[sdiy] LED driver circuit

[synth]

Do you have a website with all your designs on them?


On Nov 24, 2012, at 2:22 PM, David G Dixon wrote:

> Hi Team!
>
> I have updated my DIY 2164 4P LPF/LFO circuit with transistor-based  
> LED
> drivers for each of the eight quadrature outputs.  The problem I'm  
> having is
> that four of the LEDs are on at the same time, and I only want two  
> on at the
> same time.  I have changed the current limiting resistor (to 49.9k)  
> and now
> the brightness is where I want it.  However, I don't think I have the
> transistor base resistor sized correctly.  I started with 100k and  
> recently
> increased it to 200k with little or no change in behaviour.
>
> The positive filter outputs have NPN transistors (2N3904) with LEDs  
> driven
> from the +15V rail, and the negative filter outputs have PNP  
> transistors
> (2N3906) with LEDs driven from the -15V rail.  The transistor bases  
> are
> driven from the actual filter output opamps.  The idea is that when  
> the
> filter is self-oscillating and generating octature sine waves, one  
> will get
> an LED light-show.  This works well, but too many LEDs come on
> simultaneously.
>
> The circuit is as follows (for the positive ones):
>
> Filter Output ---> 200k ---> NPN Base
>
> +15V ---> 49.9k ---> LED ---> NPN Collector
>
> NPN Emitter ---> Ground
>
> Do I need another resistor from Base to Ground?
>
> Any suggestions would be most appreciated.  I don't want to do too  
> much more
> desoldering at this point.
>
> Thanks,
> Dave Dixon
>
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