Posting this for others. After staring more times than I care to admit at the metal inside the LEDs I could not for the life of me figure out which way was round and which was flat. After a little googling I found a good explanation and picture. A portion of the rim of the lense is flat.
I can never tell either.
I've memorised that the small element in the LED is positive, and trace back the connections to find out which one is either connected to +9V, or NOT grounded. Tedious, but works for me.
Madbean's diagrams all reference the flat/round sides of the LED - now that I know to look at the lense vs. the metal all is good.
I came across this great mnemonic over at the Guitar PCB forums:
FAT, FLAT, CAT.
To work out which is the Cathode negative leg (CAT) there are a couple of things to look for.
First you should see a flattened bottom edge to the lens case. This indicates the cathode side.
Alternatively, hold up the LED and look at the configuration of the metal frame inside. The fatter / bigger blade inside also indicates the cathode.
Remember - it is not enough to go on lead leg lengths - check the Fat, Flat, Cat way!
very cool, I am gonna try to remember it as grouchy cat
FAT, FLAT, NEGATIVE CAT!
:)
C
(also, I always test components with DMM first and LEDs make very faint light when you have it connected correctly thus indicating which leg is + and - with no real risk of harming the LED)
While I employ the FAT FLAT CAT mnemonic as well, I'd recommend building a little LED rig on a piece of vero (or perf) with a 9V battery clip, a CLR (2.2 - 4.7k), and a socket. It's a quick way to test particularly milky LEDS (yellows and reds), and it gives you an immediate visual indicator of the overall brightness of the LEDs (not all ultrabrights are created equally), thereby avoiding any disappointment when you wire up you indicators. I did this because I found the ultrabrights from Tayda to not be nearly as bright as the LEDs from satistronics, and as such wanted to know what the LED would look like once wired up (I like retina burning bright LEDs).