Identifying LED leads

[Astronaurt]

Hey, this is perhaps a quick dumb question, but I was wondering how you figure out which lead on an LED is the Cathode and which is the anode. on normal signal Diodes it's easy cuz of the little black bar, but I'm a little stumped on this one. I wouldn't be something I'd have to ask about except that I'm going to use 4 LEDs as Opto-Isolators with LDRs in an Easyvibe circuit, so I won't really be able to tell if they're lighting or not when I've already sealed them in heatshrink tubing! Thanks in advance for helping with such a N00b question!

[deadastronaut]

long leg +...
also
round edge...+



short leg  -
also
flat edge -

from one astronaut to another...

[petemoore]

  Flat edge can be seen in good light from the top [sometimes, my eyes arent that good to see the little ones] and if angled just right, from the side a reflection stands out.

[blooze_man]

Also, if you look real close at the terminals in the LED you should see that one is shaped like a flag. The flag points to positive lead.

[Astronaurt]

haha, awesome thanks guys! I just got a package from smallbear yesterday and some Heatshrink from the Hardware store today, so TIME TO BREADBOARD!

@dead astronaurt I was also planning to build your Tiny Trem circuit and a couple other projects with the same Opto-isolator set-up in the near future actually! One of my buddies is calling it "VACTROL MADNESS 2011"

[deadastronaut]

haha, awesome thanks guys! I just got a package from smallbear yesterday and some Heatshrink from the Hardware store today, so TIME TO BREADBOARD!

@dead astronaurt I was also planning to build your Tiny Trem circuit and a couple other projects with the same Opto-isolator set-up in the near future actually! One of my buddies is calling it "VACTROL MADNESS 2011"

cool, go for it..

[R.G.]

1K resistor. 9V battery.

The flat and long lead are good, but there are LEDs with the reverse polarity compared to this. I know this because I would up with 100 of them buying from a surplus seller. 

[Taylor]

1K resistor. 9V battery.

The flat and long lead are good, but there are LEDs with the reverse polarity compared to this. I know this because I would up with 100 of them buying from a surplus seller.  

Well... what percentage of parts would you say defy the normal convention? 0.001% maybe?

Since every DIYer must have a multimeter... I say, if you really want to be thorough and are too skeptical to trust short/long/flat/round, the ol' diode setting of yer multi should do the job.

[ayayay!]

but there are LEDs with the reverse polarity compared to this

True, true.  As I recall they are in the DL4. 

[dpresley58]

Isn't it possible to check LED's using the "diode" setting on most multimeters? There would probably be a faint glow when doing so, but it seems like this would work.

[CynicalMan]

I use a CR2032 battery to test LEDs. Just slip it between the leads and the LED lights up.