0V, Clear Green: 2.9-3.1V, Clear Blue, White, Pink and Violet: 3.0-3.2V. These light-emitting diodes work well for a wide variety of applications including Arduino projects."
How do I figure out what size resistor that I need for these? I believe that a parallel port put out 5v with 20ma.
I read one article that said the following (which I don't understand) "PC’s Parallel Port. It can load only 2.5mA and ~2.5 volts. It is better to use opto-couplers or ULN2803 when interfacing with an external device."

I know how to write a program to turn on/off the pins of the parallel port. I just need the above - I hope.
 

Arouse1973

Adam
Dec 18, 2013
5,177
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Dec 18, 2013
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5,177
Ok so you know the voltage, 5 Volts. What you do is subtract the LEDs forward voltage from this and divide by say 10mA. So for Red LED its (5-1.9) ÷ 0.01 =310 Ohms
 

GaryK4

Apr 25, 2015
4
Joined
Apr 25, 2015
Messages
4
Ok so you know the voltage, 5 Volts. What you do is subtract the LEDs forward voltage from this and divide by say 10mA. So for Red LED its (5-1.9) ÷ 0.01 =310 Ohms

I think that should do the trick.
Thanks.
 

Arouse1973

Adam
Dec 18, 2013
5,177
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5,177
Remember for LEDs with a higher forward voltage like green, blue and white you need to change the forward voltage.
Adam
 
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