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Netduino output pin and external LED


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#1 Paolo Patierno

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Posted 16 October 2010 - 12:39 PM

Hello, I'm a software developer and a newbie with electronic circuits. I have just bought Netduino and Arduino Starter kit (I think that it is the same for Netduino). I know that the Netduino output digital pins have the following features : 3.3 V - 8 mA. I have a red LED 10 x 5 mm with the following specs : 2.1V - 20mA. I know that I have to put a resistor in series with LED to drop the voltage from 3.3 V (pin) to 2.1 V (LED voltage). How can I do with the current ? I have 8 mA on pin but the LED works with 20 mA ! Thanks, Paolo

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#2 CW2

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Posted 16 October 2010 - 01:27 PM

I have a red LED 10 x 5 mm with the following specs : 2.1V - 20mA.

Digital pins 2, 3 and 7 can handle up to 16 mA, so you can calculate the resistor to limit current to ~15 mA: (3.3 - 2.1)/0.015 = 80 Ω. The LED will be very dim at 8 mA (if you have the datasheet, check Luminous Intensity vs. Forward Current char). You can use [general purpose NPN] transistor to switch the LED from any Netduino pin.

#3 Paolo Patierno

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Posted 16 October 2010 - 01:30 PM

Digital pins 2, 3 and 7 can handle up to 16 mA, so you can calculate the resistor to limit current to ~15 mA: (3.3 - 2.1)/0.015 = 80 Ω. The LED will be very dim at 8 mA (if you have the datasheet, check Luminous Intensity vs. Forward Current char). You can use [general purpose NPN] transistor to switch the LED from any Netduino pin.


Thank you very much...can you give me some information how I can use NPN transistor to switch the LED ?

Best regards,
Paolo

Paolo Patierno

Microsoft MVP on Windows Embedded & IoT

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Blog : Embedded101
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#4 CW2

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Posted 16 October 2010 - 01:46 PM

Thank you very much...can you give me some information how I can use NPN transistor to switch the LED ?

For example, look at the following article. You can wire the LED to Netduino exactly the same way, using the onboard 5V supply. If you wire the LED to 3.3V supply, use (3.3-2.1)/0.02 = 60 Ω resistor.

#5 Flores

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Posted 16 October 2010 - 06:42 PM

If you want to drive more leds this way (up to eight), the uln2803 could be handy.

See it in action with a netduino and an analog input here

#6 Paolo Patierno

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Posted 18 October 2010 - 12:11 PM

For example, look at the following article. You can wire the LED to Netduino exactly the same way, using the onboard 5V supply. If you wire the LED to 3.3V supply, use (3.3-2.1)/0.02 = 60 Ω resistor.


Thank you very much
Paolo.

Paolo Patierno

Microsoft MVP on Windows Embedded & IoT

Azure Advisor

Twitter : @ppatierno
Linkedin : paolopatierno
Blog : DevExperience

Blog : Embedded101
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