Let's Make Robots!

Led (anode/cathode recognition)

here's the led i bought..  the lenght of the two 'bars' (don't know their name in english) and the fact that the capsule is flat on  its right side make me believe the one on the left is the anode, but then i checked it inside...i haven't seen leds like this one but i haven't seen many of them (usually the widest part is linked to the cathode). so which one is the anode? (my guess is the left one)


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One thing I've noticed with certain LEDs is that China is starting to make them in "special" formats. I've had LEDs that look completely normal only light when you hook them up in reverse. Then I've also seen LEDs like the ones posted here that aren't quite sure what they are. I find that the only way to truly find out which way they should be hooked up is to hook 'em up. Must be all the pollution over there mutating these things...

I much prefer the curvey or flat side of the body (of the LED).

curvey=anode ...... flat side (or scoop in bottom of casing)= cathode.

Tip :- If you roll the LED on the table the flat part sits on the ground (earth)......so connect the pin next to the flat part to ground/earth/ 0V

After working with several suppliers, the flat side to ground, curvy side to positive rule seems to be the most reliable than looking at the length of the legs or the shape of the odes.  Thanks for the tip. 

Here are two LEDs. One from here in Hawaii on the right and the other from Taiwan on the left.  You will notice that on both, the Cathode tail is actually longer than the anode tail contrary to what its supposed to be.  You should also note that in order to work the one from Taiwan on the left has to have what appears to be the cathode with the longer tail on the positive side of the battery and what appears to be an anode with the shorter tail on the negative side.  On the other hand, the one I picked up in Honolulu on the right side has to have the anode with the shorter tail on the positive side of the battery and the cathode with the longer tail on the negative side.

That is unusual.

The anode is always the one with the smaller piece of material inside the lens. The anode is usuall also the longer lead, but the structure inside the lens has always been one sure way to tell for me. Can you confirm that what you are calling the anode has a smaller mass inside the lens?


If you have a measure-thingey, you can measure on it. One way current flows through, and that is the way to hook it up.

Sometimes I take off little IR LED's from old prints, and they are inside black plastic etc, so there is no way of telling what is what. If they are small and used for optical tracking, they do burn quite easlily, and are impossible to replace.

So.. Set up measure-thingey to measure resistance. Red on one leg, black on the other. If no result, make it the other way around. Then red is V and black is G.

Also see this: http://letsmakerobots.com/node/78 to know if it is working.

Sometimes the smaller ones blow if you get them too much voltage - no matter which way they are turned. 

i'm not using ir leds :)

actually i just noticed that my multimeter...ops sorry, my bad.... that my measure-thingey has a setting which says "LED" and has 2 holes when i can insert my led in and i guess i don't need to use resistors in oreder not to destroy it (by inserting it in the measure-thingey i mean)

actually you are right...nothing's gonna happen if i hook it up in the wrong way. it just won't let current pass! I'll try it now, i have some spare time.

Does it really mather to know which pin is the anode/cathode. I thought you can just hook it up, and either it works or it doesn't. But you can't break it by wiring it wrong. Am I right?

So my suggestion would be, just hook it up, and you'll see which pin is the anode/cathode. But I might be wrong about the 'won't break' part... so you better wait for someone to confirm this ;)

Well, you won't break it, but if you're debugging a circuit, you can remove 50% of the "unknowns" by knowing for a fact that your LED is the right way round!

NB I would call them "leads" or "tails."