[sdiy] Leaving off a pot?

[Dave Manley]

My point was different *color* LEDs can have different voltage drops.  
Some people like white or blue LEDs these days and these have a Vf of 
about 3.5V, while most other colors are around 2V (as low as 1.8 and up 
to 2.4) .  The other important point is the recommended current for a 
reasonably bright LED.  This is of course subjective, but could range 
upward to the typical max spec of 20 (or 40 for white or blue) mA.   I 
suggested Sam assume 15 mA, calculate the resistor, try it and see if it 
meets his needs.  If not adjust the resistor up or down with the 
knowledge that he can't hurt the TL084 because it has short cicruit 
protection. I also mentioned that the TL084 has a limited current drive 
(spec'd at 20mA) and too heavy a load will affect the ADSR output 
voltage, potentially requiring a separate buffer for the LED (as others 
have suggested).

-Dave
>
>>> LEDs have a voltage drop that varies with the color.  So the 
>>> resistor value depends in part on what color you choose.
>>
>> Wow. I had no idea. I actually thought of mentioning in my original 
>> post that I was thinking of green, but I thought it didn't matter. 
>> So, I'd need an LED whose forward voltage was rated at at least 10v, 
>> so this should work, right?:
>
> No.  A normal green LED will have a voltage drop of about 2V  That 
> means that 8V out of 10V will be dropped across the resistor.  This 
> allows you to work out the current through the LED using simple Ohms law.
>
> I=V/R
> e.g.
> I=8V/1K
> I=8mA
>
> Probably a good starting point!  My advice: just try a 1K resistor and 
> see how you like the LED brightness.  Different LEDs will have 
> different brightnesses.  The voltage drop shouldn't vary very much for 
> a green LED - all should be about 2V.
>
> Seb
>
>
>
>
>