[sdiy] LED driver circuit

[Richie Burnett]

Yes, you need a resistor from base to ground for each switching transistor. The transistors will naturally switch on when their base-emitter voltage exceeds 0.7v. Since several of your 45 degree shifted sinewaves will be above 0.7 volts at the same instant in time, you'll get several LEDs lit at once.

The base drive resistor and base- emitter resistor form a potential divider to alter the threshold voltage at which the tranny will switch on. Try 100k for the base resistor and 10k from base to emitter. That should make each transistor turn on for voltages above approximately 8v. With this much higher threshold you should see a lot less conduction overlap in transistors, and at most two LEDs lit at once as they run through their sequence.

Hope this helps,

-Richie,

David G Dixon <dixon at mail.ubc.ca> 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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