LED HELP!!! :)

lynx0v

Newly Enlightened
Joined
Jul 1, 2004
Messages
3
Hello, this is my first post.

I am getting into using LEDs for some automotive work and just would like some help in regards to it.

Basically what I am asking is what are the basic tools I need ie: soilder iron, leds, resistors, panels, etc..
and any good way to wire it all up.

I have some more questions, but they seem to skip my mind! /ubbthreads/images/graemlins/ohgeez.gif

[ QUOTE ]
Well I have a few idea's up my sleve but am trying to figure out some things about LEDs.

I've found howstuffworks.com and their info on LED's but I need some additional information to that.

First off what do I need to make one work?
Battery, resistor, panel, LED, etc...?
Plus the different sizes, So far I have found 3mm and 5mm and some 15mm and 20mm ones on the internet I can order.

I guess what I am really asking is how do I get multiple ones to light up, and change their amount of brightness, ie: a low just on color and when something touches the sensor it brightens, etc...

Thanks in Advance!!!


[/ QUOTE ]

That's what I posted in my other forum I visit(Focaljet.com Car related /ubbthreads/images/graemlins/smile.gif)

Thanks in Advanced!!
 

INRETECH

Flashlight Enthusiast
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Aug 22, 2002
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LEDs are very easy to use, but with some considerations

Mounting:

The typical 5mm LED is a piece of plastic with 2 small wires coming out of them, if you bend them too many times - you will break them off, so its best to solder the LED onto a board or mount it somehow so that this doesn't happen

Several companies make 0.100 solder-board, that is hundreds of small plated-thru holes spaced 1/10 inch apart, easy to work with - you can find it at Radio-Shack and other places

Voltage/Current:

A LED is a CURRENT device, and can easily be damaged by too much current, just don't start connecing your LEDs across the battery without having some means to limit the current, or you will quickly damage the LEDs

Not all LEDs have the same operating voltage, as the wavelength moves from the RED->GREEN->BLUE (White) the operating voltage will go up as well; for example a typical RED LED will run on approx 2.2v, but a White LED will want approx 3.2v; so you need to plan for this

Since you are running an automotive application, its a good bet your supply is going to be 12v; this will be too much voltage for a single LED - so you have to decide to "use up" the extra voltage in the dropping resistor, or just use more LEDs

The typical 5mm LED runs at 20ma (0.020A)

If you were going to run 3xWHITE LEDs - you would run them in series:

+ --(RESISTOR)------->|----->|------>|------- -

3 x 3.2v LEDs comes to 9.6v, which is under 12v
Which means - you still need to "get rid of" 2.4v

You use a dropping resistor to accomplish this, and good old Ohms Law:

I=Current
E=Voltage
R=Resistance

I=E/R

Or transposed:

R=E/I
R=2.4/.020
R=120

So, you would need a 120 Ohm resistor
Or anything in the 100-150 ohm range would be fine

You could run 4x LEDs - but that would make 12.8v no room for margin

LEDs are CURRENT devices, once you reach their operating voltage - their brightness is depending on CURRENT

Its always a good idea to have a current meter handy when working with LEDs to make sure you don't exceed their current, you can purchase a very nice cheapo DMM from arbor Freight for under $5

Tools:

Small soldering iron (NOT GUN)
High quality Solder

Clean the soldering iron frequently
 

Josey

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Jan 5, 2004
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Awesome post, Mike.

To continue your analysis, how would you deal with variable voltage, such as a 12V automotive or solar systems, which typically vary from 12.3 to 14.1 VDC?

And could you elaborate on why 12.8 V (4 white LEDs in series) leaves no room for margin?

Do LEDs regulate their own current if given the proper voltage?

Thanks, Josey
 

wholeflaffer

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Jun 20, 2004
Messages
237
Location
Pennsylvania, USA
[ QUOTE ]
Josey said:
Awesome post, Mike...could you elaborate on why 12.8 V (4 white LEDs in series) leaves no room for margin?


[/ QUOTE ]
The way I see it, if white LEDs want 3.2V each, four of them will want 12.8V. In an automotive environment, 12.8V is fine as long as the alternator is charging the battery, but if you turn off the engine, the battery may soon fall below that threshold. If you want four white LEDs to stay 'awake' consistently under these circumstances, you'll need some sort of regulation that can boost the voltage when it dips below 12.8V.
 

INRETECH

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HILLSBORO, OR
The typical automotive system runs from 12v to 14v, and sometimes even higher - depending on the charging system

To make the system work all the way down to 12v, its best to have a little margin - that is design the system to run 11.5-15v, thats why I suggested using 3x LEDs instead of 4x LEDs

The other question was

"Do LEDs regulate their own current if given the proper voltage?"

LEDs change their operating voltage as the current changes, again - do not think of them as voltage devices, think of them as current devices

The best system for running LEDs at the full brightness all the time, is to use a constant-current regulator such as the LM317


+ ---(IN OUT)---(RESISTOR)-X-->|-->|----->|------ GROUND

Connect point "X" to ADJ

This will GUAR that no matter what voltage you put on the LEDs (10-35v), they will always be full brigtness

R=1.25/.020
 

lynx0v

Newly Enlightened
Joined
Jul 1, 2004
Messages
3
Wow, thanks a lot, great information!!!
If I have any more questions can I PM you INRETECH? Thanks again
 
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