Simple LED question
 

Im working on a small circuit that requires a few infra-red LED's. Short lead is negative and long lead is positive. However when I touched one to a 9V battery, I can only get it to light when the negtive lead is touching the positive terminal. Am I an idiot or is this weird. BTW I knew it was lighting by looking at it through a camera lens.

Here you go..

http://en.wikipedia.org/wiki/LED

You'll want a current limiting resistor or you'll burn it up on the battery, BTW.

Some brands have the lead length indications reversed.

Thanks for the replies but Im still confused. The picture above confirms what I already knew: short lead with flat LED base is neg and long lead is positive. Resitors are in place in the circuit but just for kicks I took one and touched it to a battery + to + and - to - ...nothing. Switched it and it worked! Odd...

How do the ones work that change colors when polarity is reversed?

There are 2 wired in parallel except one is backward with regard to the other

Are you sure your camera will respond to the 950 nm most IR LEDs produce?

Typically Vr max is 5 volts for an LED. Do you think it emits anything if it is in avalanche breakdown?

Are you making a remote for your TV? Most of those have a 38 kHz carrier.

I used the phone on my camera. Its a very good camera for a phone...3.2MP. I could see the LED light very visibly through it. For the circuit I have a 5V regulator but I touched it to a 9V just to see what it looked like. Would that make a difference? I am making and infrared transmitter and receiver for an ultralight RC plane. The receiver weighs under 1gram. The entire plane should weigh under 12-15 grams. Made from balsa and mylar with homemade micro magnetic actuators for control and a propeller motor similar to the ones used in cell phones for vibration.

Can't help with your LED problem, but I sure would love to see some photos of your project. Sounds very cool. My Brother and I are planning to build a couple of fighting wings from scratch, maybe going electric.

Here are some pics of what I am talking about.

IR LED (long bent lead positive)
http://forums.pelicanparts.com/uploa...1234814638.jpg

9V battery (small terminal positive)
http://forums.pelicanparts.com/uploa...1234814676.jpg

LED connected (pos to pos, neg to neg)
http://forums.pelicanparts.com/uploa...1234814723.jpg

LED connected (pos to neg, neg to pos)
http://forums.pelicanparts.com/uploa...1234814755.jpg




And for those interested... here's a pic of the receiver that would mount to the plane. Motor, actuator, and battery leads get soldered to the various edges of copper pcb. Not very pretty but solding this small is very hard and I have no experience. Had to make my own soldering iron tip out of a copper wire. This thing is smaller than a penny.
http://forums.pelicanparts.com/uploa...1234814916.jpg

that diode isn't liking what you are doing to it.

Ever thought of trying these guys? $51 and you could be playing with reflow soldering in your toaster oven.
http://www.expresspcb.com/

Sorry but I'd like to keep this thread alive and try to find an answer. I just cant understand why the LED works ONLY when connected in reverse. My project is stalled until I know why this is and how to hook up the array of IR LEDS...:confused: Thanks

I have a couple of IR LEDs lying around. I'll see if I can find them and see how they work.

Thanks Dave, appreciate it.

I hooked mine up and....

long lead = pos.

http://forums.pelicanparts.com/uploa...1234934198.jpg

...so it lit up when long lead was touched to pos terminal? Spec sheet for mine say long=positive also. Maybe I should test a couple more.

Correct. I tested it a few times. Can you see through yours? You can see how the negative lead connects to the reflector dish in the pic I included.

Are they made in China? ;)