[sdiy] LED driver circuit

[David G Dixon]

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