[sdiy] led driver from negative power rail

David G. Dixon dixon at interchange.ubc.ca
Wed Sep 8 02:38:21 CEST 2010

> Can anyone point me to a better way of doing this? Otherwise, I guess
> I'll have to pull a positive lead out to the panel as well...

With the diodes pointed in the right directions, this ought to work just
fine.  You might try increasing the value of the base resistor.  I would try
100k or so.  The current through the LED is basically the base current times
h_FE (which is about 100), and brightness is going to be proportional to LED
current.  Hence, limit the base current by increasing the base resistor and
the LED should become dimmer at lower signal voltages. I would also increase
the LED current-limiting resistor (I like 2.2k in this application, to
preserve current and to render LEDs less annoyingly bright when the
transistor is saturated).

> My apologies, BTW, to the original author of the drawing, I have been
> informed it was not for public viewing. I'll get it removed ASAP.

No wonder, given its embarrassing shortcomings!  ;-) (It's not like I
haven't released a few stinkers of my own on this list over the past two
years.  He probably drew it at 2:00 am, after a hard night down the pub.)

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