[sdiy] LED won't light up :(

[Aaron Lanterman]

Meine blinkenlichten ist nicht blinken.

So, I've got my Easel Lowpass Gate board working as far as the audio  
goes (took some video today, will get on youtube at some point). It  
works without any hacks to the board itself; I just changed one  
resistor value to tweak things a bit.

But, my LED doesn't light up. Actually, none of my LEDs light up. I  
was anxious to try out the LED part, so I picked up yellow, blue, and  
green LEDs from Radio Shack.

For the original schematic, see here:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_10_200.jpg

(The two LPGs on the Easel are identical).

For this to make sense, you need to see where the "LED" pin is hooked  
to. If you see here:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_14A_200.jpg

The diode points into the circuit, and on the other end is hooked to a  
+5 V supply.

Here are my Eagle schematic and PCB layout for that part of the circuit:

http://users.ece.gatech.edu/~lanterma/lpg_led_pcb.png
http://users.ece.gatech.edu/~lanterma/lpg_led_schem.png

Since things weren't lighting up, I did a quick check of running the  
diode through a 1.3K resistor with a 5V supply.

For each LED color, I took a voltage measurement at the junction of  
the LED with the 1.3 V resistor when doing my quick-check circuit (to  
make sure the LEDs could light up, which they did in my quick-check  
circuit. I also took a measurement of the voltage at the "LED" pin of  
my LPG board.

In each case, I subtract my measurements from 5 to get the drop across  
the LED.

Here are my measurements:

green LED (specs: 2.1 v, 25 mA)
voltage at LED terminal of LPG board: 3.6 (5 - 3.6 = 2.4 V) (no light)
voltage at 1.3K resistor of quick check: 3.1 v (5 - 3.1 = 2.9 V)  
(lights up)

yellow LED (specs: 3 v, 20 mA)
voltage at LED terminal of LPG board: 3.7 v (5-3.7 = 2.3 V) (no light)
voltage at 1.3K resistor of quick check: 3.1 v (5-3.1 = 2.9 V) (lights  
up)

blue LED (specs: 3.7 v, 20 mA)
voltage at LED terminal of LPG board: 2.9 v (5-2.9 = 2.1 V) (no light)
voltage at 1.3K resistor of quick check: 2.2 v (5-2.2 = 2.8 V) (lights  
up)

It appears that there is current flowing the diode in my LPG circuit,  
but not enough to light them up.

Anyone see anything obvious that I am doing wrong?

My next step will be to breadboard just that part of the circuit and  
see if I can get the LED to light - if I can there's a problem on the  
board (but I'm can't seem to find a problem with the board).

- Aaron