Problem with Led circuit, What's wrong?

I have 12 blue leds wired in parallel in 2 groups of 6, one of the groups work just fine while the 2nd won't light up at all.

Using a 5 volt power source when I tested the 2nd group with a volt meter only .5 volts was going through the circuit. I tested each led individually and none of them will light with the same result of .5 volts.

I'm out of ideas and have no clue what's wrong with it. Seeing as how both groups are wired up the exact same and one works perfectly while the other a complete mystery.

Edit : My project is very similar to, all the wiring is basically the same set up.

https://www.instructables.com/id/Bright-USB-Powered-LED-Light/?ALLSTEPS

AndyGadget8 years ago
I'm astonished you're geting anything at all!
A typical blue LED has a forward voltage drop (Vf) of 3 to 3.5V, so you should need at least 18V for your chain of 6 to have a chance of controlling them properly. You are running them in a marginal (to say the least) state and the differences between the individual LEDs will become apparent.
Ideally you'd be running these separately with a resistor for each which would allow you to accurately set the current, but as things are *almost* working, re-hash them as three banks of four (or 4 banks of three if that doesn't work).
slayer04 (author)  AndyGadget8 years ago
If that's true than why does one chain light up as bright as can be while the other with the exact same set up are not even flicker?
If you were running these at a sensible voltage I'd say either faulty LED or one in the wrong way around. Are these actually running at a decent brightness???
As I said before, I wouldn't expect either chain to work.
But, they obviously do, so if it isn't a dodgy LED it's probably the sum of the LED Vf in that row is greater than in the other row. Individual differences in the way they come off the production line.
lemonie8 years ago
One of your LEDs is wired the wrong way around? I'm surprised that 6 will run off 5V, as that's less than 1V each. L