Guys, I am so sorry if this is spam, I pmed but no one an...

demonofaj · Post by **demonofaj** » Sat Oct 01, 2005 7:00 pm

...swered and I gotta finish this by tommorow for my Science fair.

So I got a LED which needs3.7 Volts, so I power with 3 batteries. Now I have a SPST switch with TWO prongs. I put all the wires, pos and neg, into one prong. Then with wire, solder some to one end of the led likewise with to the other, and solder both wires to the other prong. WILL IT WORK. Please, thanks\

Which end of led's are pos and neg.

short=?
long=?

G-force · Post by **G-force** » Sat Oct 01, 2005 7:07 pm

You only gave me a minute to reply! Geez... (And I DID send you a reply before I saw this.) Be more patient!

And you are going to kill your LED no matter what. I just know it

demonofaj · Post by **demonofaj** » Sat Oct 01, 2005 7:10 pm

Yeah, I'm sorry, again, but I was in panic, the last I need to know is the short/long pos or neg.

daguuy · Post by **daguuy** » Sat Oct 01, 2005 7:14 pm

i sent you a PM. i just recently saw it 'cause i was sick earlier

demonofaj · Post by **demonofaj** » Sat Oct 01, 2005 7:14 pm

Please remove this, THANKS SOOO MUCH.

codeman · Post by **codeman** » Sat Oct 01, 2005 7:16 pm

short end is negative. If your LED spec says that it drops 3.7v, you might need more than just 3 AA's. Also make sure you put a resistor in series or the diode will explode like G-force said. If you use 3 AA's you'll have approximately (4.5v - 3.7v) as voltage across your resistor. If you want 15mA through your LED you need to solve 15mA = (4.5-3.7)/R, and use R for your resistor, in this case 53.33ohms...or 51ohms is the next closest standard value.

If you dont use a resistor it WILL blow the LED, unless you're really lucky.

Hope that helps...i hated science projects

daguuy · Post by **daguuy** » Sat Oct 01, 2005 7:17 pm

you need a resistor on LEDs, no matter what voltage you put in it?

codeman · Post by **codeman** » Sat Oct 01, 2005 7:19 pm

yeah...an LED is just a diode (from the name obviously). The V-I curve is exponential, so if you put in more than its forward breakdown voltage, it'll try to conduct infintely (well, close to it) unless you limit it with a resistor

G-force · Post by **G-force** » Sat Oct 01, 2005 7:22 pm

Read this
http://www.metku.net/index.html?sect=vi ... /index_eng
And check out this
http://www.metku.net/index.html?sect=vi ... /index_eng

codeman · Post by **codeman** » Sat Oct 01, 2005 7:25 pm

yeah...thanks, that'll help. Kirchoff's laws are good to know if you're working with electronics. The only problem is, when you get to really high frequencies...they don't apply anymore! try figuring that out

Post by **bicostp** » Sat Oct 01, 2005 7:31 pm

Yeah at school they kinda figured that out the hard way .

We're making one of those maps with lights on it for the Civil War. (Just a bunch ofmomentary switches and LEDs, one of my classmates thought you needed special IC controllers to use LEDs

)

Anyways, I was making a 2x D battery holder out of 2 1x D battery holders. (the RS by the school was out, and they were cheap, and it was the school's money...

) (I just wired the positive of one to the negative of the other and melted them together.) And from across the room I hear "F**K THAT'S HOT!". Somebody taped 2 AAs together to make a battery pack and put a 1.7v LED onto it, thus frying it and their fingers.

(On a side note, I was using an el ceapo soldering iorn [the RS one that has sort of a pistol handle on it]. I put down newspaper to stop solder drips from damaging the table. But the teachers get all paranoid and make me work over a chunk of wood. Now this iorn won't burn paper, and I'm actually one of the more stable people in my class. So by the time they stopped panicking and running around finding spray bottles of water and fire extinguishers, I was done making the battery pack, and I had the iorn unplugged and sitting near the open window to cool off.

And of course I asked someone to get me a damp sponge to clean the iorn on, and they bring back paper towels with isophol alcohol on them...

DEE-DEE-DEE!)

G-force · Post by **G-force** » Sat Oct 01, 2005 7:35 pm

HA ha, nice

. I find it amazing the way people think LED's are like lamps and you can treat them the same way. Hook anything up to it, on any end, and it will magically work! (of course lamps can burn out too, but they arent quite as picky as LED's.)

soundwave · Post by **soundwave** » Sat Oct 01, 2005 7:49 pm

I havent read any of the other posts.

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(L to R) battery, switch, LED.