OK, I'm stuck. I feel a little dumb, too. My students have the LED array halfway assembled, and as a precaution, we went back trough to ensure each LED was lighting. We grounded the common wire and touched the hot lead to the exposed leg of each LED. The LEDs in the left column lit, the ones in the right did not as they should. (Note only the colums are connected. No row wires have been connected yet. In this arrangement the left column has anode up and cathode connected across to its neighbor, right column has cathode up and anode connected to it's left neighbor. The joints were then all connected with a common column wire.) Ok so far, everything is how it should be.

The the weirdness... one of my students accidentally connected an exposed leg of the left column (anode) to hot and exposed leg from the right column (cathode)to ground. The other leg of each were joined. Since current can only flow in one direction through a diode, nothing should have happened. it was flowing correctly through one led, but the other would prevent it to continue on in the circuit.

So why did both LED's light?!

What it your power supply? If a signal generator, it might be supplying some small AC current, so that led's are actually on on at a time, they just blink fast. Could you sketch a circuit of what's connected, it'd a bit hard to follow your description. Stan.

I hope you are using limiting resistors while testing.

Ralph

I'm using a 9V battery with a 3-400 ohm resistor, not sure the exact number off hand, but I've built a tester to test 1 LED at a time. Here are pics that show what is happening. if they don't insert, the links are at the bottom.

[IMG]http://i1181.photobucket.com/albums/x423/fharpster/ledhuh5.jpg[/IMG] [IMG]http://i1181.photobucket.com/albums/x423/fharpster/ledhuh3.jpg[/IMG] [IMG]http://i1181.photobucket.com/albums/x423/fharpster/ledhuh2.jpg[/IMG] [IMG]http://i1181.photobucket.com/albums/x423/fharpster/ledhuh1.jpg[/IMG] [IMG]http://i1181.photobucket.com/albums/x423/fharpster/ledhuh4.jpg[/IMG]

http://s1181.photobucket.com/albums/x423/fharpster/

OK, so inserting pics didn't work, and the photobucket album thing kinda sucks, but you can see what I uploaded. The pictures aren't good, but I put notes on them and I made a little paint picture which describes it. Any clue what's going on?

If you look at http://i1181.photobucket.com/albums/x423/fharpster/ledhuh5.jpg you will see they both light because you have them in series. Current is flowing the correct direction through both LEDs. If you connect the + to the common wire and the - to both legs you will see only one LED lights. Then connect the - to the common wire and the + to both legs and you will see the other LED lights. That is how the microcontroller controls which LED lights up.

OK, so I just figured it out, and I am an idiot. Formula 89 you are exactly right. With all of the wires in the way, it was confusing me, until I looked at the drawing I did again and all of a sudden it made sense. Hahaha, i guess my students know I'm not genius at this electronics stuff now.