[sdiy] LED won't light up :(

[Stewart Pye]

Hi Aaron,

 From your measurements it appears that in the LPG circuit you are 
getting enough voltage across the LED to light it up. (And 5 - 3.6 is 
1.4 - must be late over there...) You could check the base emitter 
voltage of the transistor to check that it's turning on.

If the transistor is simply not turning on enough, you could try 
reducing the emitter resistor (but putting a resistor between the 
collector and LED to limit current) and see if that helps...

Or, could you just drive the gate input harder?

BTW: I'm not familiar with this circuit but hopefully that was of some help.

Regards,
Stewart.






Aaron Lanterman wrote:
> 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_______________________________________________
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