# LED resistor calculation

Discussion in 'General Electronics Chat' started by yonubear, Jan 27, 2014.

1. ### yonubear Thread Starter Member

Jan 10, 2013
73
1
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I hate to be a bother but I am working on a simple ir led driver for a 10ma ir led emitter but I will admit I suck at calculating resisters for base I have a 5v source with a 360ohm resister on the collector and a 3.3v base the emitter is straight to ground what size resistor if any should I use on the base? incase it matters the base is a RPi GPIO

here is the diagram I am using if there is a better method any suggestions will be great
http://1.bp.blogspot.com/-DuyRAT_xH5Y/UQrkCocR6bI/AAAAAAAAA7A/Nqgq_O1N4Ys/s320/IRtransmit.png

Thanks,
yonu

Last edited by a moderator: Mar 7, 2014
2. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Hi Yonubear, since your IR LED only uses 10mA, you only need to put a fraction of a mA into the base.

The calculations are covered above. But roughly put; a 3.3v source driving a 0.6v NPN base gives about 2.7v on the base resistor. Since 1mA base drive will be plenty, the resistor can be;
R = E/I
R = 2.7v / 0.001A
R = 2.7k

In your case you could probably run any value from 1k to 10k in a pinch.

3. ### yonubear Thread Starter Member

Jan 10, 2013
73
1
ok thanks I had decided to just try a 1k resistor but I am getting no reaction so I guess I will go back over my board and see if I screwed something up if it matters here is what I have I have a 5v source to drive the led feeding through a 220ohm resistor network IC connected to the collector port on the 2n3904 transistor the emitter leg is tied to ground and the base is gonnected to a 1k resister on the 3.3v GPIO pin.

Thanks,
Yonu

4. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
The number one cause of transistors not working in a really simple circuit is wrongly connecting the pins.

5. ### yonubear Thread Starter Member

Jan 10, 2013
73
1
hey I wanted to say thanks it wasn't a transistor pined wrong but it was a pin issue when I soldered in the header pins the solder hadn't flowed on the pin upon looking at it with a magnifying glass I found out it looked like a little cave under the solder it wasn't even touching the pin. if I hadn't started rechecking the pins i never would have noticed it

Thanks again,
yonu

Last edited: Jan 30, 2014
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