Thread: Single LED Hookup Need Electrical Hookup Ideas?

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So I need some LED spotlighting. I wanna try something but I want only like a single LED.

I want to buy two of these guys:
http://www.allelectronics.com/make-a...BRIGHT//1.html

however instead of hooking em up to a battery case, which I did last year..I want to hook them up electrically to an electrical chord.

I don't know anything about electronics like voltage, amps, resisters, etc. I'm guessing I could use like 18/2 wire?
http://www.allelectronics.com/make-a...ER-CORD/1.html

Any other advice on how to complete this task, or what items I'd need to buy? Thanks!

Here is one of the best sources I've found for practical LED info:

http://wolfstone.halloweenhost.com/L...eringLEDs.html

You'll find some basic calculators there for what you need w/ regard to leds, capacitors, etc. Just remember that whatever you calculate your necessary voltage to be, i.e. 3v, you'll be supplying that with a wall wart (power adapter) instead of batteries.

Thanks,

I ended up getting a :

27K 1/8W RESISTOR
ULTRABRIGHT GREEN T 1 3/4 LED
YELLOW T 1 3/4 LED, ULTRABRIGHT
9 VDC 300 MA WALL TRANSFORMER 2.1mm termination endplug
CABLE W/ 2.1MM COAX POWER JACK, 6'

Hopefully this works!

Glad to see you picked up a wall wart transformer instead of going with AC from the wall. They don't call those 'suicide cords' for nuthin' The 27k resistor is going to be too high a value, though. Assuming your LEDs have 3.5V Vf for green and 2.0V for yellow....

Green will need (9V - 3.5V) / 0.02A = 275 Ohms
- and -
Yellow will need (9V - 2.0V) / 0.02A = 350 Ohms

Since resistors are only readily available in certain values, the values you'll want to look for are 300 Ohms (for green) and 360 Ohms (for yellow).

- Hook

I don't see them in those ?
http://www.allelectronics.com/make-a...sistors/1.html

I did the calculator and it said 1/8 resister should be fine? Isnt that like the smallest in size you can buy anyways? Sorry for my clueless questions, I don't know anything about this stuff whatsoever.

I bought some 1/4 resistors too just in case, but again, it's sad because I wouldnt know when to use them anyways. What's the 27K part mean anyways?

btw, these 3 lights were meant to be hooked up in a series. 2 greens and one yellow. So I was hoping they would require the same voltage, resistors, etc....please don't tell me I should seperate them. I didnt wanna buy two wall warts.

The 1/8 and 1/4 is the amount of power the resistors will dissipate and the 27k is the internal resistance or what is limiting the current flow. in your case you will need 1/4 watt resistors at least for the yellow.

huchon's right, the 1/4 or 1/2 is the power the resistor can handle. Here's the process you wanna use for this kind of thing.

1. identify what voltage you'll be using. For you, you've got 9V.
2. find the Vf for each LED you'll be using. This is also called the forward voltage.
   Here are some common ones...
   Red = 2.2V
   Green = 3.5V
   Yellow = 2.0V
   Blue = 3.5V
   White = 3.5V
3. Figure out how much current you want through your LEDs. For most typical indicator LEDs, they'll handle 20mA.
4. Enter your info into this LED calculator, and it will tell you what values to use

The calculator's got some bugs on the results page, as far as what it tells you you'll have for current and power dissipation. But it looks like the errors are on the 'safe' side. (looks like it tells you you'll have the current you asked for, rather than the current the 5% tolerance resistor value it chose for you will actually give you)

Your series LED idea (2 greens and a yellow) won't work with the supply you've got. Because your LEDs Vfs add up to 9V, your 9V supply won't have any voltage 'headroom' to be able to push current through the string. I'd recommend trying the calculator out, telling it you'd like to use either 2 greens in series, or all three separate.

- Hook

BUt like you said they all 3 add up to 9v, so why can't I use my 9v wall wart without a resistor, and have each LED just get 3v...? I'm sure a little above 2v for the yellow won't hurt it?

The resistor isn't to drop voltage, it's to limit current. There are some special circumstances where you can use LEDs without resistors, but those are because something else is limiting the current (e.g.: battery resistance in coin cells).

Think of it this way. Ohms law says that current is equal to the voltge divided by the resistance.
I= V/R

If you have zero resistance, it doesn't matter how much headroom voltage you have, you have un-limited current.
I = 0.0001/0 = infinity

That means that if the Vf of the LEDs adds to less than 9V, and you have no resistor, then you'll have too much current. On the other hand, if the Vf of your LEDs adds to equal or more than 9V, they won't light at all.

There are gray areas in engineering, though. Because of the nonlinear nature of LEDs they *might* light in a controlled manner given your setup. The problem is you can't depend on that kind of stuff. Even something as simple as the weather being 20deg colder might cause it to not work. That's hassle you just don't want to deal with on Halloween night

- Hook

Ok, well thanks for the explaination. I can't change my order, but I ordered 1/4 resistors too. There are tons of 1/4 resistors: http://www.allelectronics.com/make-a...sistors/1.html

I would have no idea what to select. Unless it was the 300OHM one like it was mentioned earlier. I don't expect me to understand all this over night, but I just want it to work. If I burn em up, whatever it was only a few bucks. 1/4 is beefier than 1/8 resistor I'm guessing, so maybe I'll just try that first huh?