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Old Oct 18, 2012, 02:08 PM
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Rhone Alps, France
Joined Aug 2008
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Originally Posted by Porthos23 View Post
I would love to see a tutorial on how to wire LEDs. I would like to add 2 blue LEDs on the front arms of my quad and 2 red LEDs on the back arms, with maybe a white LED underneath.

What I want to know is if it is better to get the power for the LEDs from the battery directly, or from somewhere else. I have a Ladybird and I found a video with somebody getting the power for their LEDs from the receiver. I would guess it could also come from a BESC or FC board.

I think it all comes down to resistors and the LED voltage range, but it has been so long since I learned all this in school. A simple tutorial explaining what to do with different battery and LED types would be great, along with some examples.
If you want to add LEDs then you need to check the LED specifications and then do a bit of mathematics. An LED has almost no internal resistance, so if you just hook it direct to a battery then the current will flow almost unimpeded and not only will you drain your battery quickly, but you will burn the LED.

This is why you have to look at the LED specifications for 2 key figures. You need to know the diode forward voltage (this is the voltage required to provide the forward bias to overcome the p-n junction) and the second is the diode forward current.

Once you have these 2 figures, you need to do some maths. A typical forward voltage would be 3.4v and typical forward current would be about 30mA. If you have a supply voltage of 4.2v (1s lipo) we subtract the forward voltage from the supply voltage (4.2-3.4) and then using ohm's law we divide that voltage by the forward current to work out what size resistor is required (0.8v/30mA 0r 0.8/0.030) which gives us the resistance that we need to put in series with the LED. In this case, the resistance = 26.666666, so we round up to a 27ohm resistor.

The power dissapated by the the resistor = I^2*R =24mW and in this case we would choose a 1/8W 27ohm resistor in series with the LED. The colour code for the LED is Red,Purple,Black.

Hope that helps.

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