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Old Apr 26, 2001, 10:38 AM
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Rhode Island
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Powering LED's

I'm building a plane which I intend to equip with LED's and try some night flying. I'll be using most likely an 8-cell pack which has a voltage swing of greater than 3V over its discharge cycle. So instead of powering the LED's directly off the battery I was thinking of using a 7805 voltage regulator. Current drain should be in the neighborhood of 200-250ma for the LED's. The question came up when I went to calculate the current limiting resistors for the LED's and I realized I didn't really know exactly what voltage to use.

Any comments?


[This message has been edited by BobK (edited 04-26-2001).]
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Old Apr 26, 2001, 12:04 PM
AMA 697691
Rochester, NY, USA
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Bob,

Your idea is sound. The 7805 will keep a constant current through the LEDs and therefor a constant brightness.

Compared to the motor draw, the LED draw is small. For the relatively short flights and small voltage drop across the 7805, you probably don't even need to heat sink it.

Good luck!
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Old Apr 26, 2001, 12:08 PM
jrb
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That darn current limiting resistor always gets me messed up when I try to do this same thing.

Are they really needed?

For example a 3 volt regulator and 3 volt LEDs?
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Old Apr 26, 2001, 12:24 PM
York Electronics
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Quote:
Originally posted by jrb:
Are they really needed?

Yes.
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Old Apr 26, 2001, 12:48 PM
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Greg,
Thanks for the reply! I agree that a heat sink probably won't be necessary since the dissipation will be around a watt or so max and the part I'm using is in a TO-220 package. I did toy with the idea of using the BEC output but I'll be using a GP C-5 or C-10 ESC, which is rated for two servos and knowing GP, that's probably marginal. Oh well, could always do it with a Jeti 350. A bit of overkill for a 280 ship though .

jrb,
Yep, they're definitely a good idea. LEDs are funny animals electrically, they act a lot like a zener diode. If your regulator was precisely 3V and the LED's voltage drop was precisely 3V as well, it'd probably work. But if the supply is actually 3.1V, or the LED was 2.9v, it would probably toast the LED.

Figuring the value of the resistor is pretty straightforward. The formula is:

R = (Vs - Vled) / Iled

where Vs is the source voltage, Vled is the LED's voltage drop, and Iled is the current. For instance, if you had a 5V supply and a 1.9V 20ma LED, it would be (5.0 - 1.9) / 0.02, or 155 ohms; a 150 ohm resistor would be close enough.
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Old Apr 26, 2001, 12:50 PM
aka: A.Roger Wilfong
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The regulator will help with maintaining the same light output level, but you could get by with out it. Just wire the LEDs to the motor battery. The current limiting resistor is easy to calculate using Ohm'm Law.

Assuming you're using the 7805, source is 5 volts, the LED drops 3 volts so you need to drop 2 volts across the resistor. The value of the resistor sets the current.

For instance if the LED wants 10 mA the formula is:

R = E/I
R = 2 V / 10 mA
R = 200 ohms

anything handy between 180 and 220 will work.

If you have a brighter, higher current LED, just substitute the LED's current rating.

- Roger
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Old Apr 26, 2001, 12:55 PM
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LED might not be bright enough to see, I use flash light blub and get the right voltage kind and string them together parallel or series to get the right brightess/voltage combination. I use transparent covering and the lightbulb will light up a whole bay in the wing. One bulb in one bay.

Brian
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Old Apr 26, 2001, 02:33 PM
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Quote:
Originally posted by BobK:
I'm building a plane which I intend to equip with LED's and try some night flying. I'll be using most likely an 8-cell pack which has a voltage swing of greater than 3V over its discharge cycle. So instead of powering the LED's directly off the battery I was thinking of using a 7805 voltage regulator. Current drain should be in the neighborhood of 200-250ma for the LED's. The question came up when I went to calculate the current limiting resistors for the LED's and I realized I didn't really know exactly what voltage to use.
I use 4 or 5 LED's in series off the flight pack. No problems. If you use a regulator, you can put it in current regulation mode.

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Old Apr 26, 2001, 05:07 PM
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Hi,

another possibility are the small, light chemical lights that are used for fishing and are available at fishing equipment shops. You crack a glass tube inside them and they will glow for some hours. You can "switch them off" for a while by deep refrigerating them and use them later again after warming up.

At least they do not use current at all .

Regards, Jochen
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Old Apr 26, 2001, 10:23 PM
RIP Ric
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Marietta, GA
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I said I'd do this a while back, and forgot. Here's a page I just put together for calculating LED load resistors.. http://www.mindspring.com/~andyw/led.html
..a
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Old Apr 26, 2001, 11:47 PM
pfg
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all the above is correct but one thing - you must use a resistor, you can not try and put at many led's across the supply to equal the supply voltage and then hope it works. led's have no resistance so long as there is enough voltage to break across the diode in them they will consume as much current as the supply can generate until they blow. the voltage across them is more like 1.2 to 1.4 volts but does depend on the device. if you can get "ultra-bright" led's - they produce alot more light for the same current. the other thing to do is make a flasher. our eyes respond to peak levels so we see a led pulsed at 1A and only 2% duty cycle much better than one run at 20ma constant. its alot more complex to do but not that hard. if your interested e-mail me and i'll explain more.
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Old Apr 27, 2001, 08:34 AM
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My first electric trainer is two years old and still flies all right. I equipped it with LEDs for night flying.

I simply hooked "triplets" of leds in series - three green for the right wing, three red for the left wing, three yellow for the front light, plus two yellow flashing ones (one on the fin the other on the belly). Never had a problem of any kind. The whole circuit is plugged into the receiver since it draws something like 60mA - more or less one more servo. If the BEC of your ESC can stand it it's all right. Power leads were coaxial cables to reduce "antenna" effect of the wires through the wings.
Very cool! Everybody at the field said it was much more beautiful by night than by day...
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Old Apr 27, 2001, 04:26 PM
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Not to throw a damper on anybody's enthusiasm for this idea but if you use a voltage regulator they can by them selves consume up to 60 mah. Resistors might be better or.. Use an old servo driver circuit and wire the LED's across the output. If you use two colors, wire them in polarity opposition to each other. High brightness White LED's and optical cables ought to be really interesting.

Looee

[This message has been edited by Looooeeee! (edited 04-27-2001).]
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Old Apr 27, 2001, 05:12 PM
aka: A.Roger Wilfong
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One last thing. If you're using multiple LEDs, use a seperate resistor for each LED. Unless LEDs have improved drastically in the last 20 years, if you wire all the LEDs in parallel and then put a simgle resistor in series with them to handle the entire current, one LED will light really bright for a little while and the others will light dimly - when that one burns out, another will take its place until they're all gone (been there, done that, sh*t canned the evidence )

- Roger
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Old Apr 27, 2001, 09:41 PM
RIP Ric
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Marietta, GA
Joined Jun 1999
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I have seen that in past years, but have no problem with that lately. Never tested it a great deal, however. I will modify my page to reflect that.
..a
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